The Geologic Time Scale

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The Geologic Time Scale
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Age of the Earth

• According to radioactive dating, the earth is
  estimated to be 4.6 billion years old.
• The Geologic Time Scale is a record of the life
  forms and geologic events in Earths history.
• Simple single celled life is estimated to have
  formed 3.5 billion years ago.
• Animals appeared 600 million years ago.

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Geologic Time Divisions

• The Geologic Timeline begins with Precambrian
  time.
• Precambrian time takes up 88 of Earths history.
  
• The 2 other basic units of geologic time are eras
  and periods.
• There are 3 eras, and the periods are smaller
  divisions within the eras.

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Paleozoic Era

• Many life forms began to evolve during the
  Cambrian Explosion.
• Life went from simple invertebrates in the
  oceans, to more complex and diverse life forms on
  land.
• The first plants, fish, amphibians, and reptiles
  evolved in the paleozoic era.

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Mass Extinction

• At the end of the Paleozoic era, a mass
  extinction occurred that killed off almost 90 of
  marine species.
• Scientists do not know what caused the
  extinction, but it could have been caused by
  climate change resulting from continental drift.

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The Mesozoic Era

• Some plants and animals did survive the Permian
  extinction, including fish, reptiles, insects and
  conifers.
• The Mesozoic Era is known as the Age of Reptiles.
  
• The first dinosaur appeared about 225 million
  years ago.

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Mesozoic Era

• Mammals first evolved during the Triassic period.
  
• During the Jurassic period dinosaurs ruled the
  Earth for about 150 million years.
• Scientists have identified several hundred types
  of dinosaurs.
• First birds also evolved during the Jurassic
  period.
• Scientists believe that birds evolved from
  dinosaurs.

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Mass Extinction Again!

• At the end of the Cretaceous period, about 65
  million years ago, a mass extinction occurred.
• Scientists hypothesize that this was caused by a
  massive object from space hitting Earth.

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The Cenozoic Era

• Called the Age of Mammals.
• They were no longer competing with large reptiles
  for food, space and survival.
• The extinction of dinosaurs created an
  opportunity for mammals.

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The Cenozoic Climate

• During the tertiary period, the Earth was
  generally warm, and on land flowering plants,
  insects, and mammals flourished. Many mammals
  became large. In the oceans whales evolved.

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The Ice Ages

• During the Quaternary period, the Earths climate
  cooled, and species had to evolve to deal with
  several ice ages.
• In the ocean, algae, coral, fish, and mammals
  thrived.
• On land, flowering plants and mammals such as
  cats, dogs, cattle, and humans became common.

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Early Humans

• The fossil record shows that modern humans may
  have evolved 100,000 years ago.
• By about 12 15,000 years ago, humans had
  reached every continent.