Early Earth History
Precambrian Time
• Precambrian Time consisted of the
Hadean, Archean, and Proterozoic Eras.
• Lasted from 4.5 billion to 544 million years
ago.
• Longest interval of geologic time
• During the first billion years there were no
life forms and it was probably just
volcanoes erupting and little else.
Not much is known about life forms
of this time because:
• Precambrian rocks have been so deeply buried
that they have been changed by heat and
pressure. Fossils cannot withstand these
conditions.
• Precambrian fossils didn’t have hard parts that
would have increased their chances of being
preserved as a fossil.
• Rocks older than 3.5 billion years are rare due to
remelting and erosion.
Early Life
• Stromalites are layered mats formed by
cyanobacteria colonies.
• Cyanobacteria- blue-green algae thought
to be one of the earliest forms of life on the
Earth.
• Appeared 3.5 billion years ago
• Have chlorophyll and carry on
photosynthesis.
Cyanobacteria cont.
• Through photosynthesis oxygen is
produced and released into the
atmosphere.
• The ozone layer then formed providing
shielding of ultra violet rays from the Earth
allowing species of one-celled organisms
to evolve into more complex organisms.
Precambrian Fossils
• Invertebrates: animals without backbones
• Appeared toward end of the Precambrian
Time
• Because they had soft bodies they did not
leave many fossils.
• Most Precambrian fossils are trace fossils.
Ediacaran Fauna
• Animals shaped like jellyfish, worms and soft
corals lived in the late Precambrian Time
• Fossils of these organisms were first found in
the Ediacara Hills in southern Australia thus the
name Ediacaran Fauna.
• These were bottom dwellers and had tough
outer coverings like air mattresses.
• Scientists are not sure why they became extinct,
it may have been the Trilobites outcompeted
them.
Paleozoic Era
• The era of ancient life, went from 544 to
248 million years ago.
• Traces of life forms are much easier to find
in Paleozoic rock than in Precambrian
rock.
• An abundance of animal with hard parts
marks the beginning of the Paleozoic Era.
• Warm shallow seas covered large parts of
the continents during this time period.
Paleozoic Era Cont.
• Much of the life was marine (ocean
dwellers). Trilobites were common
especially early on
• Other life forms developed shells
therefore, the fossil record of this era
contains abundant shells.
• Vertebrates (animals with backbones)
evolved during this era as well.
Paleozoic Life Cont.
• The first vertebrates were fish like animals
without jaws.
• Armoured fish with jaws lived during the
Devonian Period. Some were large
enough to eat large sharks with their
powerful jaws.
• Forests appeared on land by the Devonian
Period and vertebrates began to adapt to
land environments.
Life on Land
• Many fish had lungs and gills allowing
them to live in water with a low oxygen
content. They could swim to the surface
and breathe air.
• One kind of ancient fish had lungs and
leglike fins to crawl around on the ocean
floor. Paleontologists believe it may have
evolved into amphibians
Pennsylvanian Period
• Amphibians evolved an egg with a tough
skin that wouldn’t dry out, these animals
became the reptiles and they no longer
needed water to reproduce.
• Reptiles also have scales that help
prevent water loss so that they could live
farther away from water where amphibians
could not.
Mountain Building
• Several mountain building episodes
occurred during the Paleozoic Era.
• Appalachian mountains formed in several
stages:
• First the ocean separating Europe and
Africa from North America closed. The
volcanic island chain that had formed in
the ocean collided with the North American
Plate generating mountains.
Mountain Building Cont.
• The next mountain-building episode resulted
from the African Plate colliding with the North
American Plate. When this occurred rock layers
were folded and faulted.
• Rocks were pushed as much as 65 km
westward by the collision.
• Sediments were uplifted to form an immense
mountain belt, some of which remains today.
End of Paleozoic Era
• More than 90% of marine species and 70% of all
land species died off.
• One explanation is a change in climate and a
lowering of sea level.
• Near the end of the Permian Period the
supercontinent Pangaea formed. Glaciers
formed over much of its southern part.
• As continents continued to collide mountains
were formed and seas closed, while deserts
spread over North Amer. and Europe. Many
organisms especially marine organisms became
extinct.
Other Theories About Extinction
• Volcanic activity may have been great
enough to have affected the entire globe.
• A large asteroid or comet collided with the
Earth some 248 million years ago. This is
also the theory that explains the worldwide
extinction at the end of the Mesozoic Era.
• Could have been a combination of some
or all of these events happening about the
same time.