Chapter 14: History of Life

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Chapter 14 History of Life

• Biology B
• Ms. Baun
• January 24, 2008

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Early Earth

• Hot and Harsh
• Atmosphere consisted of methane, hydrogen, water
  vapor, and ammonia gas
• 3.9 billion years ago earth cooled enough for
  bodies of water to form oxygen produced

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Evidence From The Past

• Geologic Evolution the slow gradual change in
  the earth over time

• Organic Evolution the process of change in
  species from when they first appeared on earth

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Fossils
Fossil in amber (sap)
Bones

• Provides evidence for organic evolution
• Any trace or remains of an organism preserved
  through natural processes
• Usually only the hard parts are preserved
• Fossils in amber or ice
• Bones or petrifaction
• Molds, casts, and imprints

Cast Fossil
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Age of Fossils

• Index Fossil
• A fossil that can be used as a comparison for
  determining the relative age of a fossil
• Relative age
• Fossils found in sedimentary rock
• Age is determined by which rock layer it is in
• Order of events older at the bottom, younger
  closer to the top
• Absolute age
• Using radioactive dating to determine how long
  ago events occurred.

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Half life (radiometric dating)

• Half life time it takes for ½ of the atoms of a
  radioactive material to disintegrate
• Most common is Carbon-14 (determines the age of
  an organism that is less than 60,000 years old)
• Uranium-238 tells the age of objects millions of
  years ago

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Half life (radiometric dating)

• How to calculate
• Time x (1 half life/years) number of half-lives
• Example half life of C-14 is 5,730 years. If
  you have a sample of 20g, how much is left after
  17,190 years
• of half-lives 17, 190y x (1 half-life/5,730y)
  3 half lives
• 0 half-lives 20 grams left
• 1 half-life 10 grams left
• 2 half lives 5 grams left
• 3 half-lives 2.5 grams left

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Interpreting The Fossil Record

• Geologic Time Scale timetable of the earths
  history based on information obtained through
  relative and absolute dating of fossils
• Consists of several time divisions called eras
• Eras are divided into periods (epochs)

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• Pattern - Organisms go from simple to complex
• Shows mass extinction - when many species
  suddenly become extinct (ex. dinosaurs at the end
  of the cretaceous period)

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