s3. web viewunit 5—life. this word document contains all of the readings from the unit. all...

UNIT 5—LIFE

This Word document contains ALL of the readings from the unit. All readings include multiple copies at different Lexile levels. You are free to repurpose these materials as needed for your classroom. Please do remember to properly cite Big History as the source. If you modify the text, it will change the lexile level. As always, only print what you need.

LIFE AND PURPOSE.............................................................................2WHAT IS THE BIOSPHERE?................................................................12DARWIN, EVOLUTION, AND FAITH......................................................17WATSON, CRICK & FRANKLIN............................................................28SURVIVING AN EXTINCTION LEVEL EVENT..........................................38THE BIOLOGY OF AWARENESS...........................................................50CHARLES DARWIN: NATURALIST & AUTHOR.......................................57

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UNIT 5—LIFE TEXT READER 1

Life and PurposeWhat separates living things from nonliving things? It starts with two very basic activities: self-generation and self-maintenance.

Life and Purpose: A Biologist Reflects on the Qualities that Define Life (1210L)By Ursula Goodenough

What’s the difference between nonlife and life? To answer this question, we first need to define life. I’ll lay out what are to me the key hallmarks of life, and then offer a response that flows from such an understanding.

A key concept is that every organism is a self, a being. To be a “self” is to engage in two fundamental activities: self-generation and self-maintenance.

Self-generationSelf-generation entails the making of a self. If you’re a single-celled organism like a yeast, this involves starting out small, growing large, and dividing into two small daughter-yeasts that start the process again. If you’re a multicelled organism like a human, this involves starting out as a single fertilized egg, developing from an embryo to a fetus, and then taking the path from newborn to old age.

In all organisms on our planet today, the key players in self-generation are proteins. When a particular protein is made, it folds up into a particular shape, with crevices and bumps — something like a jigsaw-puzzle piece in three dimensions. These shapes allow proteins to do two major activities.

The first is to interact with other proteins, with the bumps fitting precisely into the crevices, to form the thousands of different kinds of chemical structures that make up a cell. Most parts of a cell are constructed from proteins, including the filaments that act as cellular skeletons, the channels that let ions in and out of the cells, and the receptors that let the self know what’s going on in the environment.

The second activity of proteins is to serve as enzymes, which allow chemical reactions inside the cell to take place with remarkable efficiency and accuracy. Again, shape is the key. The bumps and crevices bring together the participants in a chemical reaction and ensure that they form the proper kinds of chemical bonds with one another.

Self-maintenanceCritical to self-generation is obtaining the molecules and the energy that the self needs to run the store. One strategy is to use photosynthesis, turning the Sun’s light energy into food. The second is to ingest molecules that are made as a consequence of photosynthesis —


that is, to eat — and then break them down, using the energy released to drive self-generation. Here again, the shapes of enzymes are critical, but instead of controlling the formation of chemical bonds as in self-generation, they deftly supervise the breaking of chemical bonds, coupling this activity with the formation of energy-rich molecules like ATP (adenosine triphosphate) that keep the cell going.

Self-maintenance also entails self-protection, avoiding environmental hazards, predators, and disease.

Every organism is instructedAll the proteins we’ve been thinking about are encoded in genes embedded in DNA molecules. Each gene specifies the amino-acid sequence of a particular protein, and that sequence then defines how the protein will fold up into its functional shape.

The full set of genes necessary to pull together a self-generating and self-maintaining self is called a “genome.” A yeast genome and a human genome have many genes in common, notably those concerned with the universal project of self-maintenance, and many others that are distinctive. Daughter organisms inherit copies of genomes from parent organisms, allowing that kind of organism to continue and spread.

Embedded in the organization of genomes is the capacity to express certain genes, and hence certain proteins, on some occasions and not others. When it’s time to copy DNA into daughter molecules, the genes encoding the DNA-copying enzymes are “switched on.” When the copying process is completed, these genes are “switched off.” When it’s time for you to make red blood cells, genes encoding the hemoglobin protein are switched on in certain bone-marrow cells but remain switched off in most of the cells in your body. Thus a genome isn’t just a collection of genes; it functions continuously to instruct self-generation and self-maintenance.

Every organism can evolveAlthough DNA is copied with remarkable accuracy, mistakes sometimes happen, giving rise to mutant genes that encode variant amino-acid sequences and hence give rise to proteins with variant shapes. Also occurring are “mutations” that change the timing or magnitude of protein production.

The mutation may have no effect, at least in the short term, in which case the mutant daughter may self-organize and self-maintain just like the parent. At the other extreme, it may have disastrous consequences on self-organization and self-maintenance, and the daughter will not survive.

The most interesting mutations are those that generate instructions for a viable daughter that is somewhat different from its parent. For example, a parent duck may have delicate foot webbing while the webbing of a mutant daughter may be extra-thick. What happens next is totally dependent on environmental context. If the ducks hang out on mudflats, the mutant feet may allow for surer footing, hence better opportunities for feeding and fleeing predators, and the thick-footed trait will likely spread into future generations; if the ducks live in grasslands, the mutant feet may slow things down and the trait will be less likely to spread.


What I’ve just described is Darwinian evolution: inherited variations, coupled with natural selection. The ability of living organisms to evolve has generated the spectacular biodiversity that surrounds us, and without it, we humans would never have shown up.

Every organism has purposeSo, with this sense of what life is, can we come up with a single characteristic that distinguishes life from nonlife? Is there one towering difference between a mountain and a whale? After all, both are made of molecules. Both engage in chemistry. Both change through time.

For me, the most interesting single generalization is that organisms are purposive whereas nonlife is not. Organisms are about something, for something: muscles are for movement; eyes are for seeing. Organisms have goals. The short-term goal is to self-generate and self-maintain in a given environmental context. The long-term goal is to pass genome copies on to offspring, a goal that succeeds only if self-generation and self-maintenance succeed. Mountains are splendid, to be sure, but in the end they aren’t goal directed. They just are.

Taking this perspective, one could say that when life showed up on Earth, something completely new showed up: the emergence of purpose. Whether life, and hence purpose, exists anywhere else in the Universe is unknown and may remain a mystery. Meanwhile, we can enjoy and revel in the astonishing purposiveness that surrounds us here on Earth.

Life and Purpose: A Biologist Reflects on the Qualities that Define Life (980L) By Ursula Goodenough, adapted by Newsela

What is the difference between nonlife and life? To answer this question, we first need to define life. Below are the key features of life. Our answer will flow from understanding these features.

A key concept is that every organism is a self, a being. It takes two fundamental activities to be a self: self-generation and self-maintenance.

Self-generationSelf-generation means the making of a self. If you’re a single-celled organism like a yeast, this involves starting out small, growing large, and dividing into two small daughter-yeasts that start the process again. If you’re a multicelled organism like a human, this involves starting out as a single fertilized egg, developing from an embryo to a fetus, and then taking the path from newborn to old age.

For all organisms on our planet today, proteins are very important for self-generation. When a protein is made, it folds up into a particular shape, with crevices and bumps — something like a 3-D jigsaw-puzzle piece. These shapes allow proteins to do two major activities.


The first is to interact with other proteins. Bumps from one protein fit into the crevices of others. They combine to form the thousands of different kinds of chemical structures that make up a cell.

Most parts of a cell are made from proteins. This includes the filaments that act as cell skeletons, the channels that let ions in and out of cells, and the receptors that let the cell know what’s going on in the environment.

The second activity of proteins is to serve as enzymes. Enzymes allow chemical reactions inside the cell to happen with remarkable efficiency and accuracy. Here, the shape of proteins is important. The bumps and crevices bring together the proteins in a chemical reaction and ensure that they form the right kind of chemical bonds with each other.

Self-maintenanceThe self must get the molecules and energy that it needs for self-generation. One strategy is to use photosynthesis, turning the Sun’s light energy into food. The second is to ingest molecules that are made using photosynthesis — to eat — and then break them down, using the energy released for self-generation.

Here again, the shapes of enzymes are critical. Instead of controlling the formation of chemical bonds as in self-generation, they supervise the breaking of chemical bonds, and form energy-rich molecules to keep the cell going.

Self-maintenance also involves self-protection: avoiding environmental hazards, predators, and disease.

Every organism is instructedAll the proteins we’ve been thinking about are encoded in genes. The genes are embedded in DNA molecules. Each gene specifies the amino-acid sequence of a particular protein. That sequence defines how the protein will fold up into its functional shape.

The full set of genes necessary to pull together a self-generating and self-maintaining self is called a “genome.”

A yeast genome and a human genome have many genes in common, notably those concerned with self-maintenance. They also have many others that are distinctive. Daughter organisms inherit copies of genomes from parent organisms, allowing that kind of organism to continue and spread. A genome has the ability to express certain genes, and certain proteins, on some occasions and not others.

When it’s time to copy DNA into daughter molecules, the genes controlling the DNA-copying enzymes are “switched on.” When the copying process is finished, these genes are “switched off.”

When it’s time for you to make red blood cells, genes controlling the hemoglobin protein are switched on in certain bone-marrow cells, but they remain switched off in most of the cells in your body. Thus, a genome isn’t just a collection of genes. It functions continuously to instruct self-generation and self-maintenance.


Every organism can evolveDNA is copied with remarkable accuracy, but mistakes sometimes happen. These mistakes produce mutant genes that make proteins with different shapes.

The mutation may have no effect, at least in the short term, in which case the mutant daughter may self-organize and self-maintain just like the parent. At the other extreme, it may have disastrous consequences on self-organization and self-maintenance, and the daughter will not survive. The most interesting mutations are those that generate instructions for a daughter that is somewhat different from its parent.

For example, a parent duck may have delicate foot webbing while the webbing of a mutant daughter may be extra-thick. What happens next depends on environmental setting. If the ducks hang out on mudflats, the mutant feet may allow for surer footing and better opportunities for feeding and fleeing predators. The thick-footed trait will likely spread into future generations. If the ducks live in grasslands, the mutant feet may slow things down and the trait will be less likely to spread.

What I’ve just described is Darwinian evolution: inherited variations, coupled with natural selection. The ability of living organisms to evolve has generated the spectacular biodiversity that surrounds us, and without it, we humans would never have shown up.

Every organism has purposeSo, now we have a sense of what life is. Can we come up with a single characteristic that distinguishes life from nonlife? Is there one towering difference between a mountain and a whale? After all, both are made of molecules. Both engage in chemistry. Both change over time.

For me, the most interesting single generalization is that organisms have purpose where nonlife does not.

Organisms are about something, for something: muscles are for movement; eyes are for seeing. Organisms have goals. The short-term goal is to self-generate and self-maintain in a given environment. The long-term goal is to pass genome copies on to offspring. This goal only succeeds if self-generation and self-maintenance succeed. Mountains are splendid, to be sure, but in the end they aren’t goal directed. They just are.

If we take this view, we can say that when life showed up on Earth, something completely new showed up: the emergence of purpose. Whether life, and purpose, exist anywhere else in the Universe is unknown and may remain a mystery. Meanwhile, we can enjoy and appreciate the astonishing purpose that surrounds us here on Earth.

Life and Purpose: A Biologist Reflects on the Qualities that Define Life (860L)By Ursula Goodenough, adapted by Newsela


What’s the difference between nonlife and life? Before we answer this question, we must define life. Below are the key features of life. Once we understand these features, we will be able to answer the question.

A key idea is that every organism is a self, a being. It takes two basic activities to be a self: self-generation and self-maintenance.

Self-generationSelf-generation is the making of a self. For a single-celled organism like a yeast, this involves starting out small, growing large, and dividing into two small daughter-yeasts that start the process again.

If you’re a multicelled organism like a human, this involves starting out as a single fertilized egg, developing from an embryo to a fetus, and then taking the path from newborn to old age. For all organisms on our planet today, proteins are very important for self-generation. When a protein is made, it folds up into a particular shape. These shapes are like a 3-D jigsaw-puzzle. They have bumps that stick out and crevices, like holes. These shapes allow proteins to do two major activities.

First, they interact with other proteins. Bumps from one protein fit into the crevices of others. They combine to form the thousands of chemical structures that make up a cell. Most parts of a cell are made from proteins. These include the filaments that act as cell skeletons, the channels that let ions in and out of cells, and the receptors that let the cell know what’s going on around it.

Second, proteins serve as enzymes. Enzymes allow chemical reactions to happen inside the cell with remarkable efficiency and accuracy.

Self-maintenanceA self must get the molecules and energy it needs in order to self-generate. Some organisms use photosynthesis, turning the Sun’s light energy into food.

Others consume molecules that are made using photosynthesis. They eat, in other words. They then break down the molecules, using the energy released for self-generation.

Self-maintenance also requires self-protection: avoiding dangers in the environment, predators, and disease.

Every organism is instructedAll the proteins we’ve been thinking about are encoded in genes. The genes are embedded in DNA molecules. Each gene controls the amino-acid sequence of a particular protein. That sequence defines what shape a protein will fold into.

The full set of genes necessary to create a self-generating and self-maintaining self is called a “genome.” A yeast genome and a human genome have many genes in common, notably those concerned with self-maintenance. They also have many others that are different.


Daughter organisms inherit copies of genomes from parent organisms, allowing that kind of organism to continue and spread.

Genomes can “turn on” certain genes and proteins on some occasions and not others.

When it’s time to copy DNA into daughter molecules, the genes controlling the DNA-copying enzymes are “switched on.” When the copying process is finished, these genes are “switched off.” To make red blood cells, genes controlling the hemoglobin protein are switched on in certain bone-marrow cells, but they remain switched off in most of the cells in your body.

A genome isn’t just a collection of genes. It continuously instructs self-generation and self-maintenance.

Every organism can evolveDNA is usually copied with remarkable accuracy, but mistakes can happen. These mistakes produce mutant genes that make proteins with different shapes.

Sometimes the mutations have no short-term effect. The mutant daughter can self-organize and self-maintain just like the parent. Other times, the mutation doesn’t allow the organism to self-organize or self-maintain and it will not survive. The most interesting mutations are those that create a daughter that survives, but is somewhat different than its parent.

For example, a parent duck may have thin webbed feet. A mutant daughter may have extra-thick webbed feet. What happens next depends on the environment where the ducks live. If the ducks hang out on mudflats, the mutant feet may help the duck by creating better opportunities for feeding and fleeing predators. Here, the thick-footed trait will probably spread into future generations. But if the ducks live in grasslands, the mutant feet may slow things down. Then the trait will be less likely to spread.

I’ve just described Darwinian evolution: inherited mutations and natural selection. The spectacular biodiversity around us is thanks to living organisms’ ability to evolve. Without it, we humans never would have showed up.

Every organism has purposeNow we have a sense of what life is. Is there a single characteristic that distinguishes life from nonlife? What is the real difference between a mountain and a whale? After all, both are made of molecules. Both engage in chemistry. Both change over time.

For me, the answer is purpose. A whale has purpose, but a mountain does not.

Organisms are about something, for something. Muscles are for movement; eyes are for seeing. Organisms have goals. The short-term goal is to self-generate and self-maintain. The long-term goal is to pass the genome on to offspring. This can only happen if an organism self-generates and self-maintains. Mountains are splendid of course, but they don’t have purpose or goals. They just are.

Taking this perspective, we can say that when life showed up on Earth, something completely new showed up: purpose. Whether life, and purpose, exist anywhere else in the


Universe is unknown. It may remain a mystery. Meanwhile, we can enjoy and appreciate the astonishing purpose that surrounds us here on Earth.

Life and Purpose: A Biologist Reflects on the Qualities that Define Life (680L) By Ursula Goodenough, adapted by Newsela

What’s the difference between nonlife and life? To answer this question, we must first define life. Below are the key features of life. Answering this question depends on understanding these features.

It’s important to understand that every organism is a “self” or a “being.” A “self” performs two basic activities. It generates itself and maintains itself.

Self-generationSelf-generation is the making of a self. A yeast is an organism made up of just one cell. Self-generation for a yeast means starting out small, growing large, and dividing into two small daughter-yeasts that start the process again.

Humans are multicelled organisms. We start out as a single fertilized egg, develop from an embryo to a fetus, then take the path from newborn to old age. All organisms on our planet today use proteins to self-generate. Proteins come in particular shapes with bumps and cracks. Like a puzzle piece — the bumps stick out, and the cracks are the holes. These shapes allow proteins to do two major activities.

First, proteins interact with each other. Bumps from one protein fit into the crevices of others. They combine to form the chemical structures that make up a cell. Most parts of a cell are made from proteins. Second, proteins serve as enzymes. Enzymes allow efficient and accurate chemical reactions to happen inside the cell.

Self-maintenanceEvery self must get the energy it needs to self-generate. Some organisms use photosynthesis. They turn the Sun’s light energy into food. Other organisms eat. They consume molecules, and break them down. They use the energy released for self-generation.

Self-maintenance also requires self-protection. Each self must avoid environmental hazards, predators and disease.

Every organism is instructedThe proteins we’ve been talking about are encoded in genes. The genes are embedded in DNA molecules. Each gene controls a particular protein. Genes tell proteins what shapes to fold into.

A full set of genes that can create a self-generating and self-maintaining self is called a “genome.” A yeast genome and a human genome have many genes in common. These are


mostly the ones that deal with self-maintenance. They also have many genes that are different. Parent organisms pass down copies of their genomes to daughter organisms. This allows organisms to continue and spread.

Genomes can “turn on” and “turn off” certain genes and proteins. When it’s time to copy DNA into daughter molecules, genes controlling DNA-copying enzymes are “turned on.” When the copying is finished, these genes are “turned off.” To make red blood cells, genes controlling the hemoglobin protein are switched on in certain bone-marrow cells. However, they stay switched off in most of the cells in your body.

A genome isn’t just a collection of genes. It continuously controls self-generation and self-maintenance.

Every organism can evolveDNA is usually copied with great accuracy. But mistakes can happen. These mistakes produce mutant genes that make proteins with different shapes.

Some mutations have basically no effect. The daughter organism can survive normally. Other mutations are deadly for the daughter organism. It may die immediately. The most interesting mutations are when the daughter survives, but is a little different than its parents.

For example, a parent duck may have thin webbed feet. A mutant daughter may have extra-thick webbed feet. What happens next depends on the environment where the ducks live. If the ducks live on mudflats, the mutant feet may help the duck. It may be easier for the duck to walk on the mud, find food, and avoid predators. Here, the thick-footed trait would probably spread into future generations. But if the ducks live in grasslands, the mutant feet may make the duck slower. It might not survive. The trait is less likely to spread.

I’ve just described Darwinian evolution: passed-down mutations and natural selection. The biodiversity around us is thanks to living organisms’ ability to evolve. Without it, we humans would never have showed up.

Every organism has purposeNow we understand what life is. But is there one thing that distinguishes life from nonlife? How is a mountain really different from a whale? After all, both are made of molecules. Both engage in chemistry. Both change over time.

For me, the answer is purpose. A whale has purpose, but a mountain does not.

Organisms are about something, for something. Muscles are for movement; eyes are for seeing. Organisms have goals. The short-term goal is to self-generate and self-maintain. The long-term goal is to pass the genome on to offspring. This can only happen if an organism self-generates and self-maintains. Mountains are splendid of course, but they don’t have purpose or goals. They just are.

We can say that when life showed up on Earth, something completely new showed up: purpose. Whether life, and purpose, exist anywhere else in the Universe is unknown. It may


remain a mystery. Meanwhile, we can enjoy and appreciate the astonishing purpose that surrounds us here on Earth.


What Is the Biosphere? Geologist Eduard Seuss invented the word because he felt it was important to try to understand life as a whole rather than singling out particular organisms.

What Is the Biosphere? (1410 L)By Big History Project

Sometimes the history of a word can tell us a lot about what the word means. The study of words even has its own name: etymology. Often, a closer look at a word unfolds into another story, one that may connect to other people and other scientific studies.

The word biosphere was first used by English-Austrian geologist Eduard Suess (1831–1914) more than a hundred years ago in a four-volume work entitled The Face of the Earth (1885–1908). Suess is also credited with being the first person to propose the existence of the supercontinent Gondwanaland and the ancient Tethys Ocean, based upon his work studying fossils in the Alps and his knowledge of the fossils of Glossopteris ferns that were found on several different continents.

At the time, no one knew about plate tectonics. German meteorologist Alfred Wegener didn’t put forth his theory on continental drift until 1912, a couple of years before Suess died, and the best explanation Suess could offer for the presence of marine fossils in the mountains was that the waters of the Tethys Ocean had flooded the whole Earth, not that the continents had actually drifted apart and changed. This is a great example of how limited evidence can sometimes lead scientists to settle on incorrect conclusions. It also demonstrates how the work of one person can build on that of others, collectively leading to new discoveries about the world around us.

Suess combined bio, meaning “life,” and sphere, referencing the Earth’s rounded surface, to express the portion of the Earth that supported life. He invented the word because he felt it was important to try to understand life as a whole rather than singling out particular organisms. He wrote in The Face of the Earth:

The plant, whose deep roots plunge into the soil to feed, and which at the same time rises into the air to breathe, is a good illustration of organic life in the region of interaction between the upper sphere and the lithosphere, and on the surface of continents it is possible to single out an independent biosphere.

As our knowledge of life on the planet evolves, we’ve come to use the word biosphere as a way of explaining the entire intertwined network of life on Earth. This concept combines an understanding of geology, knowledge of the distinct layers that make up the Earth and its atmosphere, and an awareness of the biodiversity surrounding us. We can think of the biosphere as the habitat, or home, for all life on our planet, in all its forms, and with all its intricate biological and geological relationships.


Biosphere = the network of all life on Earth

Worlds within worldsThe biosphere is incredibly small — just a thin layer around a medium-size planet. But it’s also incredibly large, when you consider all of the different living things and our planet’s vast expanses of water and land. As with most things that seem large and encompassing, it’s possible to break down the biosphere and to use other words to describe specific environments or habitats.

These smaller areas are called “ecosystems,” and they are characterized by particular geologic or climatic features that accommodate certain forms of life. Oceans, jungles, and mountain ranges can be ecosystems, but even more specific places can be their own ecosystems. Think of a cave, a river or river valley, a coral reef, a city, or the “vent communities” that surround black smokers on the ocean floor. Altitude, latitude, longitude, climate, soils, and terrain can all contribute to the distinct features of an ecosystem — the Earth’s geologic processes have produced a multitude of diverse environments. The biosphere boasts incredible diversity and, even in extreme environmental conditions, astounding examples of life’s flexibility and determination.

Every organism — from baboons to bacteria — has a specialized way to make a living as it vies for resources and energy and reproduces within its own environment. Examining these individual ecosystems, using biology and geology, reveals the many complex relationships between life and the planet we all share.

What Is the Biosphere? (1210 L)By Big History Project

Sometimes the history of a word can tell us a lot about what the word means. Often, a closer look at a word unfolds into another story, one that may connect to other people and other scientific studies.

The word biosphere was first used by English-Austrian geologist Eduard Suess (1831–1914) more than a hundred years ago in a four-volume work called The Face of the Earth (1885–1908).

Suess combined bio, meaning “life,” and sphere, referencing the round Earth, to describe the part of the Earth that supported life. He invented the word because he felt it was important to try to understand life as a whole rather than singling out particular organisms. He wrote in The Face of the Earth:

The plant, whose deep roots plunge into the soil to feed, and which at the same time rises into the air to breathe, is a good illustration of organic life in the region of interaction between the upper sphere and the lithosphere, and on the surface of continents it is possible to single out an independent biosphere.

As we learn more about the planet, we’ve come to use the word biosphere as a way of explaining the entire interconnected network of life on Earth. This concept combines an


understanding of geology, knowledge of the different layers that make up the Earth and its atmosphere, and an awareness of the biodiversity surrounding us. We can think of the biosphere as the habitat, or home, for all life on our planet, in all its forms, and with all its intricate biological and geological relationships.


Worlds within worldsThe biosphere is incredibly small. It’s just a thin layer around a medium-sized planet. But it’s also incredibly large when you consider all the different living things and our planet’s vast areas of water and land. As with most things that seem very large, it’s possible to break down the biosphere and use other words to describe specific environments or habitats.

These smaller areas are called “ecosystems.” An ecosystem is a unique area that supports certain forms of life. Oceans, jungles, and mountain ranges can be ecosystems, but even more specific places can be their own ecosystems. Think of a cave, a river or river valley, a coral reef, a city, or the “vent communities” that surround hydrothermal vents on the ocean floor.

Altitude, latitude, longitude, climate, soils, and terrain can all contribute to the distinct features of an ecosystem. The Earth’s geologic processes have produced many diverse environments. The biosphere boasts incredible diversity and, even in extreme environmental conditions, astounding examples of life’s flexibility and determination.

Every organism has a specialized way to make a living. Baboons and bacteria both fight for resources and energy in their own way. They all reproduce within their own environment. Examining these individual ecosystems, using biology and geology, reveals the many complex relationships between life and the planet we all share.

What Is the Biosphere? (950L)By Big History Project

Sometimes the history of a word can tell us a lot about what the word means. A closer look at a word can unfold into another story. This story may connect to other people and other scientific studies.

The word biosphere was first used by English-Austrian geologist Eduard Suess. It appeared more than a hundred years ago in his book The Face of the Earth. Suess combined two words to make biosphere. Bio means life. Sphere refers to the round Earth.

The biosphere refers to the part of our planet that supports life. Suess invented the word because he felt it was important to understand life as a whole — not just particular organisms.

In The Face of the Earth, Suess used a plant as an example of organic life interacting between the upper sphere and the lithosphere, or Earth's crust. The plant feeds itself through its roots deep in the soil. At the same time, it rises into the air to breathe. Seuss


wrote that on "the surface of continents it is possible to single out an independent biosphere.”

As we learn more about life on Earth, we’ve come to use the word biosphere as a way of explaining how life is interconnected. This idea combines the study of geology, the Earth’s atmosphere, and an awareness of the different types of life that surround us. We can think of the biosphere as the habitat, or home, for all life on the planet. The biosphere is home to life in all its forms, with all its biological and geological relationships.


Worlds within worldsThe biosphere is incredibly small. It’s just a thin layer around a medium-sized planet. But it’s also incredibly large when you consider all the living things on Earth and our planet’s huge areas of water and land.

As with most things that seem very large, it’s possible to break down the biosphere and use different words to describe smaller environments and habitats. These smaller areas are called "ecosystems." An ecosystem is a unique area that supports certain forms of life. Oceans, jungles and mountain ranges can be ecosystems, but so can more specific places. Think of a cave, a river, a coral reef, a city, or the “vent communities” that surround hydrothermal vents on the ocean floor.

Altitude, latitude, longitude, climate, soils, and terrain can all make an ecosystem unique. The Earth’s geologic processes have produced many different environments. The biosphere has incredible diversity. Even in extreme environmental conditions, we see amazing examples of life’s flexibility and determination.

Every organism has a different way to make a living. From baboons to bacteria, all life forms must fight for resources and energy. They must all reproduce within their own environment. Examining these individual ecosystems, using biology and geology, reveals the many complex relationships between life and the planet we all share.

What Is the Biosphere? (760L)By Big History Project

Sometimes the history of a word can tell us a lot about what the word means. The word biosphere was first used by English-Austrian geologist Eduard Suess. It appeared more than a hundred years ago in his book The Face of the Earth. Suess combined two words to make biosphere. Bio means life. Sphere refers to the round Earth.

The biosphere is the part of our planet that supports life. Suess made up this word because he wanted people to see how all life on Earth is related. He did not want to just focus on living things separately. We now use the word biosphere to explain how all life on Earth is connected.

In The Face of the Earth, Suess used a plant as an example of his idea. The plant feeds itself through its roots deep in the soil. At the same time, it rises into the air to breathe. The


soil and the air are part of two different spheres, yet the plant interacted in both. Seuss wrote that on "the surface of continents it is possible to single out an independent biosphere.”

The biosphere is the habitat for all life on the planet. The biosphere is home to life in all its forms, with all its relationships.

Biosphere = the network of all life on Earth.

Worlds within worldsThe biosphere is incredibly small. It’s just a thin layer around a medium-sized planet. But it’s also incredibly large when you consider all the living things on Earth. It includes huge land areas and the massive oceans. The biosphere can be broken down in smaller areas. These smaller areas are called "ecosystems." An ecosystem is a unique area that supports certain forms of life.

Oceans, jungles, and mountain ranges can be ecosystems, but so can smaller places. Think of a cave, a river, a coral reef, or a city. The “vent communities” that surround hydrothermal vents on the ocean floor are ecosystems.

Each ecosystem is different. Its location on Earth, its climate, soils, terrain and other things make it one of a kind. The Earth has many different environments. The biosphere has incredible diversity. Even in extreme environmental conditions, we see amazing examples of life’s flexibility and determination.

Every organism has a different way to make a living. Every species must fight for resources and energy. All must reproduce in their own environment. Through biology and geology, we can study these individual ecosystems. What we learn reveals the many complex relationships between life and the planet we all share.


Darwin, Evolution, and Faith Nothing in modern science has proved more challenging to religious believers than evolution. Disputes about Darwin’s ideas have run for more than 150 years, and they are as heated today as ever.

Darwin, Evolution, and Faith (1350L)By John F. Haught

Nothing in contemporary science has proved more challenging to religious believers than evolutionary biology. Disputes about the religious and theological implications of Darwin’s ideas have been going on now for more than a century and a half, and they are as heated today as ever.

Why has Darwin’s science been such a religiously troubling idea? In those parts of the world influenced by the Bible and the Qur’an, we may point to at least five reasons: (1) Darwinian biology tells a whole new story of creation, one that cannot be literally reconciled with religious creation stories such as those narrated in the book of Genesis; (2) the evolutionary notion of natural selection seems to eliminate the role of God in creating the various species of life; (3) Darwin’s theory of human descent from nonhuman forms of life raises questions about traditional beliefs in human uniqueness, such as the biblical claim that human beings are created “in the image and likeness of God”; (4) the prominent role of chance or accidents in evolution raises questions about whether a creator truly cares for the world; and (5) the competitive “struggle for existence” inherent in evolution seems at odds with a Universe created by God.

What did Darwin think about God? After returning, in 1836, from his sea voyage, he spent the next 20 years or so brooding about the theological implications of his discoveries. He had once taken for granted, as almost everyone else did at the time, that all living species came into being by God’s special creation in the beginning. However, reflecting on what he had observed during his sea voyage, Darwin began to wonder how his Christian faith could be true. His doubts continued to grow, probably reinforced by the anguish he experienced at the deaths of his father and 10-year-old daughter, Annie. In his autobiography Darwin writes: “Disbelief crept over me at a very slow rate, but was at last complete. The rate was so slow that I felt no distress, and have never since doubted for a single second that my conclusion was correct.”

Still, Darwin never considered himself the outright atheist that some modern writers have made him out to be. He continued to refer occasionally to the work of a “Creator” who fashioned the Universe and its general laws but who then left its living outcomes to a combination of chance and natural selection. In any case, the religious world of his time was ill prepared for his ideas. Even now, some people are still reeling from the shock Darwin


seems to have delivered to traditional beliefs. For others, however, an appreciation of his ideas deepens and widens their faith in God.

Three approachesWhen Darwin’s On the Origin of Species first appeared, most people in Europe and America read the biblical accounts of origins literally. They thought the world was only around 6,000 years old and all living species had been created separately and in a fixed way at the time of the world’s origins. So, can ancient scriptural accounts of the world’s creation by God be reconciled with Darwin’s new story? Here are three responses to the question:

1. ConflictThis approach, whose adherents include both religious believers and skeptics, maintains that evolution by natural selection can never be reconciled with belief in God. Conflict comprises two main groups. On one side are “creationists” and proponents of “intelligent design.” Both groups reject evolution as scientifically misguided. Creationists are Christians, and Muslims, who consider their holy books to be the source of true science and who therefore reject Darwinian evolution as simply wrong. Proponents of intelligent design do not necessarily read the scriptural texts literally, but they consider the complexity of life and subcellular mechanisms too staggering to be the result of natural causes alone. They argue that a supernatural agency is responsible for the complex “design” that exists in the domain of life.

There are also those who believe strongly in evolution and use it in their arguments against the existence of a creative and providential deity. These people use the conflict position to reject both creationism and intelligent design as wishful thinking incompatible with evolutionary biology. Especially in the United States, the sense of a conflict between evolution and faith continues to dominate public discussions. There are other ways, however, of looking at the issue.

2. ContrastThis approach claims that science and faith are responding to completely different kinds of questions, and so there can be no genuine conflict between evolution and theology. The contrast approach argues that people should simply acknowledge that sacred scriptures are not science and that Darwinian science has nothing to do with faith. In the Roman Catholic Church, for example, Pope Leo XIII in 1893 instructed the faithful not to look for scientific information in biblical texts. Galileo had given his fellow Catholics the same advice back in the seventeenth century. As far as evolution is concerned, therefore, Darwin’s theory of life’s descent and diversity should never be placed in competition with biblical creation narratives. The creation stories in the Bible were not intended to satisfy scientific curiosity but to urge devotees to be grateful for the richness of creation. The Bible’s intention is to answer questions such as “Why is there anything at all rather than nothing?” and “Is there an eternal reason for trusting that life is worthwhile?”

For the most part, Roman Catholics and other mainstream Christians have avoided confusing science with faith and theology by recognizing that they answer different questions and serve different needs. Nevertheless, major strands of fundamentalist and evangelical


Protestantism still view the Bible as scientifically accurate, and they consider Darwin’s science to be incompatible with biblical “science.” According to the contrast position, however, reading the Bible as a source of scientific information, whether by creationists or religiously skeptical evolutionists, misses the whole point of the ancient religious literature.

3. ConvergenceThis approach sees truth in both science and religion, and since truth cannot contradict truth, scientific and religious truths must be reconcilable. It adds that in the real world science and faith can enrich each other. This means that, after Darwin, people of faith cannot have exactly the same thoughts about God as before. Religious believers and theologians need to readjust their thinking about God after Darwin no less than they did after Copernicus’s demonstration of a Sun-centered Solar System. Challenges such as evolution are essential to keeping faith and theology alive and healthy. Theology was eventually able to adjust to a heliocentric Universe, so it can now adjust to evolution. The theory of evolution and faith in a creative and providential deity are not mutually exclusive and numerous theologians and scientists have found ways to reconcile these beliefs. In their view, there is no necessary danger to religious faith in thinking bold new thoughts about God after Darwin. After all, even the idea of God, whether people are aware of it or not, has evolved over the course of time, and it will continue to do so. If we take the time to think about God in terms of evolution, convergence argues, religious understanding will have everything to gain and nothing to lose.

Reconciling evolution and faithEver since Darwin, many Christians and other religious people have been enthusiastic about the discovery of evolution. For example, immediately after On the Origin of Species was published, the learned Anglican priest and theologian Charles Kingsley gave thanks to Darwin for demonstrating how ingenious and creative evolution is, and how the exciting new picture of life had enlarged his understanding of the Creator. A God who can make a Universe that can make itself by way of natural processes, Kingsley proposed, is much more impressive and worthy of worship than one who is always tinkering with the world or keeping it tied to divine puppet strings.

Likewise, the Catholic priest and renowned geologist and paleontologist Pierre Teilhard de Chardin (1881–1955) wrote many works arguing that his own faith makes more sense after Darwin than it did before. As one of the first exponents of Big History, Teilhard emphasized that evolutionary biology — along with geology, astrophysics, and cosmology — clearly demonstrated that the Universe is still coming into being. The fact that this process involves struggling, chance, failure, and loss — along with grandeur and beauty — is completely consistent with the fact that the Universe remains unfinished. The role of a creator, Teilhard proposed, is not to force the Universe to fit tightly and immediately into a prefabricated mold, but to open it to an ever-widening range of new possibilities as it moves toward a fresh future.

God creates this open Universe through natural processes rather than magic. As an evolutionist and a devout Christian, Teilhard saw no contradiction in interpreting the whole of


cosmic history as the response to an invitation by God. God, he insisted, is not a dictator but the ultimate and everlasting goal of cosmic process. God always makes room for freedom. Moreover, Teilhard suggested that, with evolution, the meaning of human life and moral action includes each one of us contributing to the great work of ongoing creation.

Darwin, Evolution, and Faith (1100L)By John F. Haught, adapted by Newsela

Nothing in modern science has proved more challenging to religious believers than the theory of evolution. For more than a century and a half people have fought over what Darwin’s ideas mean for religious belief, and the debate is just as heated today as ever.

Why has Darwin’s science been such a troubling idea for religious believers? In those parts of the world influenced by the Bible and the Qur’an, we may point to at least five reasons:

1. Darwinian biology tells a whole new story of creation. This story is very different than religious creation stories, such as those in the book of Genesis;

2. Evolution and natural selection seem to eliminate the role of God in creating the various species of life;

3. Darwin’s theory assumes humans descended from nonhuman forms of life. This idea raises questions about traditional beliefs in human uniqueness. It calls into doubt the biblical claim that human beings are created “in the image and likeness of God”;

4. Accidents play an important role in evolution. They raise questions about whether a creator truly cares for the world;

5. The competitive “struggle for existence” in evolution doesn’t seem to fit with a Universe created by God.

What did Darwin think about God? After returning from his sea voyage, he spent the next 20 years thinking about what impact his discoveries would have on religion. He had once believed that all living species were created by God in the beginning. However, what he learned on his sea voyage caused him to begin to doubt his Christian faith.

His doubts were probably strengthened by the deaths of his father and 10-year-old daughter. In his autobiography, Darwin writes: “Disbelief crept over me at a very slow rate, but was at last complete. The rate was so slow that I felt no distress, and have never since doubted for a single second that my conclusion was correct.”

Still, Darwin never considered himself an atheist. He occasionally referred to a “Creator” who made the Universe and its general laws, but left living things to chance and natural selection.

In any case, the religious world of Darwin’s time was not prepared for his ideas. Even now, some people are still recovering from the shock Darwin delivered to traditional beliefs. For others, an appreciation of his ideas deepens and widens their faith in God.

Three approachesWhen Darwin’s On the Origin of Species first appeared, most people read the biblical creation account literally. They thought the world was only around 6,000 years old. They


believed all living species had been created separately and permanently at the world’s origins.

So, can ancient religious accounts of the world’s creation by God be reconciled with Darwin’s new story? Here are three responses to the question:

1. ConflictThis approach is taken by both religious believers and skeptics. It maintains that evolution and natural selection can never fit with belief in God. Conflict is made up of two main groups.

On one side are “creationists” and supporters of “intelligent design.” Both groups reject evolution. Creationists are Christians and Muslims who consider their holy books to be the source of true science. They therefore reject Darwinian evolution as simply wrong.

Supporters of intelligent design don’t always read the holy book literally, but they consider the complexity of life too complicated to be the result of natural causes alone. They argue that only a supernatural force is responsible for the “design” of life.

There are also people who believe strongly in evolution and use it to argue against the existence of God. These people reject both creationism and intelligent design. Especially in the United States, a sense of conflict continues to dominate public discussions. However, there are other ways of looking at the issue.

2. ContrastThis approach claims that science and faith are answering different questions, so there can be no real conflict between evolution and religion. The contrast approach argues that people should acknowledge that sacred scriptures are not science. It also asserts that Darwin's theory has nothing to do with faith.

Pope Leo XIII in 1893 instructed the faithful not to look for scientific information in biblical texts. Galileo had given his fellow Catholics the same advice back in the seventeenth century. In this approach, Darwin’s theory of evolution should not be placed with competition with biblical creation narratives. The creation stories in the Bible were not intended to teach science, but to urge believers to be grateful for the richness of creation.

The Bible’s intention is to answer questions such as “Why is there anything at all rather than nothing?” and “Is there an eternal reason for trusting that life is worthwhile?”

For the most part, Roman Catholics and other mainstream Christians have avoided confusing science with religion by recognizing that they answer different questions and serve different needs. Still, many fundamentalist groups still view the Bible as scientifically accurate. They consider Darwin’s science to be incompatible with biblical “science.”

According to the contrast position, reading the Bible as a source of scientific information misses the whole point of the ancient religious literature.

3. Convergence


This approach sees truth in both science and religion. Since truth cannot contradict truth, scientific and religious truths must both be valid. In this model, science and faith can enrich each other.

After Darwin, people of faith must readjust their thinking about God. This is similar to what happened after Copernicus’s demonstration of a Sun-centered Solar System. Challenges such as evolution are important for keeping faith alive and healthy. Religion was able to adjust to a heliocentric Universe, so it can now adjust to evolution.

The theory of evolution and faith in God do not contradict each other. Many religious scholars and scientists have found ways to balance both beliefs. In this view, there is no danger to faith in thinking bold new thoughts about God after Darwin. After all, the idea of God has evolved over the course of time and will continue to do so.

Reconciling evolution and faithSince Darwin, many Christians and other religious people have been enthusiastic about the discovery of evolution.

For example, Anglican priest and theologian Charles Kingsley gave thanks to Darwin. He said Darwin’s picture of ingenious and creative evolution enlarged his understanding of the Creator. Kingsley proposed that a God who can make a Universe that can make itself by natural processes is more impressive than a God who keeps it tied to divine puppet strings.

Likewise, the Catholic priest and renowned geologist and paleontologist Pierre Teilhard de Chardin wrote many works arguing that his own faith makes more sense after Darwin than it did before. Teilhard said that evolutionary biology, geology, astrophysics, and cosmology clearly demonstrated that the Universe is still being created.

The role of a creator, Teilhard proposed, is not to force the Universe to fit neatly into a certain shape, but to open it to a widening range of possibilities as it moves toward a fresh future. God creates this open Universe through natural processes rather than magic. Teilhard suggested that with evolution, the meaning of human life includes contributing to the great work of ongoing creation.

Darwin, Evolution, and Faith (960 L)By John F. Haught, adapted by Newsela

Nothing in modern science is more challenging to religious believers than the theory of evolution. For more than 150 years people have fought over what evolution means for religion. The debate is just as heated today as ever.

Why has Darwin’s science been such a troubling idea for religious believers? For Christians and Muslims, we can point to at least five reasons:

1. Darwinian biology tells a story of creation that is very different than religious creation stories;

2. Evolution and natural selection seem to make God unnecessary in creating the various species of life;


3. Darwin argued that humans descended from nonhuman forms. Traditional religious belief holds that humans are unique, and created “in the image and likeness of God”;

4. In evolution, chance and accidents are important. This raises questions about whether a creator truly cares for the world; and

5. The competitive “struggle for existence” in evolution doesn’t seem to fit with a Universe created by God.

What did Darwin think about God? After his sea voyage, he spent 20 years thinking about what impact his discoveries would have on religion. Darwin once believed that all living species were created by God. However, what he learned on his sea voyage caused him to doubt his Christian faith. The deaths of his father and 10-year-old daughter probably increased his doubts.

In his autobiography, Darwin wrote: “Disbelief crept over me at a very slow rate, but was at last complete. The rate was so slow that I felt no distress, and have never since doubted for a single second that my conclusion was correct.”

Still, Darwin never considered himself an atheist. He sometimes referred to a “Creator” who made the Universe and its general laws, but left living things to chance and natural selection. In any case, the religious world in Darwin’s time was not prepared for his ideas. Even now, some people are still recovering from the shock Darwin delivered to traditional beliefs. For others, an appreciation of his ideas deepens and widens their faith in God.

Three approachesWhen Darwin’s On the Origin of Species first appeared, most people read the biblical creation account word for word. They thought the world was only around 6,000 years old and all living species had been created separately and permanently at the world’s beginning.

Can ancient religious accounts of the world’s creation by God fit with Darwin’s new story? Here are three ways to answer that question:

1. ConflictThis approach says that Darwin’s theories can never fit with belief in God. Both religious believers and skeptics can take this approach.

Among religious believers, there are “creationists” and supporters of “intelligent design.” Both groups have problems with evolution. Creationists consider their holy books to be the source of true science. They therefore reject Darwinian evolution as simply wrong.

Supporters of intelligent design don’t always read their holy book literally. But they consider the complexity of life too detailed to be the result of natural causes alone. They argue that only a supernatural force could be responsible for the “design” of life.

There are also people who believe strongly in evolution and use it to argue against the existence of God. These people reject both creationism and intelligent design. Conflict surrounds public debate on evolution, especially in the United States. However, there are other ways of looking at the issue.

2. Contrast


This approach claims that science and faith answer different questions. Here, there can be no real conflict between evolution and religion. The contrast approach argues that people should understand that holy books are not science. Similarly, Darwinian science has nothing to do with faith.

Pope Leo XIII told his followers not to look for scientific information in biblical texts. Galileo had given his fellow Catholics the same advice back in the seventeenth century. In this approach, Darwin’s theory of evolution should not compete the Bible's origin stories. The creation stories in the Bible were not meant to teach science. Instead, they were meant to urge believers to be grateful for the richness of creation.

The Bible never meant to answer questions such as “Why is there anything at all rather than nothing?” and “Is there an eternal reason for trusting that life is worthwhile?”

For the most part, mainstream Christian churches have recognized that science and religion answer different questions and serve different needs. Still, some fundamentalist groups view the Bible as scientifically accurate. They think Darwin’s science cannot exist alongside biblical “science.”

According to the contrast position, reading the Bible as a source of scientific information misses the point of the ancient religious literature.

3. ConvergenceThis approach sees truth in both science and religion. Scientific and religious truths are both valid. In this model, science and faith enrich each other. After Darwin, people of faith must adjust their thinking about God. The same thing happened when Copernicus showed a Sun-centered Solar System.

Challenges such as evolution are important for keeping faith alive and healthy. Religion was able to adjust to a heliocentric Universe. It can now adjust to evolution. In this model, the theory of evolution and faith in God do not contradict each other. Many religious scholars and scientists have found ways to balance both beliefs.

In this view, there is no danger to thinking bold new thoughts about God after Darwin. After all, the idea of God has evolved over time and will continue to do so. For example, Anglican priest and religious scholar Charles Kingsley thanked Darwin. He said Darwin’s ideas of ingenious and creative evolution helped enlarge his understanding of the Creator.

Kingsley liked the idea of a God who created the Universe, but made it so that it could maintain itself through natural processes. He said this was more impressive than a God who controlled every little thing that happened.

Likewise, Catholic priest and renowned geologist and paleontologist Pierre Teilhard de Chardin said that his own faith makes more sense after Darwin. Teilhard argued that modern science demonstrated that the Universe is still being created. This process involves struggle, chance, failure and loss — as well as beauty and majesty.

Along with Kingsley, he believed God creates this Universe through natural processes. Teilhard suggested that the meaning of human life includes our contributions to the great work of ongoing creation.


Darwin, Evolution, and Faith (850 L)By John F. Haught, adapted by Newsela

Nothing in modern science is more challenging to religious believers than the theory of evolution. For more than 150 years people have fought over its meaning for religion. The debate is still ongoing.

Darwin’s theories of evolution and natural selection have been difficult to accept for many religious believers. His ideas have been controversial for more than a century and a half. Today, the controversy is as strong as ever.

Why has Darwin’s science been so troubling for religious believers? We can point to at least five reasons:

1. Darwinian biology tells a creation story that is very different from religious ones;2. Evolution and natural selection seem to make God unnecessary in the creating of

life;3. Traditional religion teaches that humans are unique and created in the image of God.

Darwin proposed that humans descended from other animals;4. In evolution, chance and accidents are important. This raises questions about

whether a creator truly cares for the world; and5. In evolution there is a constant struggle to survive. This doesn’t seem to fit with a

Universe created by God.

What did Darwin think about God? After his famous sea voyage, he spent 20 years thinking about what effect his discoveries would have on religion. Darwin originally believed that God had created all life in the beginning. But what he learned on his sea voyage made him question this. The deaths of his father and daughter also may have increased his doubts about his faith.

In his autobiography, Darwin wrote: “Disbelief crept over me at a very slow rate, but was at last complete. The rate was so slow that I felt no distress, and have never since doubted for a single second that my conclusion was correct.”

Still, Darwin never thought of himself as an nonbeliever. He sometimes referred to a “Creator.” This Creator made the Universe and its general laws, but left living things to chance and natural selection.

In any case, the religious world in Darwin’s time was not ready for his ideas. Even now, some people are still recovering from the shock Darwin gave to traditional beliefs. For others, appreciating Darwin’s ideas deepens their faith in God.

Three approachesAt the time Darwin published On the Origin of Species, most people took the biblical creation story literally. They thought the world was only around 6,000 years old. They believed all living species had been created at the world’s beginning.


Can ancient religious accounts of the world’s creation fit with Darwin’s new story? Here are three approaches to that question:

1. ConflictThis approach says that Darwin’s theories can never fit with belief in God. Both religious believers and skeptics can take this approach. Among religious believers, there are “creationists” and supporters of “intelligent design.” Both groups have problems with evolution.

Creationists consider their holy books to be the source of true science. They reject Darwinian evolution as simply wrong.

Supporters of intelligent design don’t always read their holy book literally. But they think that life is too complex to be the result of natural causes alone. They argue that a supernatural force must be responsible for the “design” of life.

There are also people who believe strongly in evolution and use it to argue against the existence of God. These people reject both creationism and intelligent design. Especially in the United States, this issue causes conflict. However, there are other ways to look at it.

2. ContrastThis approach claims that science and faith answer different questions. So, there can be no real conflict between evolution and religion.

Holy books are not science. Likewise, Darwinian science has nothing to do with faith, in this view. Pope Leo XIII told his followers not to look for scientific information in the Bible. Galileo gave his fellow Catholics the same advice back in the seventeenth century.

In this approach, Darwin’s theory of evolution does not need to compete with biblical creation stories. The creation stories in the Bible were not meant to teach science. They were meant to urge believers to be grateful for the richness of creation.

The Bible seeks to answer “Why is there anything at all rather than nothing?” It tries to explain whether there is "an eternal reason for trusting that life is worthwhile?”

Most mainstream Christian churches have recognized that science and religion answer different questions. Still, some fundamentalist groups view the Bible as scientifically accurate. They think Darwin’s science cannot go alongside biblical “science.”

In the contrast approach, reading the Bible as science misses the point of ancient religious literature.

3. ConvergenceThis approach sees truth in both science and religion. Scientific and religious truths are both valid. In this model, science and faith complement each other.

After Copernicus proposed a Sun-centered Solar System, religious people had to adjust their thinking. They must do the same thing after Darwin, this view says. Challenges such as evolution are important for keeping faith alive and healthy. Religion was able to adjust to a heliocentric Universe. It can now adjust to evolution.


In this model, evolution and religion are not in conflict. Many religious scholars and scientists have found ways to balance both beliefs. In this view, there is no danger in thinking bold new thoughts about God after Darwin. After all, the idea of God has evolved over time. And it will keep evolving still.

Anglican priest Charles Kingsley thanked Darwin. He said Darwin’s ideas of creative evolution helped enlarge his understanding of the Creator. Kingsley liked the idea of a God who created the Universe, but made it so that it could take care of itself through nature. He said this was more impressive than a God who controlled every little thing that happened.

Likewise, Catholic priest Pierre Teilhard de Chardin said that his own faith made more sense after Darwin. Teilhard was also a famous geologist and paleontologist. He argued that modern science shows that creation is still happening. This process involves struggle, chance, failure and loss. But it also involves beauty and majesty.

Along with Kingsley, he believed God creates this Universe through natural processes. Teilhard suggested that our contributions to the great work of creation gives human life its meaning.


Watson, Crick & FranklinDNA is found in all life on Earth. It contains the information that allows organisms to pass down traits to offspring. In the early 1950s, scientists raced to understand the structure of DNA.

Watson, Crick, and Franklin: The race to discover the structure of DNA (1220L)By Cynthia Stokes Brown

In 1953, three English biochemists helped unlock the mystery of life by determining the double helix structure of the DNA molecule. Found in all life on Earth, DNA contains the information by which an organism regenerates its cells and passes traits to its offspring.

Despite his success in formulating the theory of natural selection, Charles Darwin did not yet understand how characteristics are passed from parent organisms to their offspring with the slight changes that make evolution possible and identify each individual.

By the middle of the twentieth century this was still not well understood. The first part of the century had seen major breakthroughs in physics, such as Einstein’s Theory of Relativity and atomic bombs that used the energy of nuclear fusion. After World War II, scientists turned to understanding the physical basis (atomic and molecular) of biological phenomena.

In the 1950s, biochemists realized that DNA, short for deoxyribonucleic acid, delivered the instructions for copying a new organism. A yard of DNA is folded and packed into the nucleus of every cell in pairs called “chromosomes,” with one exception: in the reproductive cells, where the pieces of DNA are not paired.

DNA has three constituents: 1) a type of sugar called “ribose”; 2) a phosphate (phosphorous surrounded by oxygen) responsible for its acidity; and 3) four kinds of bases — adenine (A), thymine (T), guanine (G), and cytosine (C). Since these four bases seemed too simple to be able to pass on all the information needed to create a new organism, biochemists were baffled about DNA’s structure and how it worked. However, these four bases combine like letters of an alphabet to describe complex variations in genetic traits.

The question became how to study the DNA molecule. Biochemists believed that understanding its structure would reveal how the molecule coded the instructions for copying a new organism. They began taking X-ray images of crystals of DNA, believing that its crystallization meant it must have a regular structure. The pattern of the X-rays bouncing off atoms (a phenomenon called “diffraction”) gave information about their location in the molecule. One of the pioneers of this technique, called “X-ray crystallography,” was Linus Pauling, who worked at the California Institute of Technology in Pasadena. In the early 1950s, Pauling, a prominent chemist doing molecular research in the States, seemed a likely


candidate to unlock the mystery of life, since he had already concluded that the general shape of DNA must be a helix, or spiral.

The raceThe victory, however, went to three people working in England, in one of the great scientific races of all time. One, Rosalind Franklin, was working at King’s College at the University of London. The other two, James Watson and Francis Crick, were friends and lab mates some 50 miles away at the Cavendish Laboratory at Cambridge University, where they worked cooperatively and shared their ideas.

Franklin was from a wealthy, influential family in London. She had earned her PhD in 1945 from Cambridge in physical chemistry. Starting at King’s College in 1951 at the age of 31, she was focused on studying DNA. She became extremely skilled in X-ray crystallography, able to produce clear and accurate diffraction images of DNA crystals by using fine-focus X-ray equipment and pure DNA samples.

Over in Cambridge, biochemists were supposed to leave the study of DNA to the lab at King’s College. Francis Crick, age 35 in 1951, was working on his PhD in the crystallography of proteins. He had grown up in a small English village and, since he had failed to qualify for Cambridge, took his undergraduate degree in physics from the University of London. Watson, only 23 in 1951, was at Cambridge as a post doctorate fellow in biology with limited knowledge of chemistry. He had grown up in Chicago, performed on the national radio show “Whiz Kids,” entered the University of Chicago at age 15, and secured his doctorate from the University of Indiana at just 22. He was at the Cambridge lab to learn crystallography.

Between 1951 and January 1953, Franklin reasoned through her precise X-ray diffraction images that: 1) DNA takes two forms (shorter-dryer and longer-wetter), 2) the sugar-phosphate backbones must be on the outside, and 3) the molecule looks the same upside down or right side up. In late 1952, she recorded an especially clear X-ray diffraction image that her colleague, Maurice Wilkins, later showed to Watson in January 1953 without telling Franklin or asking her permission. Franklin and Wilkins did not always communicate well, so his actions were perhaps not surprising.

Watson knew at once from seeing Franklin’s photograph that DNA had to be a helix with certain dimensions. He was so excited that he returned to his lab to draw up plans for models that the machine shop would construct out of sheet metal and wire.

In building their models, Watson and Crick had to find the answers to several questions. How many strands did the helix have? Which direction did the strands run? Were they on the inside or the outside? How were the four chemical bases arranged?

While Franklin believed the answers would come with more X-ray images of better quality, Watson and Crick recognized they were racing against Linus Pauling for a solution and thought that making a model would speed up the answers. First, they tried using two strands, putting them in the center of the model with the bases on the outside; however, this did not produce a chemically acceptable structure.


Next, they played around with the shapes of the four bases, using paper models and combining them in different ways. Finally, they visualized a structure that solved the puzzle: If two of the bases were bonded in pairs (G with C), they took up the same space as the other pair (A with T). Hence, they could be arranged like steps on a spiral staircase inside of two strands of sugar-phosphates running in opposite directions.

These insights occurred to Crick and Watson between February 4 and 28, 1953, when they announced at lunch in their usual pub that they had found the secret of life.

The news gets outThe April 25, 1953, issue of Nature published Crick and Watson’s 900-word article, “A Structure for Deoxyribose Nucleic Acid.” Wilkins and Franklin, who both accepted Crick and Watson’s solution, wrote accompanying articles. By the 1960s, scientists generally embraced the double helix as the structure of DNA, and in 1962, Wilkins, Watson, and Crick received the Nobel Prize in medicine/physiology for their work.

Franklin could not share in the prize as it cannot be granted to someone who has passed away. She had died from ovarian cancer at the age of 37 on April 16, 1958, in London. She had a family history of cancer, but her exposure to X-rays may have contributed to her death. And in any case, she may not have had the chance for the award had she been alive. Crick and Watson never told Franklin that they had used her images. She was mentioned only in passing by Crick and Watson in Nature. Nor did Watson explain this in his popular account of their discovery, The Double Helix (1968).

It wasn’t until much later that Watson finally admitted in public that he and Crick could not have found the double helix in 1953 without Franklin’s experimental work. If she had survived, would she have been acknowledged and shared in the prize?

In their 1953 article, Watson and Crick did not discuss how DNA copies itself. They simply included this sentence: “It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.”

Five weeks after their first article in Nature, Crick and Watson published another article proposing the idea that, to make a copy, the double helix unzips, or separates, into two strands — each a backbone of sugar-phosphates with the four bases attached in some sequence. Then the cell uses each strand as a template to assemble another DNA strand from free-floating complementary bases: A picks up T, while C picks up G. This would result in two identical DNA molecules, one a copy of the other. Occasional mistakes in copying enable evolution to occur and each organism to be unique. This idea has been confirmed, while the means for carrying it out have proved to be immensely complex.

Crick continued his research in England until 1976, when he moved to the Salk Institute for Biological Studies in La Jolla, California, where he died in 2004. Watson returned to the United States, researching at Harvard from 1956 to 1976. He helped establish the Human Genome Project in the early 1990s and served as president of the Cold Spring Harbor Laboratory on Long Island, New York, until his retirement in 2007.


Watson, Crick, and Franklin: The race to discover the structure of DNA (1070L)By Cynthia Stokes Brown, adapted by Newsela

In 1953, three English biochemists helped unlock the mystery of life by determining the structure of the DNA molecule. Found in all life on Earth, DNA contains the information by which an organism regenerates its cells and passes traits to its offspring.

Darwin had successfully proposed the theory of natural selection, but he didn’t understand how parents pass characteristics to their offspring. Slight changes when passing down traits made evolution possible.

By the middle of the twentieth century this was still not well understood. There were major breakthroughs earlier in the century in physics, such as Einstein’s Theory of Relativity and atomic bombs that used nuclear fusion.

After World War II, scientists began trying to understand the physical basis (atomic and molecular) of biology. In the 1950s, biochemists realized that DNA delivered the instructions for copying a new organism. A yard of DNA — deoxyribonucleic acid — is folded and packed into the nucleus of every cell in pairs called “chromosomes.”

DNA has three parts:

1. a type of sugar called “ribose”2. a phosphate responsible for its acidity3. four kinds of bases — adenine (A), thymine (T), guanine (G), and cytosine (C)

These four bases seemed too simple to pass on all the information needed to create a new organism. Biochemists didn’t understand DNA’s structure and how it worked. However, these four bases combine like letters of an alphabet to describe complex variations in genetic traits.

The question became how to study the DNA molecule. Biochemists wanted to understand its structure. They thought this was the key to understand how it coded the instructions for copying a new organism.

They began taking X-ray images of crystals of DNA, believing that its crystallization meant it must have a regular structure. The pattern of the X-rays bouncing off atoms gave information about their location in the molecule.

One of the pioneers of this technique, called “X-ray crystallography,” was Linus Pauling, who worked at the California Institute of Technology in Pasadena. In the early 1950s, Pauling, a prominent chemist, seemed likely to unlock the mystery of life, since he had already concluded that the general shape of DNA must be a helix, or spiral.

The raceThe victory, however, went to three people working in England, in one of the great scientific races of all time. One, Rosalind Franklin, was working at the University of London. The other


two, James Watson and Francis Crick, were friends and lab mates some 50 miles away at Cambridge University, where they worked cooperatively and shared their ideas.

Franklin was from a wealthy, influential family in London. After earning a PhD from Cambridge in physical chemistry, she began to study DNA at the University of London in 1951. Franklin became extremely skilled in X-ray crystallography. She was able to produce clear and accurate images of DNA crystals by using fine-focus X-ray equipment and pure DNA samples.

Over at Cambridge, Francis Crick was 35, working on his PhD in the crystallography of proteins. He had grown up in a small English village.

Watson was only 23 in 1951. He had grown up in Chicago, performed on the national radio show “Whiz Kids,” entered the University of Chicago at age 15, and secured his doctorate from the University of Indiana at just 22. He was at the Cambridge lab to learn crystallography.

Between 1951 and 1953, Franklin examined her precise X-ray diffraction images. She reasoned that 1) DNA takes two forms (shorter-dryer and longer-wetter), 2) the sugar-phosphate backbones must be on the outside, and 3) the molecule looks the same upside down or right side up.

In late 1952, she recorded an especially clear X-ray image. Her colleague Maurice Wilkins showed the image to Watson in 1953 without telling her or asking her permission.

When Watson saw the image, he knew at once that DNA had to be a helix. He returned to his lab to begin making models out of sheet metal and wire.

Watson and Crick built models to try to visualize DNA. How many strands did the helix have? Which direction did the strands run? Were they on the inside or the outside? How were the four chemical bases arranged?

Franklin believed more X-ray images of better quality would answer the questions. But Watson and Crick knew they were racing against Pauling. They felt making models would speed up the answers.

Using paper models and combining them in different ways, they visualized a structure that solved the puzzle. If two of the bases were bonded in pairs (G with C), they took up the same space as the other pair (A with T). Hence, they could be arranged like steps on a spiral staircase inside of two strands of sugar-phosphates running in opposite directions.

These insights occurred to Crick and Watson in February 1953. They announced at lunch in their usual pub that they had found the secret of life.

The news gets outThe April 25, 1953, issue of Nature published Crick and Watson’s article, “A Structure for Deoxyribose Nucleic Acid.” Wilkins and Franklin, who both accepted Crick and Watson’s solution, wrote accompanying articles.


By the 1960s, scientists had accepted the double helix as the structure of DNA. In 1962, Wilkins, Watson, and Crick received the Nobel Prize in medicine/physiology for their work.

Franklin could not share in the prize. She had passed away in 1958 of ovarian cancer. She was just 37. Franklin had a family history of cancer, but her exposure to X-rays may have contributed to her death.

In any case, she may not have had the chance for the award had she been alive. Crick and Watson never told Franklin that they had used her images. In Nature, Watson and Crick only mentioned her briefly. She wasn’t credited in Watson’s book about the discovery, The Double Helix (1968).

It wasn’t until much later that Watson finally admitted in public that he and Crick could not have found the double helix in 1953 without Franklin’s experimental work. If she had survived, would she have been acknowledged and shared in the prize?

In their 1953 article, Watson and Crick did not discuss how DNA copies itself.

Five weeks after their first article in Nature, Crick and Watson published another article proposing the idea that, to make a copy, the double helix unzips, or separates, into two strands. Each strand is a backbone of sugar-phosphates with the four bases attached in some sequence.

Then the cell uses each strand as a template to assemble another DNA strand from free-floating complementary bases: A picks up T, while C picks up G. This would result in two identical DNA molecules, one a copy of the other. Occasional mistakes in copying enable evolution to occur and each organism to be unique. This idea has been confirmed, while the means for carrying it out have proved to be quite complex.

Crick continued his research in England until 1976, when he moved to the Salk Institute for Biological Studies in California, where he died in 2004. Watson returned to the United States, researching at Harvard from 1956 to 1976. He helped establish the Human Genome Project in the early 1990s and served as president of the Cold Spring Harbor Laboratory in New York, until his retirement in 2007.

Watson, Crick, and Franklin: The race to discover the structure of DNA (880)By Cynthia Stokes Brown, adapted by Newsela

In 1953, three biochemists helped unlock the mystery of life. They had determined the structure of the DNA molecule. Found in all life on Earth, DNA contains the information that allows organisms to regrow cells and pass on traits to offspring.

Darwin explained natural selection, but he didn’t know how the traits were passed down exactly. Traits are handed down from parent to child, over generations. Sometimes these traits change. Slight changes in traits as they are passed down made evolution possible.

A century later, this was still not well understood. After World War II, scientists began to study the physical basis (atomic and molecular) of biology. In the 1950s, biochemists


realized that DNA delivered the instructions for copying a new organism. A yard of DNA (deoxyribonucleic acid) is folded and packed into the nucleus of every cell in pairs called “chromosomes.”



These four bases seemed too simple to pass on all the information needed to create a new organism. Biochemists didn’t understand DNA’s structure or how it worked. However, these four bases combine like letters of an alphabet to describe complex variations in genetic traits.

Scientists wondered how to study the DNA molecule. Biochemists knew that if they could understand its structure, they would understand how it coded the instructions for copying a new organism.

They began taking X-ray images of crystals of DNA. One of the pioneers of this technique, called “X-ray crystallography,” was Linus Pauling. A prominent chemist working at the California Institute of Technology, Pauling seemed likely to unlock the mystery of life. He had already decided that the general shape of DNA must be a helix, or spiral.

The raceBut the victory in one of the great scientific races of all time went to three people working in England. The first was Rosalind Franklin. She was working at the University of London. The other two, James Watson and Francis Crick, were friends and lab mates some 50 miles away at Cambridge University.

Franklin was from a wealthy family in London. After earning a PhD from Cambridge in physical chemistry, she began to study DNA at the University of London in 1951. She became extremely skilled in X-ray crystallography. She was able to produce clear and accurate images of DNA crystals by using fine-focus X-ray equipment and pure DNA samples.

At Cambridge, Francis Crick was 35, working on his PhD in the crystallography of proteins. He grew up in a small English village.

Watson was only 23 in 1951. He grew up in Chicago, and entered the University of Chicago at age 15. Watson earned his doctorate from the University of Indiana at just 22. He was at the Cambridge lab to learn crystallography.

Franklin examined her precise X-ray images between 1951 and 1953. She began to make hypotheses about the shape and structure of DNA. In late 1952, she took an especially clear X-ray image. Her colleague Maurice Wilkins showed the image to Watson in 1953 without telling her or asking her permission.

When Watson saw the image, he knew at once that DNA had to be a helix. He returned to his lab to begin making models out of sheet metal and wire. Watson and Crick built models


to try to visualize DNA. How many strands did the helix have? Which direction did the strands run? Were they on the inside or the outside? How were the four chemical bases arranged?

Franklin believed more X-ray images of better quality would answer the questions. But Watson and Crick knew they were racing against Pauling. They felt making models would speed up the answers.

Crick and Watson used paper models and combined them in different ways. Finally, they visualized a structure that solved the puzzle. If two of the bases were bonded in pairs (G with C), they took up the same space as the other pair (A with T). Hence, they pictured DNA like a spiral staircase. The pairs of bases made up the steps. Two strands of sugar-phosphates running in opposite directions were the sides, or railings, of the staircase.

The two men made this discovery in February 1953. They announced at lunch in their usual pub that they had found the secret of life.

The news gets outCrick and Watson published “A Structure for Deoxyribose Nucleic Acid” in Naturein April 1953. Wilkins and Franklin both wrote accompanying articles.

By the 1960s, scientists had accepted the double helix as the structure of DNA. In 1962, Wilkins, Watson, and Crick received the Nobel Prize in medicine/physiology for their work.

Franklin could not share in the prize. She had passed away in 1958 of ovarian cancer. She was just 37. Franklin had a family history of cancer, but her exposure to X-rays may have contributed to her death.

Even if she had lived, she may not have shared in the prize. Crick and Watson never told Franklin that they used her images. They didn’t give her much credit in their Nature article or in a book about the discovery.

It wasn’t until much later that Watson admitted in public that he and Crick could not have found the double helix in 1953 without Franklin’s experimental work. If she had survived, would she have been acknowledged and shared in the prize?

In their 1953 article, Watson and Crick did not discuss how DNA copies itself. Later, they published another article proposing that DNA’s double helix unzips into two strands to make a copy. Each strand has a backbone of sugar-phosphates with the four bases attached.

Crick continued his research in England until 1976, when he moved to the Salk Institute for Biological Studies in California. He died there in 2004. Watson returned to the United States. He did research at Harvard from 1956 to 1976. In the early 1990s, he helped establish the Human Genome Project. Watson retired in 2007.

Watson, Crick, and Franklin: The race to discover the structure of DNA (740L)By Cynthia Stokes Brown, adapted by Newsela


In 1953, three biochemists helped unlock the mystery of life. They had determined the structure of the DNA molecule. Found in all life on Earth, DNA contains the information that allows organisms to regrow cells and pass on traits to offspring.

Darwin explained natural selection and evolution. But he didn’t know how traits were passed down. Traits are handed down from parent to child, through generations. Over time, some traits change. Slight changes in traits as they get passed down are what makes evolution possible.

This process was still not well understood a whole century after Darwin. Then after World War II, scientists began to study biology in a new way.

In the 1950s, biochemists realized that DNA contained the instructions for copying a new organism. A yard of DNA (deoxyribonucleic acid) is folded. It is packed into the nucleus of every cell in pairs called “chromosomes.”



These four bases seemed too simple. How could they pass on all the information needed to create a new organism? The key was that the bases combine in different patterns to mark different genetic traits.

Scientists wondered how to study the DNA molecule. Biochemists wanted to understand its structure. Then they could understand how a new organism is copied. They began taking X-ray images of crystals of DNA.

Linus Pauling worked at the California Institute of Technology. He was a pioneer in “X-ray crystallography.” Pauling seemed likely to unlock the mystery of life. He had already concluded that the general shape of DNA must be a helix, or spiral.

The raceThe competition to unravel DNA was one of the great scientific races of all time. Victory went to three people working in England. The first was Rosalind Franklin. She worked at the University of London. The other two were James Watson and Francis Crick. They worked together at Cambridge University, just 50 miles away from Franklin.

Franklin was from London. She earned a PhD from Cambridge in chemistry. While studying DNA, she became an expert in X-ray crystallography. She was able to produce clear and accurate images of DNA crystals using precise X-ray equipment and pure DNA samples.

At Cambridge, Francis Crick was 35. He was working on his PhD in the crystallography of proteins. He grew up in a small English village.

Watson was only 23 in 1951. He grew up in Chicago. He entered the University of Chicago at age 15, and got his doctorate from the University of Indiana at just 22. He was at Cambridge to learn crystallography.


Using her precise X-ray images, Franklin began to make hypotheses about the structure of DNA. In late 1952, she took an especially clear X-ray image. Her colleague Maurice Wilkins showed the image to Watson. He did so without telling Franklin or asking her permission.

When Watson saw the image, he knew at once that DNA had to be a helix, or spiral. He returned to his lab to begin making models out of metal and wire. Watson and Crick built models to try to visualize DNA. How many strands did the helix have? Which direction did the strands run? Were they on the inside or the outside? How were the four chemical bases arranged?

Franklin believed better quality X-ray images would solve the problem. But Watson and Crick knew they were racing against Pauling. They felt making models would lead them to an answer quickly. Using paper models, Crick and Watson finally visualized a structure that solved the puzzle. They pictured DNA like steps on two spiral staircases. The bases bonded together in pairs. They made up the steps in the staircase. The ribose sugar made up the sides, or railings, of the stairs.

The two men made this discovery in February 1953. Sitting at lunch in their usual pub, they announced that they had found the secret of life.

The news gets outCrick and Watson published their discovery in Nature magazine in April 1953. Wilkins and Franklin wrote articles to go along with it.

In 1962, Wilkins, Watson, and Crick received the Nobel Prize for their work. Franklin could not share in the prize. She had died in 1958 of cancer. She was just 37. Exposure to X-rays may have contributed to Franklin's death.

Even if she had lived, she may not have shared in the prize. Crick and Watson never told Franklin they used her images. They didn’t give her much credit in their Nature article. Much later, Watson admitted that they could not have solved the problem without Franklin’s work.

Watson and Crick also addressed how DNA copies itself. They proposed that DNA’s double helix unzips into two strands to make a copy. Each strand has a backbone of sugar-phosphates with the four bases attached.

Crick continued his research in England until 1976, when he moved to California. He died there in 2004. Watson returned to the United States. He did research at Harvard from 1956 to 1976. He helped establish the Human Genome Project in the early 1990s. He retired in 2007.


Surviving an Extinction Level EventLife on Earth has experienced repeated periods when a large portion of its species died off, followed by a recovery and the emergence of a newly shaped tree of life.

Surviving an Extinction Level Event (1190 L)By Cynthia Stokes Brown, adapted by Newsela

Life on Earth has experienced repeated periods when a large portion of its species died off, followed by a recovery and the emergence of a newly shaped tree of life.

Extinction events are periods in Earth’s history during which a sharp decrease in the diversity and abundance of living organisms occurs. This is measured by the easily observable life forms, and does not include the bacterial ones (which constitute a great portion, perhaps even the majority, of Earth’s biodiversity and biomass). During these periods, the rate of extinctions greatly exceeds the normal, slow pace that regularly occurs as new species emerge.

The people who study extinctions are geologists and paleontologists; they examine the history of our planet as recorded in sedimentary rocks. They use fossils as evidence, especially marine fossils, since those are the most abundant. Only since the 1970s have scientists agreed that numerous extinction events have occurred, and only since the early 1980s have they agreed on what the five major ones were.

The “crater of doom”One fine day about 65.5 million years ago, while dinosaurs were grazing and hunting around the world, an object the size of Mount Everest came hurtling through space. Only a seven-minute window existed during which the object’s path could intersect with Earth’s orbit around the Sun.

Although the chances seem to have been slight, the object hit Earth. (It may have been a comet, made of dirty ice, or an asteroid, made of rock.) The object landed just off the coast of what is now the Yucatán Peninsula in Mexico, at an estimated velocity 150 times the speed of a jet airliner.

The impact made a hole the size of Belgium, throwing up debris that rose high into the atmosphere and circled around the Earth. The collision generated so much initial heat that continental forests burned, putting more particulates in the atmosphere. With the Sun’s rays blocked by smoke and debris, photosynthesis slowed or stopped, the temperature cooled, and the amount of rainfall decreased significantly for a few months at least. Plants and animals died. All the dinosaurs, except some avian dinosaurs, which were on their way toward evolving into birds, died. An estimated 75 percent of all species disappeared. Among


the survivors were crocodiles, turtles, and small, rodent-like mammals, which were our ancestors.

Geologists call this extinction event the “K-T event” because it marked the end of one geologic period, the Cretaceous (spelled with a “K” in German), and the beginning of the next, the Tertiary.

The story of the K-T event is quite well understood after years of patient detective work. It began in the mid-1970s with a young geologist, Walter Alvarez, in the mountains of Italy, near the town of Gubbio. There he found a thin layer of clay a centimeter thick between the layers of Cretaceous and Tertiary limestone; the Cretaceous layer contained many fossilized single-celled marine organisms, while very few appeared in the Tertiary layer. In the stratum between, Alvarez’s associates found iridium, an element extremely rare in the Earth’s crust but more common in meteorites. This suggested an impact by an asteroid or comet around the date of the extinction. In 1980, the Alvarez team presented its hypothesis that an asteroid/comet had hit and had caused massive, rapid extinction by altering the air and water. Further research around the world showed that high levels of iridium existed in the rock record at other K-T boundary sites.

Within two years the evidence persuaded most geologists to accept this hypothesis. Others were unsure. If a massive asteroid/comet had hit, where was the crater? No known depression on land seemed large enough for such a massive object; hence, the crater must be under water. Large objects that hit water create huge tsunami waves, which leave telltale signs in the rock record, sometimes well inland from the coast. A worldwide search turned up evidence of such a large tsunami on the shores of Texas, across the Gulf of Mexico from the Yucatán Peninsula.

Much earlier, in 1950, geologists working for the Mexican national oil company, PEMEX, had mapped a 120-mile crater underwater, off the coast of the Yucatán Peninsula. To find this crater, they had charted tiny variations in the pull of gravity, which reflected variations in rock density. From these maps, geologists could tell where the dense and light rocks were located beneath the sea. But not until 1991 did the K-T researchers get together with the PEMEX geologists, who tended not to publish their information, and realize that the “crater of doom” had been found. They named it Chixculub (a Mayan word pronounced cheek-shoe-lube), after the small coastal town nearby.

Other extinction eventsPaleontologists and geologists have identified four other major extinction events, all of which predate the K-T extinction. Named for the geologic times they correspond to, they are the End-Triassic, the End-Permian, the Late Devonian, and the Ordovician.

Of the five major extinctions, the End-Permian proved to be the most massive — the mother of all extinction events. An estimated 95 percent of marine species and 70 percent of land species were lost. This dying-off lasted for about 165,000 years and included both gradual and sudden environmental changes that greatly altered conditions on the Earth.


Very few creatures made it through the End-Permian extinction. Cockroaches did — and ginkgo trees and horseshoe crabs. So did our ancestors, small protomammals that had evolved from reptiles: they were furry and warm-blooded, but still laid eggs.

Possible causes of extinctionsOnce most geologists and paleontologists agreed that the cause of the K-T extinction was an asteroid/comet hitting Earth, many of them first hypothesized that objects from space had caused all the major extinctions. That proved false when studies of fossil layers from the times of earlier extinctions showed that life forms had disappeared gradually, not suddenly as they did in the layers of sediment dated 65.5 million years ago.

The discussion about what causes mass extinctions continues. Scientists do not yet fully understand the reasons for them. Some possible explanations are:

Sudden massive volcanic activity, as evidenced by vast areas of lava plains that date to coincide with extinction events. Volcanoes emit carbon dioxide, which results in global warming; they also emit dust and aerosols that inhibit photosynthesis, causing food chains to collapse.

Rapidly changing climate. Impact or multiple-impact events. Anoxic events (the middle or lower layers of ocean becoming deficient or lacking in

oxygen). Ever-changing position of oceans and continents (plate tectonics).

It seems likely that some combination of these possible causes may have taken place at certain times. One reputable paleontologist, Peter Ward, made the following hypothesis in 2006 to explain the four major extinctions other than the K-T event:

A “sudden” increase of carbon dioxide and methane in the atmosphere occurred, caused by vast volcanic lava beds. The warmer world disrupted ocean circulation patterns and the position of the currents that convey downward warm surface water with oxygen and upward the cold bottom water with less oxygen. Without the mixing of the ocean layers, the bottom water became anoxic, without oxygen. This allowed green sulfur bacteria, which live on sulfur not oxygen, to expand. They produced hydrogen sulfide, which bubbled up, killing much of life and destroying the ozone layer, which protected life against ultraviolet rays from the Sun.

Ward’s discussion, and the conclusions of some other scientists, suggests that humans must reduce the carbon dioxide that we are emitting, or we may set off a similar chain of events.

A sixth major extinction?Many biologists agree that a sixth major extinction is currently underway. This one is unique because it is the result of humans degrading and destroying the habitats of other life forms. This extinction apparently began about 50,000 years ago when humans moved into Australia and the Americas, causing the disappearance of many species.


No one knows how many species currently exist on Earth. The best estimate is about 8.7 million, not counting microorganisms. To date, only a small fraction of these estimated species have been identified, but new ones are constantly discovered and named.

This gives the impression that new species are appearing as fast as old ones are disappearing. A 2003 study by the World Conservation Union suggested that one in four known mammal species is threatened with extinction in the next several decades, while one in eight known bird species is at risk.

If the present trend continues, biologists fear that we could lose 50 percent of all known living species by the end of this century.


Life on Earth has seen several periods when a large portion of its species died off. Yet, they've always led to a recovery and the rise of a newly shaped tree of life.

An extinction event is a time in the Earth’s history when many living organisms die off. Species are always going extinct, but during an extinction event, it happens much more quickly than usual.

Geologists and paleontologists study extinction events. They study sedimentary rocks to learn about the history of our planet. Marine fossils give them important clues about extinction events.

In the 1970s, scientists agreed that numerous extinction events had occurred. In the 1980s, scientists agreed on the five major ones.

The “crater of doom”One fine day about 65.5 million years ago, while dinosaurs were grazing and hunting around the world, an object the size of Mount Everest came flying through space. There were only seven minutes when the object’s path could cross Earth’s orbit.

The odds were against it, but the object hit Earth. It may have been a comet, made of dirty ice, or an asteroid, made of rock. The object landed just off the coast of what is now the Yucatán Peninsula in Mexico, traveling about 150 times as fast as a jet.

The impact made a hole the size of Belgium, throwing up debris that rose high into the atmosphere and circled around the Earth. The collision created so much heat that huge forests burned, sending more particulates into the atmosphere.

The Sun’s rays were blocked by smoke and debris so photosynthesis slowed or stopped. The temperature cooled and the amount of rainfall decreased for a few months at least.

Plants and animals died. Almost all the dinosaurs died. Some avian dinosaurs survived. These were already evolving into birds. About 75 percent of all species disappeared.


Among the survivors were crocodiles, turtles, and small, rodent-like mammals, which were our ancestors.

Geologists call this extinction event the “K-T event” because it marked the end of one geologic period, the Cretaceous (spelled with a “K” in German), and the beginning of the next, the Tertiary.

The story of the K-T event is quite well understood thanks to years of patient detective work. In the mid-1970s, young geologist Walter Alvarez made a discovery in the mountains of Italy. He found a thin layer of clay a centimeter thick between the layers of Cretaceous and Tertiary limestone. The Cretaceous layer contained many marine fossils but the Tertiary layer had fewer.

In the layer of clay, Alvarez’s associates found iridium. This element is extremely rare on Earth, but more common in meteorites. This suggested an impact by an asteroid or comet around the date of the extinction.

In 1980, Alvarez and his team reported that an asteroid or comet had hit and caused massive, rapid extinction by altering the air and water.

Within two years the evidence persuaded most geologists to accept this hypothesis. Others were unsure. If a massive asteroid or comet had hit, where was the crater? No known depression on land seemed large enough for such a massive object. It seemed the crater must be under water.

Large objects that hit water create huge tsunami waves, which leave telltale signs in the rock record, sometimes well inland from the coast. A worldwide search turned up evidence of such a large tsunami on the shores of Texas, across the Gulf of Mexico from the Yucatán Peninsula.

Much earlier, in 1950, geologists working for the Mexican national oil company, PEMEX, had mapped a 120-mile underwater crater, off the coast of the Yucatán Peninsula.

It wasn’t until 1991 that the K-T researchers got in touch with the PEMEX geologists, who usually didn’t publish their information. The K-T researchers realized that the “crater of doom” had been found. They named it Chixculub (a Mayan word pronounced cheek-shoe-lube), after the small coastal town nearby.

Other extinction eventsPaleontologists and geologists have identified four other major extinction events. All of these came before the K-T extinction. Each is named for the geologic time it corresponds to. They are: the End-Triassic, the End-Permian, the Late Devonian and the Ordovician.

The End-Permian was the most massive of the five major extinctions. It was the mother of all extinction events.

In it, about 95 percent of marine species and 70 percent of land species were lost. The dying off lasted for 165,000 years and included both gradual and sudden environmental changes that greatly changed conditions on the Earth.


Very few creatures made it through the End-Permian extinction. Cockroaches did — and ginkgo trees and horseshoe crabs. So did our ancestors, small protomammals (early mammals) that had evolved from reptiles: they were furry and warm-blooded, but still laid eggs.

Possible causes of extinctionsOnce most geologists and paleontologists agreed that the cause of the K-T extinction was an asteroid or comet hitting Earth, many of them hypothesized that objects from space had caused all the major extinctions.

That proved false. Studies of fossil layers from earlier extinctions showed that life forms had disappeared gradually, not suddenly, as they had in the K-T event.


Sudden massive volcanic activity. Scientists have found huge lava plains that coincide with extinction events. Volcanoes emit carbon dioxide, which results in global warming. They also emit dust and aerosols that slow photosynthesis, causing food chains to collapse.

Rapidly changing climate. Impact or multiple-impact events. Anoxic events (the middle or lower layers of ocean becoming deficient or lacking in

oxygen). Changing position of oceans and continents (plate tectonics).

It seems likely that some combination of these causes may have taken place. One reputable paleontologist, Peter Ward, made the following hypothesis in 2006 to explain the four major extinctions other than the K-T event:

A “sudden” increase of carbon dioxide and methane in the atmosphere occurred, caused by vast volcanic lava beds. The warmer world disrupted ocean circulation patterns and currents. Without the mixing of the ocean layers, the bottom water became anoxic, without oxygen. This allowed green sulfur bacteria, which live on sulfur not oxygen, to expand. They produced hydrogen sulfide, which bubbled up, killing much of life and destroying the ozone layer, which protected life against ultraviolet rays from the Sun.

Ward’s discussion, and the conclusions of some other scientists, suggests that humans must reduce the carbon dioxide that we are emitting, or we may set off a similar chain of events.

A sixth major extinction?Many biologists agree that a sixth major extinction is currently underway. This one is unique because it is the result of humans degrading and destroying the habitats of other life forms. This extinction apparently began about 50,000 years ago when humans moved into Australia and the Americas, causing the disappearance of many species.



It may seem that new species are appearing as fast as old ones are disappearing. A 2003 study by the World Conservation Union suggested that one in four known mammal species is threatened with extinction in the next several decades, while one in eight known bird species is at risk.

If the present trend continues, biologists fear that we could lose 50 percent of all known living species by the end of this century.


Life on Earth has seen several periods when a large portion of its species died off. Yet, they've always led to a recovery and the rise of a newly shaped tree of life.

An extinction event is a time in the Earth’s history when many living organisms die off. Species are always going extinct, but it happens much more quickly during an extinction event.

Geologists and paleontologists study extinction events. They investigate sedimentary rocks and marine fossils to find important clues about the history of our planet.

It wasn’t until the 1980s that scientists agreed on the five major extinction events.

The “crater of doom”One day about 65.5 million years ago, dinosaurs were grazing and hunting around the world. An object the size of Mount Everest came flying through space.

The odds were against it, but the object hit Earth. It may have been a comet, made of dirty ice, or an asteroid, made of rock. The object landed just off the coast of what is now the Yucatán Peninsula in Mexico. It was traveling about 150 times as fast as a jet.

The impact made a hole the size of Belgium. It kicked up debris that rose high into the atmosphere and circled around the earth. The collision created so much heat that huge forests burned. This sent more pollution into the atmosphere.

The Sun’s rays were blocked by smoke and debris. Photosynthesis slowed or stopped. The temperature cooled and the amount of rainfall decreased.

Plants and animals died. Almost all the dinosaurs died. The only ones to survive were some avian dinosaurs that were evolving into birds. About 75 percent of all species disappeared. Crocodiles, turtles, and small, rodent-like mammals survived. The small mammals are our ancestors.


Geologists call this extinction event the “K-T event.” It marked the end of the Cretaceous period and the beginning of the Tertiary period. (Cretaceous is spelled with a “K” in German.)

The story of the K-T event is well understood thanks to years of patient detective work. It started when young geologist Walter Alvarez made a discovery in the mountains of Italy. He found a thin layer of clay between the layers of Cretaceous and Tertiary limestone. The Cretaceous layer contained many more marine fossils than the Tertiary layer.

Alvarez’s team found iridium in the layer of clay. Iridium is an extremely rare element on Earth. It's more common in meteorites. The iridium suggested an impact by an asteroid or comet around the date of the extinction. They reported in 1980 that an asteroid or comet had hit and caused massive extinction by changing the air and water.

Most geologists accepted this hypothesis based on strong evidence. Others were unsure. If a massive asteroid or comet had hit, where was the crater? No known depression on land seemed large enough for such a massive object. It seemed the crater must be under water.

Large objects that hit water create huge tsunami waves. These waves leave unique signs in the rock record. Rocks on the shores of Texas showed these signs. In 1950, across the Gulf of Mexico, geologists from an oil company mapped a 120-mile underwater crater off the coast of the Yucatán Peninsula.

It wasn’t until 1991 that the K-T researchers got in touch with the oil-company geologists. The K-T researchers realized that the “crater of doom” had been found. They named it Chixculub (a Mayan word pronounced cheek-shoe-lube), after the small coastal town nearby.

Other extinction eventsPaleontologists and geologists have identified four other major extinction events. All of these came before the K-T extinction. Each is named for the geologic time it corresponds to. They are: the End-Triassic, the End-Permian, the Late Devonian and the Ordovician.

The End-Permian was the hugest of the five major extinctions. It was the mother of all extinction events.

In it, about 95 percent of marine species and 70 percent of land species were lost. The dying off lasted for 165,000 years. Environmental changes both sudden and gradual greatly changed conditions on Earth.

Very few creatures made it through the End-Permian extinction. Cockroaches did — and ginkgo trees and horseshoe crabs. So did our ancestors, small protomammals (early mammals) that had evolved from reptiles: they were furry and warm-blooded, but still laid eggs.


Possible causes of extinctionsMost geologists and paleontologists agreed that the cause of the K-T extinction was an asteroid or comet hitting Earth. Many of them hypothesized that objects from space had caused all the major extinctions.

That proved false. Studies of fossil layers from earlier extinctions showed that life forms had disappeared gradually, not suddenly, as they had in the K-T event.


Volcanic activity. Scientists have found huge lava plains that coincide with extinction events. Volcanoes give off carbon dioxide, which results in global warming. They also send out dust and aerosols that slow photosynthesis, causing food chains to collapse.

Rapidly changing climate. Impact or multiple-impact events. Anoxic events — ocean layers losing their oxygen. Changing position of oceans and continents (plate tectonics).

Some combination of these causes may have taken place. Paleontologist Peter Ward made this hypothesis in 2006 to explain the four other major extinctions:

A “sudden” increase of carbon dioxide and methane in the atmosphere occurred, caused by volcanoes. The warmer world disrupted ocean circulation patterns and currents. Without the mixing of the ocean layers, the bottom water became anoxic, without oxygen. This allowed green sulfur bacteria, which live on sulfur not oxygen, to expand. They produced hydrogen sulfide. The hydrogen sulfide bubbled up, killing much of life and destroying the ozone layer. Without the ozone, life was unprotected from the Sun's ultraviolet rays.

Ward’s hypothesis suggests that humans must reduce the carbon dioxide we produce. If we don’t, we may start a similar chain of events.

A sixth major extinction?Many biologists agree that a sixth major extinction is happening today. This one is the result of humans degrading and destroying the habitats of other life forms.

This extinction apparently began about 50,000 years ago. Humans moving into Australia and the Americas caused the disappearance of many species.


Today it is thought that one in four known mammal species is threatened with extinction in the next several decades. One in eight known bird species is at risk. Biologists fear that we could lose 50 percent of all known living species by the end of this century.



Life on Earth has seen several periods when a large portion of its species died off. Yet, life on our planet has always bounced back. Each time, a newly shaped tree of life has come into being.

An extinction event is when many species die off at once. It is normal for species to go extinct sometimes. But during an extinction event it happens more quickly than usual.

Scientists now agree that five major extinction events have occurred.

The “crater of doom”Sixty-five million years ago, dinosaurs were hunting and grazing around the world. An object the size of Mount Everest came flying through space and hit Earth. The huge object (a comet or asteroid) landed just off the coast of Mexico. It was traveling 150 times as fast as a jet.

The impact made a hole the size of Belgium. Dust and debris rose high into the atmosphere and spread around the world. Forest fires filled the skies with smoke. The Sun’s rays were blocked. Without sunlight, photosynthesis stopped or slowed. Temperatures cooled and rainfall decreased.

Plants and animals died, including almost all the dinosaurs. About three-quarters of all species disappeared. Crocodiles, turtles, and small, rodent-like mammals survived. The small mammals are our ancestors.

This is called the K-T event because it marked the end of the Cretaceous period and the beginning of the Tertiary period. (Cretaceous is spelled with a “K” in German).

We understand the K-T event thanks to years of patient work by scientists. It began when young geologist Walter Alvarez made a discovery in the mountains of Italy.

Alvarez found a thin layer of clay between the layers of Cretaceous and Tertiary limestone. The Cretaceous layer had many more fossils than the Tertiary layer. In the layer of clay was iridium. This element is very rare on Earth. It's more common in meteorites. This suggested an asteroid impact around the date of the extinction.

Alvarez and his team announced in 1980 that an asteroid or comet had hit the Earth. They said that was the cause of a massive extinction.

Most geologists accepted this hypothesis. Others were unsure. If a massive asteroid or comet had hit, where was the crater? There was no crater on land that was large enough. It seemed the crater must be underwater.

Large objects that hit water can create huge tsunami waves. These waves leave behind unique signs in the rock record. Rocks on the shore of Texas, across from Mexico, showed these signs.


Actually, a massive crater had been discovered decades earlier. Oil-company geologists had discovered and mapped a 150-mile crater off the coast of Mexico in 1950.

K-T researchers contacted the oil-company geologists in 1991. The “crater of doom” had been found.

Other extinction eventsScientists have identified four other major extinction events. All came before the K-T extinction. Each is named for the geologic time it corresponds to. They are: the End-Triassic, the End-Permian, the Late Devonian and the Ordovician.

The End-Permian was the largest of the five major extinctions. It was the mother of all extinction events.

About 95 percent of marine species were lost. Seventy percent of land species disappeared. The dying off lasted 165,000 years.

Very few creatures made it through the End-Permian extinction. Cockroaches did. Ginkgo trees and horseshoe crabs did too. So did our ancestors, small protomammals (early mammals) that had evolved from reptiles. They were furry and warm-blooded, but still laid eggs.

Possible causes of extinctionsMost scientists agreed that an asteroid or comet caused the K-T extinction. Some of them hypothesized that all major extinctions were caused by space objects.

That wasn’t true. The other extinctions happened gradually. The K-T event happened suddenly.

Scientists still aren’t sure what caused the four other extinction events. Some possible explanations are:

Volcanoes. Scientists have found evidence of large eruptions at the same time as extinctions. Volcanoes give off carbon dioxide, which causes global warming. They also send out dust that blocks the sun. Without sunlight, plants die and food chains collapse.

Climate change. Impact or multiple-impact events. Ocean layers losing their oxygen (anoxic events). Changing position of oceans and continents (plate tectonics).

Some combination of these may have taken place. Paleontologist Peter Ward made this hypothesis in 2006 to explain the four other major extinctions:

Volcanoes caused a sudden increase in carbon dioxide and methane in the atmosphere. The warmer temperatures changed ocean currents. This caused some layers of the ocean to lose their oxygen. This allowed green sulfur bacteria to expand. The bacteria produced hydrogen sulfide. The hydrogen sulfide bubbled up, killing life on Earth and destroying the


ozone layer. Without the ozone, life on Earth was unprotected from the Sun’s ultraviolet rays.

If we continue to put carbon dioxide into the atmosphere, we risk starting a similar chain of events.

A sixth major extinction?Many biologists argue that a sixth major extinction is happening today. This one is being caused by humans. We are destroying the habitats of other life forms.

No one knows how many species currently exist on Earth. The best estimate is about 8.7 million.

Experts believe that one in four known mammal species is threatened with extinction in the next several decades. One in eight known bird species is at risk.

Biologists fear we could lose half of all known living species by the end of this century.


The Biology of AwarenessA biologist reflects on the qualities that define life.

The Biology of Awareness (1100L)By Ursula Goodenough

One of the amazing complexities of life is an organism’s ability to recognize and react to its surroundings. This trait of awareness exists in the smallest single-celled organism as well as in the most complex of creatures — that is, Homo sapiens, like you and me.

EmergenceA living organism has thousands of different kinds of protein shapes, enabling it to self-organize and self-maintain, to reproduce and adapt. How is this even possible inside of a single cell? One useful way to think about this is with a concept called “emergence.” There’s a T-shirt slogan for emergence that goes, “You get something else from nothing but.” What does that mean? Well, the nothing buts are important parts of a system that form relationships with, and organize themselves with respect to, one another. When that happens, the something else that wasn’t there before, a new property or capability, pops through.

Let’s think of some examples. We might consider an emergent property like blood circulation. The nothing buts are things like the heart, the arteries, and the capillaries that, working collectively, allow for blood circulation to take place.

There’s a useful term in biology for emergent properties: traits. So blood circulation is a trait. Human-style motility is a trait, and there are countless others. An organism is thus a collection of traits. When we speak of traits, we are thinking of the large emergent part rather than the nothing buts, but we can go all the way down to the molecules, breaking the trait into little pieces, or numerous nothing buts. Then, when we put Humpty Dumpty back together again, when we assemble all the pieces and get the emergent property, we start talking about the traits that are generated.

Critical to all of this is the idea of natural selection — the process that brings new kinds of organizations and traits from one generation to the next. Natural selection is looking at traits such as blood circulation and mobility. It’s not looking at the proteins and protein shapes. It can’t see the genes. It has no idea how the traits are generated. The criterion for selection is whether a particular version of a trait is adaptive (favorable) or not adaptive (unfavorable) to the particular environmental context where the organism is making its living.

Traits materialize and then evolve. An example of one trait that’s evolving — and that we’re particularly interested in — is what we call “awareness.” So how does awareness work?


AwarenessProbably the very first organisms that were ever successful already had some ability to be aware of their environment: to figure out where a food source is, or where light is. And the basic organization of awareness is shared throughout the biological world.

First, some sort of an outside signal exists for an organism to be aware of. Then, the organism must have a receptor looking for that signal. The receptor is usually a protein, and when the signal and the receptor interact, the protein changes its shape. In your nose, for example, the odor receptors are one shape when you’re not smelling anything; but when a particular odorant comes in, certain receptors bind to that odorant and change their shape.

What happens next is a whole cascade of what we call “downstream events.” The cell notices the shape change, and more shape changes are stimulated. Finally, there’s an adaptive response. If an organism has smelled something, it gets the response to either go toward the thing (if it decides that it is good and wants to eat it), or to move away from it (if it’s the smell of, say, a predator or some toxic substance). Such changes and responses occur even in bacteria, which happen to be very aware of their environment.

Neurons and brainsYou can see the evolution of awareness throughout the single-celled world, but animals took the whole awareness idea to another level by inventing particular kinds of cells called “neurons.” Many neurons tell muscles whether to contract or not. They’re called motor neurons. Another group of important neurons, called sensory neurons, have sensory receptors.

In animals, sensory neurons are almost always hooked up to the brain. The brain is a collection of additional neurons that interacts with the input pouring in from the sensory neurons and integrates the signals. A brain makes things multichanneled and allows for multitasking. You can see a signal. You can smell a signal. You can touch a signal. You can taste a signal. All of these inputs come into the brain, and the neurons in the brain hook all of that information together and figure out the most appropriate response. Are you in the presence of predator or prey?

Learning and memoryAnother wondrous trait is the brain’s ability to learn and remember. We once believed only highly developed animals could store information, but recent research has shown that even something rather simple, like a clam, with only about 20,000 neurons in its brain, can remember for several days stimuli it received.

Animals like mammals can have millions or even billions of neurons in their brains and any one neuron in the brain is in contact with, and can be stimulated by, a thousand other neurons. Complicating things further, some of the neurons that relate to a given neuron stimulate it and cause it to fire, while others prevent it from firing. So whether the neuron actually fires is a result of their collective input.


Imagine that multiplied by about 100 billion, and you’ll begin to see why it’s hard to puzzle out how a complex mammalian brain might work. In fact, there’s very little that we do understand about it. We know that because brains can remember, a mammal with a huge memory store is not only aware of what it’s sensing in the moment, it’s also aware of all the things that it remembers. So it’s much more knowledgeable about the world than if it just had a protein receptor reacting to an outside signal, as with a single-celled organism.

Language and the selfWhat about human brains? How are they different? Well, they don’t look very different than other mammalian brains, and they control most of the same activities, like breathing and body temperature. But they do other things as well. The most important and interesting new feature is our unique mode of communication, called “symbolic language.” We have a way of thinking that generates abstract ideas, and we can remember these abstract ideas and put them together in spoken and written language. We also use language to teach one another, rather than learning only from experience and imitation, and to transmit and evolve our ideas from generation to generation via the social system we call “culture.” Another crucial feature is our storytelling ability — we are narrative creatures, and each of us has a self-narrative. Our “I-self” wakes up in the morning, goes to bed at night, and remembers things about its life. This I-self is crucial to our experience, and it probably distinguishes us from all of the other animals on the planet.

The Biology of Awareness (850L)By Ursula Goodenough, adapted by Newsela

One of the amazing complexities of life is that an organism can recognize and react to its surroundings. This is awareness.

Even the smallest single-celled organisms are aware. So are the most complex creatures, humans like you and me.

EmergenceA living organism has thousands of different protein shapes. These allow it to self-organize, self-maintain, reproduce and adapt. How is this possible inside of a single cell?

We can answer this question with a concept called “emergence.” Organisms have different parts that work together. Sometimes, when certain parts combine in a certain way, a new trait can emerge.

For example, the heart, the arteries, and the capillaries work together to allow for blood circulation to take place. Blood circulation is an emergent property. Another name for emergent properties is traits. An organism is a collection of traits.

Natural selection is the process that brings new kinds of organizations and traits from one generation to the next. Traits are not good or bad. They are just favorable or unfavorable for a certain environment. Traits favorable in a certain environment are likely to be passed on.


Traits materialize and then evolve. We are particularly interested in the trait of awareness. So how does awareness work?

AwarenessThe first organisms that ever survived must have been aware of their environment. They would need to figure out where a food source is, or where light is. The basic organization of awareness is shared among all living things.

First, there must be an outside signal for an organism to be aware of. Then, the organism must have a receptor looking for that signal. The receptor is usually a protein. When the signal hits the receptor, the protein changes shape.

In your nose, for example, the odor receptors are one shape when you’re not smelling anything. When you smell something, receptors attach to the odorant and change their shape.

Finally, there’s an adaptive response. If an organism has smelled something, it gets the response to either go toward the thing (if it decides that it is good and wants to eat it), or to move away from it (if it’s the smell of, say, a predator or some toxic substance). Such changes and responses occur even in bacteria, which are very aware of their environment.

Neurons and brainsSingle-celled organisms are aware, but animals took awareness to another level thanks to “neurons.” There are different kinds of neurons. Motor neurons tell muscles whether to contract or not. Sensory neurons have sensory receptors.

In animals, sensory neurons are almost always hooked up to the brain. The brain receives input from the sensory neurons and organizes the signals.

A brain allows for multitasking. You can see a signal. You can smell a signal. You can touch a signal. You can taste a signal. All of these inputs come into the brain and the brain analyzes the signals and figures out the most appropriate response. Are you in the presence of predator or prey?

Learning and memoryAnother amazing trait is the brain’s ability to learn and remember. We used to believe that only highly developed animals could remember things. But recent research has shown that even a simple animal like a clam, with only 20,000 neurons in its brain, can remember things for several days.

Animals like mammals have millions or even billions of neurons in their brains. Any one neuron can be stimulated by a thousand others.

Imagine that multiplied by about 100 billion, and you’ll begin to see why it’s hard to puzzle out how a complex mammalian brain might work. In fact, there’s very little that we do understand about it.


Thanks to memory, a mammal can be aware of the present moment, as well as everything it remembers. This makes it more knowledgeable about the world than if it just had a protein receptor reacting to an outside signal, as with a single-celled organism.

Language and the selfHow are human brains different? They don’t look very different than other mammalian brains. They control the same activities like breathing and body temperature.

But our brains do other things as well. The most important and interesting is our unique mode of communication called “symbolic language.”

We can generate abstract ideas, remember them, and put them together in written and spoken language. We also use language to teach one another, rather than learning only from experience and imitation.

We also transmit and evolve our ideas from generation to generation via the social system we call “culture.”

Another crucial feature is our storytelling ability. We are narrative creatures. Each of us has a self-narrative. Our “I-self” wakes up in the morning, goes to bed at night, and remembers things about its life. This I-self is crucial to our experience, and it probably distinguishes us from all of the other animals on the planet.

The Biology of Awareness (710L)By Ursula Goodenough, adapted by Newsela

One of the amazing mysteries of life is awareness. Living things can recognize and react to their surroundings.

Even the smallest single-celled organisms are aware. So are the most complex creatures, humans like you and me.

EmergenceA living organism has thousands of different protein shapes. These allow it to self-organize, self-maintain, reproduce and adapt. How is this possible inside of a single cell?

We can answer this question with a concept called “emergence.” Organisms have different parts that work together. Sometimes, when certain parts combine in a certain way, a new trait can emerge.

For example, the heart, the arteries, and the capillaries work together to allow for blood circulation. Blood circulation is an emergent property. Another name for emergent properties is traits. An organism is a collection of traits.

Natural selection is the process that brings new traits from one generation to the next. Traits are not good or bad. They are just favorable or unfavorable for a certain environment. Traits favorable in a certain environment are likely to be passed on.


Traits appear and then evolve. We are interested in the trait of awareness. So how does awareness work?

AwarenessThe first successful organisms must have been aware of their environment. They needed to find food and light. All living things share the basic organization of awareness.

First, there must be an outside signal for an organism to be aware of. Then, the organism must have a receptor looking for that signal. The receptor is usually a protein. When the signal hits the receptor, the protein changes shape.

In your nose, for example, receptors change shape when you smell something. After the signal, there is a response. Is it a good smell or bad smell? Good smell — move toward it. Bad smell — move away.

Such responses happen even in bacteria, which are very aware of their environment.

Neurons and brainsSingle-celled organisms are aware, but animals take awareness to another level thanks to “neurons.” There are different kinds of neurons. Motor neurons tell muscles whether to contract or not. Sensory neurons have sensory receptors.

In animals, sensory neurons are almost always hooked up to the brain. The brain receives input from the sensory neurons and organizes the signals.

A brain allows for multitasking. You can smell a signal. You can touch a signal. You can taste a signal. All of these inputs come into the brain. The brain analyzes the signals and decides on a response. Are you going to eat, or be eaten?

Learning and memoryAnother amazing trait is the brain’s ability to learn and remember. We used to believe that only highly developed animals could remember things. But recent research has shown that even a simple animal like a clam, with only 20,000 neurons in its brain, can remember things for several days.

Animals like mammals have millions or even billions of neurons in their brains. Any one neuron can be stimulated by a thousand others.

Imagine that multiplied by about 100 billion. You’ll begin to see why it’s hard to figure out how complex mammalian brains work. In fact, there’s very little that we understand about them.

Thanks to our memory, a mammal can be aware of the present moment, as well as everything it remembers. This makes it more knowledgeable about the world than a single-celled organism. A single-celled organism just has a protein receptor that reacts to an outside signal.


Language and the selfHow are human brains different? They don’t look very different than other mammalian brains. They control the same activities like breathing and body temperature.

But our brains do other things as well. The most interesting is our unique mode of communication called “symbolic language.”

We use written and spoken language to express abstract ideas and concepts. We also use language to teach one another. Other animals learn only from experience and by imitation. We also pass down our ideas from generation to generation via the social system we call “culture.”

Another crucial feature is our storytelling ability. We are narrative creatures. Each of us has a self-narrative. Our “I-self” wakes up in the morning, goes to bed at night, and remembers things about its life. This I-self is crucial to our experience, and it probably distinguishes us from all of the other animals on the planet.


Charles Darwin: Naturalist & AuthorBefore the 1800s, scholars mainly assumed that living organisms remained as they were created, never changing. Darwin shattered this idea by showing evidence that species do change over time.

Charles Darwin: Naturalist & Author (1200L)By Cynthia Stokes Brown

Before the nineteenth century, scholars generally assumed that living organisms remained as they were created, never changing. Charles Darwin shattered this idea by presenting evidence that species do change over time, in a process he called natural selection.

Setting out to seaCharles Robert Darwin was born the same day that Abraham Lincoln was born in Kentucky. Both of these men helped reshape the way we look at the human race.

Darwin was the fifth of six children. His father, Robert Darwin, was a successful doctor who became wealthy investing in new factories. Darwin’s mother, Susannah, was the daughter of the famous manufacturer of pottery and china, Josiah Wedgwood. She died when Darwin was 8, and his older sisters took charge of raising him. His family was Unitarian and Abolitionist, both minority beliefs in their day. Darwin considered himself a religious man.

Darwin’s father steered him first into medicine, then into the ministry, but Darwin had his heart set on becoming a naturalist. He went to university in Edinburgh, Scotland, and finished at Cambridge University in England. When he was 22, eager to see nature beyond Europe, he signed on to accompany the captain of a ship, the HMS Beagle, on a two-year voyage surveying the coast of South America. The trip extended to almost five years around the world and gave Darwin the opportunity to observe natural life in a great variety of settings.

Among the places that Darwin visited were the Galápagos Islands, a unique set of 14 islands about 600 miles off the coast of Ecuador, on the western side of South America. It was there that Darwin found animals and birds slightly different from each other on each island. He collected many specimens to take back to England, among them examples of different finches.

On returning to England, Darwin showed his finches to expert ornithologists (biologists specializing in birds). They told him that he had about a dozen species of finches, different from those seen anywhere else. When Darwin examined the variations in their beaks, he began to see how they were adapted to dealing specifically with the different kinds of seeds available on the birds’ various native islands. The design of each type of beak had developed from the need for food.


Darwin wrote reports about his trip, which qualified him to be elected to the prestigious Royal Society of London when he was not quite 30 years old. Shortly after, Darwin married a cousin of his, Emma Wedgwood. They had 10 children, two of whom died in infancy and one at age 10. Emma and Charles were immensely happy together; near the end of his life he told his children that she had been his greatest blessing.

Darwin’s key ideaBy the time he was about 30, Darwin had already formulated his key idea — that small variations occur in reproduction and that the individuals with the variations most suited to their changing environment live to reproduce the same characteristics. Over enough time and with enough isolation these variations result in new species, as had happened on the Galápagos Islands.

Darwin did not rush into print with his idea. He wanted to gather more evidence, and he didn’t want to upset people who believed in divine creation at a single moment. Darwin himself struggled with this issue and searched for ways to reconcile his own faith with the scientific evidence he had uncovered. Almost 20 years went by without his publishing his theories.

Then in 1858, Alfred Russel Wallace, an English naturalist traveling in Indonesia at the time, sent Darwin a letter outlining theories similar to Darwin’s that Wallace had developed from his own years of field research. Such a story, where two researchers thousands of miles apart independently come to similar conclusions, is a recurring one in the history of science. Wallace’s letter, sent aboard a Dutch mail ship from the small island of Ternate on March 9, 1858, clearly motivated Darwin to quickly put his own ideas into print. His book, called On the Origin of Species, appeared in 1859. The introduction stated Darwin’s main idea:

As many more individuals of each species are born than can possibly survive; and as, consequently, there is a frequently recurring struggle for existence, it follows that any being, if it vary however slightly in any manner profitable to itself, under the complex and sometimes varying conditions of life, will have a better chance of surviving, and thus be naturally selected. From the strong principle of inheritance, any selected variety will tend to propagate its new and modified form.

Darwin presented three kinds of evidence in support of his theory of natural selection. First, fossils showed that species have changed over time. Second, geographical distribution showed that species are descended from local ancestors, even if the environment is similar elsewhere. Third, he identified unexpected similarities between species, such as the fact that cats, whales, bats, and humans all have fingers. The finger bones showed that these species, despite their huge differences, are all related to each other.

Darwin’s book ignited a widespread controversy, but within a decade many scientists accepted his ideas. Since that time, new evidence has clearly supported Darwin’s concepts. For example, biologists have been able to watch species change in relation to their environment. They have discovered the structure of the genetic code, DNA, and understand how it works to pass on traits and to produce a few variations, called “mutations.” Genetic and fossil evidence has proved that the human species emerged in Africa and is most


closely related to chimpanzees. By the end of the twentieth century, Darwin’s theory of evolution had been buttressed by so much confirming evidence that scientists accepted it as fact, and it has become the foundation of modern biology.

Darwin did not know the Earth’s age. Educated people in England thought that maybe it was tens of thousands of years old, but Darwin realized that more time than that was needed for species to evolve; he estimated the Earth’s age at about 300,000 years. Not until 1953 was the Earth proved to be 4.56 billion years old — plenty old enough for evolution to occur.

Darwin also did not understand how variations occurred in reproduction, as the structure of DNA was not known until 1953 (although occurring in the same year, radiometric dating of the Earth and the discovery of the structure of DNA were unrelated). Yet Darwin knew that variations occurred and could be magnified, as seen in what he termed “artificial selection:” say, when dog or pigeon breeders select animals with the traits they want. They then let these animals reproduce together but not with ones that don’t carry the traits, eventually resulting in new breeds of dogs and pigeons. Darwin called his idea “natural selection” to contrast it with artificial selection, or breeding.

A quiet life of observationDarwin spent most of his life on his farm with his family, writing and studying. He liked solitude and routine, and he loved playing with his children. He visited his extended family, traveled to London occasionally, and wrote thousands of letters to people all over the world, asking questions and seeking information. Penny postage stamps were introduced in Britain in 1840 (the first in the world), and communication improved steadily as the British built their empire. Darwin made remarkable use of these improvements.

Darwin developed his unusual talent for observing natural phenomena and recording what he saw. The one subject that had fired his imagination at Cambridge University was collecting beetles. During his adult years he studied barnacles, bees, orchids, ants, rabbits, pigeons, earthworms, and insectivorous plants. He tramped over his countryside collecting samples and even paid his staff to help. He filled his hothouse with seedlings. He begged information and samples from correspondents everywhere. His eyes seemed always in tune with the vast diversity of life.

After On the Origin of Species, Darwin’s other famous work was The Descent of Man, and Selection in Relation to Sex (1871). In this book, he made explicit his argument that humans descended from apes, which caused his critics to depict him in cartoons with a monkey’s body. Darwin guessed that this evolution must have occurred in Africa, since that’s where the apes lived, but he did not yet have any concrete evidence for this.

Darwin wrote clearly and memorably. His books sold well and his total earnings on sales came close to a half-million dollars in modern terms, an impressive sum for science books. His best-known sentence is this final one at the end of On the Origin of Species:

There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved.


Last days, lasting legacyDarwin suffered all his adult life from stomach problems and heart palpitations, which were never explained in his day or since. In 1882, he died at the age of 73, carefully nursed by his wife. Darwin had expected to be buried in the churchyard in his village of Downe, but his contemporaries insisted that he be buried in Westminster Abby in London, not far from the tomb of Sir Isaac Newton. Time has confirmed that Darwin did indeed open a new era in our understanding of the living world. He showed us how organisms of today are the product of a long process of change, the grandeur of evolution itself.

Charles Darwin: Naturalist & Author (930L)By Cynthia Stokes Brown, adapted by Newsela

Before the 19th century, scholars generally assumed that living organisms remained as they were created, never changing. Charles Darwin shattered this idea by presenting evidence that species do change over time, in a process he called natural selection.

Charles Robert Darwin was born the same day that Abraham Lincoln was born in Kentucky in 1809. Both of these men helped reshape the way we look at the human race.

Darwin was the fifth of six children. His father was a successful doctor and businessman. His mother came from a wealthy family. She died when Darwin was 8, and his older sisters raised him. His family belonged to the Unitarian Church and was anti-slavery. Both of these were uncommon in that time. Darwin considered himself a religious man.

Darwin’s father pushed him toward medicine, and then religious studies, but Darwin had his heart set on becoming a naturalist. He went to university in Scotland, and finished at Cambridge University in England.

When he was 22, Darwin joined a ship going on a two-year voyage along the coast of South America. The trip on the HMS Beagle ended up lasting almost five years and going around the world. It gave Darwin the opportunity to observe natural life in many different settings.

Darwin visited the Galápagos Islands, a unique set of 14 islands about 600 miles off the coast of Ecuador, on the western side of South America. He found animals and birds slightly different from each other on each island. He collected many specimens to take back to England. Famously, he brought back many different finches.

After returning to England, Darwin showed his finches to bird experts called ornithologists. They told him that he had about a dozen species of finches, different from those seen anywhere else.

Darwin examined the birds’ beaks. He saw how each bird’s beak was adapted to the type of seeds available on its island. The shape of each beak had developed from the need for food.

Darwin wrote reports about his trip. Soon he was elected to the prestigious Royal Society of London, a group of famous scientists. Shortly after, Darwin married a cousin of his, Emma Wedgwood. They had 10 children. Two died shortly after birth and one died at age 10.


Emma and Charles were very happy together. Near the end of his life, he told his children that she had been his greatest blessing.

Darwin’s key ideaDarwin was only 30, but he had already formulated his key idea: small changes occur when creatures reproduce. Some creatures are helped by these changes. Those creatures are more likely to reproduce and pass on those characteristics to their offspring.

Over enough time, these changes can result in new species, as happened on the Galápagos Islands.

Darwin did not rush to publicize his idea. He wanted to gather more evidence, and he didn’t want to upset people who believed that God created the world in a single moment, called divine creation. Darwin struggled with his findings. He tried to find ways to balance his faith with the scientific evidence he had uncovered.

It took him almost 20 years before he published his theories.

In 1858, another English naturalist, Alfred Russel Wallace, sent Darwin a letter outlining his own theories, which were similar to Darwin’s. Like Darwin, Wallace had developed his theories from years of research in the field.

Wallace’s letter clearly motivated Darwin to put his own ideas into print. His book, On the Origin of Species, appeared in 1859.

The introduction stated Darwin’s main idea:

More individuals of each species are born than can possibly survive. Consequently, there is a constant struggle for existence. Any being that has an advantage, even a small one, will have a better chance of surviving. Thus it is naturally selected. These individuals that survive will tend to pass down their traits to their offspring.

Darwin presented three kinds of evidence in support of his theory of natural selection. First, fossils showed that species have changed over time. Second, geographical distribution showed that species are descended from local ancestors. Third, he found unexpected similarities between species.

For example, cats, whales, bats and humans all have fingers. The finger bones showed that these species, despite their huge differences, are all related to each other.

Darwin’s book caused huge controversy, but many scientists had accepted his ideas within a decade. Since then, new evidence has clearly supported Darwin’s theories.

Biologists have been able to watch species change in relation to their environment. Scientists have discovered the structure of DNA and understood how it passes down traits with occasional errors, or mutations. Genetic and fossil evidence have proved that the human species emerged in Africa and is most closely related to chimpanzees.

By the end of the twentieth century, Darwin’s theory of evolution had been supported by a great deal of evidence. Scientists now accepted his theory as fact. His ideas have become the foundation of modern biology.


Darwin did not know the Earth’s age. Educated people in England thought that maybe it was tens of thousands of years old, but Darwin realized that more time than that was needed for species to evolve. He estimated the Earth’s age at about 300,000 years. The Earth was, much later, proven to be 4.56 billion years old. That’s more than enough time for evolution to occur.

Darwin also didn’t understand how variations occurred in reproduction. He knew variations happened and could be magnified. Dog or pigeon breeders did this when they selected animals with the traits they want. They then let these animals reproduce together but not with ones that don’t carry the traits. This results in new breeds of dogs or pigeons. Darwin called his idea “natural selection” to contrast it with artificial selection, or breeding.

A quiet life of observationDarwin spent most of his life on his farm with his family, writing and studying. He liked solitude and routine, and he loved playing with his children. He wrote thousands of letters to people all over the world, asking questions and seeking information.

Darwin continued developing his talent for observing the natural world and recording what he saw. During his life he studied barnacles, bees, orchids, ants, rabbits, pigeons, earthworms, and insect-eating plants.

Darwin’s other famous work was The Descent of Man, and Selection in Relation to Sex (1871). In this book, he argued that humans descended from apes. His critics drew him in cartoons with a monkey’s body. Darwin guessed that this evolution must have occurred in Africa, since that’s where the apes lived, but he did not yet have any concrete evidence for this.

Darwin wrote clearly and memorably. His books sold well and his total earnings on sales came close to $500,000 in modern terms, an impressive sum for science books.

His best-known sentence is this final one at the end of On the Origin of Species:


Last days, lasting legacyAll his life, Darwin suffered from stomach problems and heart palpitations. They were never explained. He died in 1882 at the age of 73. He was buried in Westminster Abby in London, not far from the tomb of Sir Isaac Newton.

Time has confirmed that Darwin did indeed open a new era in our understanding of the living world. He showed us how organisms of today are the product of a long process of change, the greatness of evolution itself.


Charles Darwin: Naturalist & Author (790L)By Cynthia Stokes Brown, adapted by Newsela

Before the 1800s, scholars generally assumed that living organisms remained as they were created, never changing. Charles Darwin shattered this idea by presenting evidence that species do change over time. He called the process that species go through "natural selection."

Charles Robert Darwin was born the same day as Abraham Lincoln in 1809. Both of these men helped reshape the way we look at the human race.

Darwin was the fifth of six children. His father was a doctor and businessman. His mother came from a wealthy family. She died when Darwin was 8, and his older sisters raised him. His family was Unitarian (a Christian sect) and abolitionist (anti-slavery). Both of these were uncommon in that time. Darwin considered himself a religious man.

Darwin’s father pushed him toward medicine or religious studies, but Darwin had other ideas. His heart was set on becoming a naturalist. He went to university in Scotland, and finished at Cambridge University in England.

When he was 22, Darwin joined a ship going on a voyage to South America. It was only supposed to sail for two years. However, the trip on the HMS Beagle ended up lasting almost five years and going around the world. It gave Darwin the opportunity to observe natural life in many different places.

Darwin visited the Galápagos Islands, 14 unique islands about 600 miles off the coast of Ecuador in western South America. He found animals and birds slightly different from each other on each island. Darwin collected many samples to take back to England.

Famously, he brought back different finches. A finch is a small bird. Darwin showed his finches to bird experts, called ornithologists, in England. They told him that he had about a dozen species of finches. No one had ever seen them anywhere else.

Darwin examined the birds’ beaks. He noticed that each bird’s beak had adapted to the type of seeds available on its island. The need for food had caused the shape of each beak to develop over time.

Darwin wrote reports about his trip. Soon he was elected to the famous Royal Society of London. It is a group of famous scientists. Shortly after, Darwin married a cousin of his, Emma Wedgwood. They had 10 children. Two died as infants and one died at age 10. Emma and Charles were very happy together. Near the end of his life, he told his children that she had been his greatest blessing.

Darwin’s key ideaDarwin was only 30, but he had already developed his key idea: small changes occur when creatures reproduce. Some creatures are helped by these changes. The creatures that benefit from the changes are more likely to reproduce and pass on those characteristics to their offspring.


Over time, these changes can result in new species. This was just what he believed had happened on the Galápagos Islands.

Darwin did not rush to publish his idea. He wanted to gather more evidence. He didn’t want to upset people who believed that God had created the world in a single moment. Darwin struggled with his findings. He tried to find a way to balance his religious beliefs with the scientific evidence he had uncovered.

It took him almost 20 years before he published his theories.

In 1858, another English naturalist, Alfred Russel Wallace, sent Darwin a letter outlining his own theories. Wallace had come up with ideas similar to Darwin’s. Like Darwin, Wallace had developed his theories from years of research out in nature.

Wallace’s letter clearly motivated Darwin to put his own ideas into print. His book, On the Origin of Species, appeared in 1859.

The introduction stated Darwin’s main idea:

More individuals of each species are born than can possibly survive. Consequently, there is a constant struggle for existence. Any being that has an advantage, even a small one, will have a better chance of surviving. Thus it is naturally selected. These individuals that survive will tend to pass down their traits to their offspring.

Darwin presented three kinds of evidence in support of his theory of natural selection. First, fossils showed that species have changed over time. Second, species are descended from local ancestors. Third, he found unexpected similarities between species. For example, cats, whales, bats, and humans are very different animals. Yet, they all have fingers. The finger bones showed that these species are all related to each other.

Darwin’s book caused a huge controversy. Still, within 10 years, many scientists accepted his ideas. New evidence since then has clearly supported Darwin’s theories.

Biologists have been able to watch species change in relation to their environment. Scientists have discovered the structure of DNA. They now understood how it passes down traits through generations. Occasionally errors, called mutations, get passed down as well. Genetic and fossil evidence has proved that the human species emerged in Africa. Scientists now believe we evolved from chimpanzees.

By the end of the twentieth century, Darwin’s theory of evolution had plenty of evidence supporting it. His theory was now accepted as fact. Darwin's ideas became the foundation of modern biology.

Darwin did not know the Earth’s age. Educated people in England thought that maybe it was tens of thousands of years old. Darwin realized that it must be older. More time would have been needed for species to evolve. He estimated the Earth’s age at about 300,000 years. Much later, the Earth was proven to be 4.56 billion years old. That’s more than enough time for evolution to occur.

Darwin also didn’t understand how changes occurred in reproduction. He knew changes happened and could be magnified. Dog breeders did this when they selected animals with


the traits they wanted. They let animals with traits they liked reproduce together. They didn't let them reproduce with ones that didn’t carry the traits. Over time this led to new breeds of dogs. It was called artificial selection, or breeding. Darwin called his idea “natural selection” to contrast it with this planned evolution.

A quiet life of observationDarwin spent most of his life on his farm with his family, writing and studying. Darwin wrote thousands of letters to people all over the world. He was always asking questions and seeking information.

Darwin continued observing the natural world and recording what he saw. During his life he studied bees, orchids, ants, rabbits, pigeons, earthworms, and insect-eating plants.

Darwin’s other famous work was The Descent of Man, and Selection in Relation to Sex (1871). In this book, he argued that humans descended from apes. His critics drew him in cartoons with a monkey’s body. Darwin guessed that this evolution must have occurred in Africa, since that’s where the apes lived. But Darwin never had any solid evidence for this.

Darwin wrote well and his books sold well. His total earnings on sales came close to $500,000 in modern terms, an impressive sum for science books.

His best-known sentence is this final one at the end of On the Origin of Species:


Last days, lasting legacyAll his life, Darwin suffered from stomach problems and heart palpitations. They were never explained. He died in 1882 at the age of 73. He was buried in Westminster Abby in London, not far from the tomb of Sir Isaac Newton.

Time has confirmed that Darwin opened a new era in our understanding of the living world. He showed us how organisms of today are the product of a long process of change, the greatness of evolution itself.


s3. web viewunit 5—life. this word document contains all of the readings from the unit. all...

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