The History of Life

Chapter 17

Pgs. 417-420; 435-440

17-1 The Fossil Record

• Palentologists: Scientists who study fossils and infer what past life was like

• The Fossil Record: Shows that:– Most organisms that ever lived on earth are

now extinct – Modern organisms have unicellular (single-

celled) ancestors– Fossils occur in a particular order with older

fossils in older rocks

17-1The Fossil Record

• Most fossils form in sedimentary rock

• Examples of fossils include:– Eggs– Footprints– Body Parts

• When rock erodes from wind, rain and shifting mountain ranges, older fossils can be exposed which can then be studied

17-1 Fossils- Relative Dating

• Paleontologists use two techniques to determine the age of fossils:– Relative Dating– Radioactive Dating

• Relative Dating: – The age of a fossil is determined by comparing its

placement with fossils in other layers of rock – Scientists can use index fossils to compare the

relative ages of fossils– Allows paleontologists to estimate fossil’s age

compared to other fossils

17-1 Fossil Dating

• Radioactive Dating: Uses radioactive decay to assign time ranges to rocks– Uses half-life toe determine the length of time

required for ½ of the radioactive atoms in a sample to decay

– Scientists calculate the age of a fossil by the amount of remaining radioactive isotopes it contains

17-1 Geologic Time Scale

• Geologic Time Scale: Scientists divisions of the geologic time scale to represent evolutionary time– Studied rock layers and index fossils

worldwide and put them in order according to relative age

– Precambrian Time covers about 88% of Earth’s history

– After Precambrian Time the basic division are eras and Periods

17-1 Geologic Time Scale

• Eras: There are 3 eras between the Precambrian Era and the present:– Paleozoic Era (500-250 million years ago)– Mesozoic Era (250-65 million years ago)– Cenozoic Era ( Earth’s most recent period: 65

million years ago-present)

• Periods: Each of the 3 eras are subdivided into periods (pg.421 fig. 17-5)– Earths most recent period is the Quaternary

17-2 Earth’s Early History

• The Formation of the Earth occurred nearly 4.6 billion years ago:– Early atmosphere contained poisonous gasses– Earth was struck by objects that generated heat and melted it– Earth cooled enough for rocks to form 4 billion years ago– Volcanic Activity began and made the Earth very hot– Comets and asteroids bombarded its surface

• Oceans did not exist until 3.8 billion years ago, because the Earth was too hot and water remained a gas

17-2 The Puzzle of Life’s Origin

• One necessary condition for the presence of the first life on Earth was the presence of liquid water– Microspheres: 1st molecules that had some

characteristics of living systems• Had permeable membranes, simple energy

systems• Were similar to modern-day bacteria

17-2 Origin of Eukaryotic Cells

• Eukaryotic Cells: Cells that have a nucleus– Original prokaryotic cells began developing cell

membranes– Other prokaryotic cells invaded and began living in

the original prokaryotic cells– These cells developed a symbiotic relationship with

one another

• Endosymbiotic Theory: Eukaryotic cells developed from a symbiosis of several different prokaryotic organisms

17-2 Origin of Eukaryotic Cells

• The Endosymbiotic Theory gained support in the 1960’s by looking at chloroplasts and mitochondira. Lynn Margulis of Boston University made the following observations:– Mitochondrial DNA and chloroplast DNA

resemble bacterial (prokaryotic) RNA– Their ribosomes resemble the ribosomes of

bacteria– Like bacteria, they reproduce by mitosis

17-4 Patterns of Evolution

• Macroevolution:Large-scale evolutionary patterns and processes that occur over long periods of time. There are six of these:– Extinction – Adaptive Radiation– Convergent Evolution – Coevolution– Punctuated Equilibrium– Changes in Developmental Genes

17-4 Patterns of Evolution

• Extinction: More than 99% of all species that have ever lived are now extinct. – There are several hypotheses to explain this

• EX: large asteroids hitting earth wiped out the dinosaurs, errupting volcanoes and the continents moving

– Each time one species became extinct it opened up habitats for new species to develop which cleared the way for evolution to continue

17-4 Patterns of Evolution

• Adaptive Radiation: A single species that has evolved into several different forms that live in different ways– EX: Darwin’s finches evolved into more than a

dozen different species from one original species, Dinosaurs evolved at the same time as 1st mammals, but eventually ruled the earth until extinction

17-4 Patterns of Evolution

• Convergent Evolution: The process by which unrelated organisms come to resemble one another because they live in similar environments with similar environmental demands– EX: Aquatic birds & swimming mammals like

Sharks, dolphins, seals and penguins all live in water environments and have streamlined bodies

17-4 Patterns of Evolution

• Coevolution: The process by which two species evolve in response to one another over time.– An evolutionary change in one organism may

be followed by a change in another organism• EX: A flower can reproduce only if it attracts a

specific type of pollinator

17-4 Patterns of Evolution

• Punctuated Equilibrium: Long stable periods of evolutionary equilibrium interrupted by brief periods of more rapid change or evolution– Can occur because:

• Small population becomes isolated from the main part of the population

• Small group of organisms migrates to a new environment- EX: Darwin’s finches