History of Earth: Geologic Time Line

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History of Earth Geologic Time Line
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Geologic time scale-another view
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Hadean Era (Earth formed 4.63 BYA)

• Earth is molten
• Compounds, but no life exists

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Archaean Era (Begins 3.73 BYA)

• First Cell forms
• Photosynthesis begins
• Endosymbiotic theory

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EndOsymbiotic theory
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Archaean era

• Meteor bombardment ends
• Sun 30 dimmer, but atmosphere consisted of
  nitrogen, water vapor, methane, and CO2, so earth
  did not freeze over
• Oceans formed as rain fell

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Proterozoic era (from 1 bya)

• Bacteria, protozoa, algae, corals, worms,
  jellyfish, sponges and fungi exist.
• Photosynthesis on land begins
• Most life was unicellular because there was no
  protective ozone layer to filter UV rays.
• Most life was aquatic because water filtered the
  UV rays.
• First snowball earth ice agepossible because
  continents were all close to equator at that
  time.

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Snowball Earthwhat saved us??

• The carbon cycle has always moderated our
  temperatures!!

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How did life survive?

• Single-celled organisms survived in water
  channels within ice and deep sea vents

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Paleozoic era--age of land plants
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Paleozoic era

• Begins with the Cambrian explosionexplosion of
  new life formsevery phylum is represented within
  50 million years
• Multicellular life requires a stable environment
  and oxygen to survive. Scientists arent sure
  what caused rise in oxygen.
• Hard shells and bones developed, so the fossil
  record is abundant.
• Land plants emergedcooled planet by accelerating
  carbon cycle

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Late paleozoic era

• Largest mass extinction to date85 of life
  became extinct
• Scientists arent sure of causedeep seas became
  anoxic and climate warmed (global warming?)
• Could have been caused by massive volcanic
  eruptions

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Mesozoic era-The age of dinosaurs
FYINewest chemical analyses suggest that
dinosaurs had brightly colored patterns on their
skin and many of them had feathers around their
feet and legs.
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Mesozoic era

• Age of reptiles
• Amphibians and small mammals also flourished
• Mass extinction at end of Mesozoic eraprobably
  caused by large meteor impact close to Yucatan
  peninsula
• Dinosaurs died out

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Cenozoic erathe age of mammals
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Cenozoic era

• Age of mammalsmammals radiated widely after
  competition from dinosaurs was diminished
• Endothermicallowed them to live in a variety of
  environments
• Angiosperms also spreadprotected seeds made them
  fit for environment
• Continents drifted apart, currents trapped around
  Antarctica made it get colder and freeze, which
  cooled atmosphere, causing another ice age

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Cenozoic EraHumans emerge

• Last mass extinctions (mammoth, saber toothed
  tigers) caused by overhunting by humans
• Humans then began to domesticate crops and
  animals.

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Fossilsour peak into the past

• Fossil formation
• Requires the organism to be buried fairly quickly
  before it is decomposed entirely
• This type of fossilization only preserves hard
  tissue (bones), so scientists must hypothesize
  what soft tissue is like

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Dating Fossilsrelative dating

• One way to date fossils is by relative dating.
• Fossils found toward the surface are assumed to
  be younger than deeper fossils
• Since some rock layers will have washed away,
  exposing older layers, scientists will confirm
  dating by using an index fossila fossil whose
  age is known to date the layers fossils are found
  in

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Dating fossilsradioactive dating

• Some elements are radioactive and will decay
  (become un-radioactive) at a predictable pace
• One of these elements is carbon-14, which is in
  the tissues of every living organism.
• When an organism dies, the carbon-14 in its body
  begins to decay
• Scientists can look at how much carbon-14 is left
  in a fossil to determine its age
• Scientists are also now looking at rubidium
  content (another radioactive isotope) because it
  allows accurate dating of older specimens