Download Earth History.

Lecture 32
Earth History
Chapter 15.13  15.18
• Geologic Epochs
• Human History
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.
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.
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.
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.
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.
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.
Summary
Rocks and fossils provide a source of knowledge about the
Earth’s 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