1. The History of Life
Chapter 17
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2. 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

3. 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

4. 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

5. 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

6. 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

7. 17-1 Geologic Time Scale
Eras: There are 3 eras between the Precambrian Era and the present:
Paleozoic Era ( million years ago)
Mesozoic Era ( 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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. 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