Earth History

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Lecture 32

• Earth History

Chapter 15.13 ? 15.18

• Geologic Epochs
• Human History

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Precambrian Time
The Earth was formed about 4.5 billion years ago.
Already 4 billion years old rocks show a
sedimentary component, which implies the presence
of erosion.
Nothing is known about the continental locations,
but there is evidence of plate tectonics at that
time. At least 2 glacial periods were identified
in the Precambrian.
Bacterial life structures found in Greenland were
dated to live about 3.6 billion years ago. These
are primitive cyanobacteria which formed colonies
in the oceans and survived intense solar UV
radiation.
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The Paleozoic Era
The history of the Paleozoic Era is well-studied
because of a wealth of fossils available.
The oldest fossils of this era are marine
invertebrates, creatures without internal
skeletons but with external shells.
Small corals, ancestors of starfish and sea
urchins, a variety of fishes, and first
amphibians (relatives of modern frogs and
salamanders) were present in the Devonian
Period. Fossils of reptiles were found in the
Carboniferous Period. Coal and petroleum were
formed in this period from remains of plants and
marine creatures.
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The Mesozoic Era
This era began 225 million years ago, when the
modern continents were joined together in Pangaea.
A bit later Pangaea split into Laurasia and
Gondwana. North America parted from Europe 120
million years ago.
Reptiles and dinosaurs (descendants of primitive
reptiles that had survived from the Paleozoic and
became extinct 65 million years ago) developed
in the Mesozoic Era. Flowering plants also
existed in this period. The first birds with
feathering wings arose from dinosaur ancestors in
the Jurassic. First (small) mammals appeared in
the Triassic period.
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The Cenozoic Era

• Main features of this era which began 65 million
  years ago
• No shallow seas
• Widespread volcanic activity
• Continuous tectonic disturbance (the reason for
  dividing the Cenozoic from the Mesozoic era)

In mid-Tertiary period the largest European (the
Alps) and Asian (the Himalayas) mountains were
folded and uplifted.
Mountains in the North America were repeatedly
uplifted during the Cenozoic, and erosion
following the uplifts shaped the present day
landscapes.
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Mammals
Some mammals survived the mass extinction that
ended the Mesozoic era thanks to their great
adaptability. They significantly evolved because
of the dinosaur extinction. Mammals dominated the
Earth by mid-Tertiary period.
Every 100,000 years glaciers advanced from the
mountains to the plains of Europe and North
America. However, most mammals survived the ice
ages. The last glacial period ended 10,000 years
ago and suggested another one to come in the near
future.
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Human History
Humans seem to appear a few million years
ago. There is biological evidence that the
descendants of a common ancestor split into 2
branches (humans and apes) about 6 million years
ago.
By 2.4 million years ago the line from which
modern humans could come had large brains.
Modern humans, Homo sapiens or wise man, appeared
a few 100,000 years ago. They seem to spread from
Africa into Europe and Asia. Humans came to North
America 15,000-20,000 years ago.
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Summary
Rocks and fossils provide a source of knowledge
about the Earths history The Earth was formed
about 4.5 billion years ago Stable life appeared
on Earth in the form of simple bacteria about 3.8
billion years ago The first humans appeared a
few million years ago and their population
continues to grow