Chapter 12: The History of Life

12.1: The Fossil Record• Objectives: Describe the ways that

fossils can form.• Identify the use of relative dating and

absolute dating techniques.• Warm Up: What are some of the

ancient life forms you remember? • Words to Know: Relative Dating,

Radiometric Dating, Isotope, Half-Life

Fossils Can form in Several Ways• There are many processes that make fossils.

– 1. Permineralization – occurs when minerals carried by water are deposited around a hard structure.

– 2. Natural Casts – form when flowing water removes all of the original bone or tissue, leaving just an impression in sediment. This can be filled by minerals recreating the original shape of the organism.

– 3. Trace Fossils – record the activity of an organism. They include nests, burrows, imprint of leaves and footprints.

– 4. Amber-preserved Fossils – organisms that become trapped in tree resin that hardens into amber after the tree gets buried underground.

– 5. Preserved Remains – form when an entire organism becomes incased in material such as ice or volcanic ash or immersed in bogs.

• Most fossils form in sedimentary rock, which is made by many layers of sediment or small rock particles.

• The environments for fossil creation are wetlands, bogs river mouths, lakebeds, and floodplains.

• Why re so few complete fossils discovered?

Dating • Relative Dating

estimates the time during which an organism lived by comparing the placement of fossils of the organism with the placement of fossils in other layers of rock.

• Relative dating allows scientists to infer the order in which groups of species existed, but does NOT give actual ages.

Dating • Radiometric Dating is a technique

that uses the natural decay rate of unstable isotopes found in material in order to calculate the age of that material.

• Isotopes are atoms of an element that have the same number of protons, but a different number of neutrons.

• Ex: Carbon 14 is a common tool used for dating fossils.

• The decay rate of many radioactive isotopes has been measure and is expressed as the isotope’s half-life.

• A Half-Life is the amount of time it takes for half of the isotope in a sample to decay into a different element.

• Radiocarbon Dating• A fossil’s age can be estimated by

comparing the ratio of stable isotope, such as Carbon 12, to the difference between the amounts of Carbon 12 and Carbon 14 there will be.

Determining Earth’s Age• Scientists have used radiometric dating to determine the age

of Earth.• Meteorites do not break down like rocks on Earth and are

believed to have been on Earth from the beginning.• Meteorites provide an unspoiled sample for radiometric

dating.• Scientists measure the Earth’s age at about 4.5 Billion years.• Why are meteorites helpful for determining the age of Earth?

12.2: Geologic Time Scale• Objectives: Recognize the role of index

fossils in determining the age of rocks.• Identify the major intervals of the

geologic time scale.• Warm Up: What are some reasons that

species become extinct?• Words to Know: Index Fossil, Geologic

Time Scale, era, Period, Epoch, mass extinction, adaptive radiation.

Index Fossils • Index fossils are fossils of

organisms that existed only during specific spans of time over large geographic areas.

• Using index fossils for age estimates of rock layers is not a new idea (been around since the 1700’s).

• The best index fossils are common, easy to identify, found widely around the world, and only existed for a short period of time.– Ex: Fusulinids disappeared

after a mass extinction and indicates that a rock layer must be between 248 million and 360 million years old.

• Could a rock layer with fusulinid fossils be 100 million years old? Explain.

The Geologic Time Scale • The Geologic Time Scale is a representation of the

history of Earth.• It organizes Earth’s history by major changes or events

that have occurred, using evidence from the fossil and geologic records.

• The time scale is divided into units:– 1. Eras – last tens to hundreds of millions of years and consist

of two or more periods.– 2. Periods – the most commonly used units of time on the

geologic time scale, lasting tens of millions of years. Each period is associated with a particular type of rock.

– 3. Epochs – the smallest units and last several million years.• Why do adaptive radiations often occur after mass

extinctions?

The Geologic Time Scale

The Geologic Time Scale

12.3: Origin of Life• Objectives: Describe the conditions on

Earth Billions of years ago.• Summarize the main hypotheses of

how life began on Earth.• Warm Up: What scientific theories or

ideas have you heard of that deal with the formation of Earth and the solar system?

• Words: Nebula, Ribozyme

Earth was very Different Billions of Years ago.

• Most scientists agree on two points when it comes to the origin of the Earth:

• 1. Earth is billions of years old.• 2. The conditions of the early

planet and its atmosphere were very different from those of today.

• Today, the most widely accepted hypothesis of Earth’s origins suggests that the solar system was formed by a condensing Nebula, a cloud of gas and dust in space.

• This idea is supported by research and suggests that the Earth is 4.6 Billion years old.

• Early Earth was violent and very hot for the first 700 million years.

• Hydrogen, carbon monoxide and nitrogen gas were in the atmosphere.

• What was not present was OXYGEN.• As the planet cooled, water vapor

condensed and fell as rain that collected as pools.

• Once water was present, organic compounds could form.

• Describe the nebular hypothesis of Earth’s origin.

Organic Molecule Hypothesis• There are two general

hypotheses about how life-supporting molecules appeared on early Earth.

• Miller-Urey Experiment– In 1953, Miller and Urey

designed an experiment to test a hypothesis first proposed in the

1920’s.– Miller and Urey built a system to

model early Earth. – They demonstrated that organic

compounds could be made by passing an electrical current

through a closed system of early gases.

– These gases were Methane, ammonia, hydrogen and water vapor.

Meteorite Hypothesis• Analysis of a meteorite

that fell in Australia in 1969 revealed that amino acids are present on meteors.– This evidence suggests

that amino acids could have been present when Earth formed, or that these organic molecules may have arrived on Earth through meteorite or asteroid impacts.

Early Cell Structure Hypotheses• Iron-Sulfide Hypothesis• Martin and Russel noted

that hot iron sulfide rising from below the ocean floor combines with the cooler ocean water to form chimney-like structures made of many compartments.

• They proposed that 4 billion years ago, biological molecules combined in the compartments of these chimneys.

Early Cell Structure Hypotheses• Lipid Membrane Hypothesis

– Several scientists have proposed that the evolution of lipid membranes was a crucial step for the origin of life.

– Lipid molecules spontaneously form membrane-enclosed spheres, call liposomes.

– In 1992, Harold Morowitz tested the idea that at some point liposomes were formed with a

double, or bilayer, lipid membrane.

– These liposomes could then form around a variety of organic molecules, such as amino acids,

fatty acids, sugars and nucleotides.

– The liposomes would act as membranes and would later give rise to the first true cells.

RNA as Early Genetic Material• A hypothesis that has become much supported recently

states that RNA, rather than DNA, was the genetic material that stored information in living things on early Earth.

• In the 1980’s it was discovered that RNA can catalyze reactions.

• Ribozymes are RNA molecules that can catalyze specific chemical reactions.

• RNA can copy itself, chop itself into pieces, and from these pieces make even more RNA.

12.4: Early Single-Celled Organisms

• Objectives: Recognize the role microbes played in shaping life on Earth.

• Summarize the theory of Endosymbiosis.• Relate increased biodiversity to sexual

reproduction.• Warm Up: If you put photosynthetic

organisms together with water what do you get?

• Words to Know: Cyanobacteria, Endosymbiosis

Microbes • Single-celled organisms changed

Earth’s surface by depositing minerals.• They changed the atmosphere by

giving off oxygen as a by-product of photosynthesis.

• Before photosynthesis evolved, the first prokaryotes would have anaerobic, living without oxygen.

• Scientists have found evidence that photosynthetic life evolved more than 3.5 billion years ago.

• These fossils are of Cyanobacteria, which are bacteria that could carry out photosynthesis and release oxygen.

• Some cyanobacteria live in colonies.• Higher oxygen levels in the

atmosphere and ocean allowed the evolution of aerobic prokaryotes, which need oxygen to live.

• How are stromatolites evidence of Earth’s early life?

Eukaryotic Cells • The fossil record shows that eukaryotic organisms evolved 1.5 billion years

ago.• Eukaryotes have a nucleus and other membrane bound organelles.• One hypothesis of eukaryotic evolution is the theory of endosymbiosis.• Endosymbiosis is a relationship in which one organism lives within the

body of another, and both benefit from that relationship.• The Theory of Endosymbiosis suggests that early mitochondria and

chloroplasts were once simple prokaryotic cells that were taken up by larger prokaryotes around 1.5 billion years ago.

• Instead of being digested, some of the smaller prokaryotes may have survived inside of the larger ones.

• If it the cell took in a prokaryote that acted like a mitochondria, the larger cell got the energy from ATP.

• If the cell took in a prokaryote that acted like a chloroplast, the larger cell got nutrients through photosynthesis.

• In exchange, the mitochondria and chloroplasts found a stable environment and nutrients.

• Support for this theory is based on the fact that both chloroplasts and mitochondria have their own DNA and are the same size as prokaryotes.

• What evidence supports the theory of endosymbiosis?

Eukaryotic Cells

Evolution of Sexual Reproduction

• The first prokaryotes and eukaryotes could only reproduce asexually.

• Sexual reproduction may have resulted in an increase in the rate of evolution by natural selection.

• Sexual reproduction creates more genetic variation.

• Sexual reproduction may have been the first step in the evolution of multicellular life.

• How can mutations be beneficial to organisms?

12.5: Radiation of Multicellular Life.

• Objectives: Summarize the key events in the Paleozoic, Mesozoic, and Cenozoic eras.

• Identify how changes in environmental conditions affected the evolution and radiation of animal groups.

• Warm Up: To what kind of habitat were dinosaurs and reptiles of the Mesozoic era adapted? Why aren’t there dinosaurs on Earth anymore?

• Words to Know: Paleozoic, Cambrian Explosion, Mesozoic, and Cenozoic.

Life Moved onto Land • One hypothesis suggests that it was an

advantage for early one-celled organisms to increase in size by becoming multicellular.

• Cells that cooperated could compete more effectively for limited resources.

• Multicellular organisms first appeared during the Paleozoic Era, which began 544 million years ago.

• Members of every major animal group evolved within only a few million years.

• The era ended 248 million years ago with a mass extinction.

• More than 90% of marine animal species and 70% of land animal species of that time became extinct.

• The earliest part of the Paleozoic Era, the Cambrian Period is often called the Cambrian explosion.

– A huge diversity of animal species evolved.

• The middle of the Paleozoic Era was a time of great diversity as life moved onto land.

• The number and variety of plant groups greatly increased.

• Four-legged vertebrates, such as amphibians, became common.

• Why is the Cambrian period also called the Cambrian Explosion?

Reptiles Radiated During the Mesozoic Era

• The Mesozoic Era began 248 million years ago and ended 65 million years ago.

• Called the Age of Reptiles because the dinosaurs roamed Earth during this era.

• The Mesozoic era also feature birds and flowering plants.

• The oldest direct ancestor of mammals first appeared.

• The Mesozoic era is divided into three periods: Triassic, Jurassic and Cretaceous.

– Life took off slowly in the early Triassic.– The Jurassic was marked by the

dinosaurs.– The peak in dinosaur diversity though

is the Cretaceous.• This era ended with the most famous mass

extinction when it is believed a meteorite struck the Earth and blocked the sun.

• How had life on earth changed from the beginning of the Paleozoic era to the end of the Mesozoic?

Mammals Radiated during the Cenozoic Era.

• The Cenozoic Era began 65 million years ago and continues today.

• It is divided into two periods: Tertiary and Quaternary.

• During the Tertiary Period, placental mammals and monotremes evolved and diversified.

• During the Tertiary period, birds, ray-finned fishes, and flowering plants also underwent dramatic radiations.

• The earliest ancestors of the modern humans evolved near the end of the Tertiary.

• However, homo sapiens anatomically modern humans, did not appear until about 100,000 years ago.

• Why is the Cenozoic era sometimes referred to as the age of mammals?

12.6: Primate Evolution • Objectives: Examine the evolutionary

relationships between humans and other primates.

• Recognize the names and relative ages of extinct hominids.

• Summarize the events and forces that shaped human evolution.

• Warm Up: What living primate species is the closest relative to our species?

• Words to Know: Primate, Prosimian, Anthropoid, Hominid, Bipedal

Humans Share a Common Ancestor …

• The common ancestor of all primates probably arose before the mass extinction that closed the Cretaceous period 65 million years ago.

• Primates make up a category of mammals with flexible hands and feet, forward-looking eyes, and enlarged brains relative to their body size.

• Primates also have arms that can rotate in a circle around their shoulder joint, and many primates have opposable thumbs.

Primate Evolution• The relationship of primate evolution forms a multi-branched tree.• Prosimians are the oldest living primate group, and most are small

and active at night.– Ex: lemurs, lorises, and tarsiers.

• Anthropoids, the humanlike primates, are further subdivided into the New World monkeys, and hominoids.– Many species have prehensile, or grasping, tails, that allows them

to hang.– They have larger brains and can manipulate objects.

• Hominids walk upright, have long lower limbs, thumbs that oppose, four other fingers and relatively large brains.

• This group includes ALL the human lineage, both modern and extinct.Walking Upright• Fossil discoveries have revealed that one trait had a huge impact

on development – walking on 2 legs.• Bipedal is an adjective that describes two-legged or upright

walking.• What is another common animal that is bipedal?

Primate Evolution

There are Many Fossils of Extinct Hominids

• Most hominid species are classified into two group: the genus Australopithecus and the genus Homo

• The earliest member of the genus Homo was Homo habilis. (“handy man”).

• He lived 2.4-1.5 million years ago in what are now Kenya nd Tanzania.

• Made stone tools and more closely resembled the modern human brain in shape.

• H. neanderthalensis lived from 200,000 – 30,000 years ago, in Europe.

• Some evidence suggests that he existed with Homo sapiens (modern day humans).

• What type of evidence could indicate that H. sapiens and H. neanderthalensis coexisted?

Modern Humans Arose about 100,000 Years Ago.

• Fossil evidence reveals that the first Homo sapiens (modern humans) appeared in Ethiopia.

• The Role of Culture• Human evolution is influenced by culture.• Objects such as tools demonstrate a steady trend of

increasing sophistication and usefulness.• Evolution of the Human Brain• The human brain and skull size have both increased.• These traits evolved much faster in human than in other

hominids.• When might having an increasingly larger brain size no

longer be a selective advantage?