The Precambrian Era
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The Precambrian Era covers seven-eighths of Earth’s history
although paleontologists have found very few fossils of
Precambrian organisms. The Precambrian Era was marked by
the formation of Earth 4.6 billion years ago, and ended with the
explosion of hard-shelled organisms 600 million years ago. It is
divided into the earlier Archaean period, when no life existed,
and the later Proterozoic Period, in which life did exist. The
state of the Earth was much different back then; and was
probably ripe enough for life to spring out of non-living
particles. Most life forms during this era were in the form of
prokaryotic single-celled organisms, but fossil finds started to
increase at the end of the period as complex, soft-bodied
invertebrates began to develop. Life never spread out to land
during this time, because oxygen and ozone levels in the
atmosphere were not abundant yet, but life did thrive in the seas
at the very least. Stromatolites are among the most famous
“organisms” during the Precambrian era, and are formed from
the growth of cyanobacteria, which are bacteria with
chlorophyll and were instrumental in developing the Earth’s
first supply of oxygen. We do not know of any major
extinctions during the Precambrian Era due to a lack of fossils,
but we do know that the era ended with the explosion of
invertebrate life in the seas.
Stromatolites
Bacteria
The Paleozoic Era
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The Paleozoic Era was an era that lasted from around 600
million years ago to 230 million years ago. During the
Cambrian Era, the first period of the Paleozoic Era, there was
an abundance of hard-shelled organisms, including the famous
trilobite, which has been found in many areas around the world.
Additionally, the first vertebrate, a small fish, came into
existence during the Cambrian Epoch, although life was still
restricted to the seas. During the next period, the Ordovician
period, seas had spread across much of North America, and
many new animals, including shellfish, coral, and fishlike
vertebrates had developed. Life finally spread onto the land in
the following Silurian period, and the land had now exhibited
arachnids and simply plants. The following period, the
Devonian period, was also known as “The Age of Fishes” due
to the proliferation of fish in the seas. The same period also
marked the proliferation of plant life on land as well as the first
amphibians. The first reptiles on the Earth appeared in the
following Carboniferous period, which is responsible for many
of the oil deposits that we have today. The climate of the Earth
was very hot and moist, but cooled down in the following
Permian period, which was characterized by the first conifers (a
type of gymnosperm) as well as the formation of glaciers in
many areas. The Appalachian Mountains also formed during
this period, but a mass extinction spelled the end of this period
and of the Paleozoic Era, as 90% of all species of life on Earth
had disappeared in this mass extinction; all within a few million
years.
Trilobite
Coelacanth
Ammonites
Diplocaulus
The Mesozoic Era
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The Mesozoic Era covered the period from 230 to 65 million
years ago.
The Mesozoic Era is often known as the “Age of Dinosaurs”
because these reptiles were the largest, the most powerful, and
the most conspicuous animals of the era. They basically
dominated the Earth in the Mesozoic Era, which is divided into
three periods: The Triassic, the Jurassic, and the Cretaceous
Periods.These three periods exhibited their own unique share of
dinosaurs, although the most conspicuous dinosaurs, such as the
Tyrannosaurus Rex, were found in the Cretaceous Period, the
period in which dinosaurs were at their peak. The era also
marked the birth of the Rocky Mountains, as well as the first
birds, and mammals, both of which come from reptiles.
Flowering plants (angiosperms) also first appeared in the
Mesozoic Era. While the early Mesozoic was dominated by
unusual plants, the first modern gymnosperms began to appear
during this era.
The end of the Mesozoic Era is among the most famous events
in the history of the Earth. 65 million years ago, 75% of the
species on Earth went extinct, including the dinosaurs, while the
cause of the mass extinction remains controversial.
During the Mesozoic Era, Pangaea began to split apart, and seas
were forming between the continents. Europe was flooded
during this era, and became dry again only during the
Cretaceous period. A great rift zone helped drive Europe and
North America apart, while Northern Africa also started to rift
apart. Tectonic activity on the Earth was particularly active
during the late Mesozoic Era, and there were many other events
related to the shifting of the continents.
Tyrannosaurus Rex
Archaeopteryx
Ichthyosaur
Conifers
The Cenozoic Era
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The Cenozoic Era enclosed the period of time from the
extinction of the dinosaurs 65 million years ago to the present
day, and is divided into two periods: The Tertiary Period, and
the Quaternary Period. The era is often called the “Age of
Mammals” because mammals thrived and became widespread
across the Earth during this era. Birds and flowing plants
(angiosperms) also thrived during this era, while reptiles were
now only living in a fraction of their former glory. The Earth
developed to the point as we know it today, and several
mountain ranges, including the Alps and the Himalayas formed.
The climate of the Cenozoic era varied greatly; the Earth was
relatively warm at some times, but the era had also possessed
numerous ice ages. We may currently be living in an
intergalactic ice age in fact; which is basically a time period in
between ice ages. In the beginning of the period, the Earth was
recovering from the mass extinction at the end of the Mesozoic
Era, but the survivors of the extinction thrived, with few
competitors or predators. Small mammals eventually evolved
into the larger mammals that we recognize today, although
many small mammals still obviously exist today. Additionally,
this period also marked the evolution of apelike creatures into
humans, who now have so much control over the world that the
world is now in the mercy of the hands of humans. There has
been a recent mass extinction that has just begun a century ago,
as more and more species become extinct due to the actions of
impetuous and selfish humans. The mass extinction is still
continuing today, and could possibly be an extinction similar to
those of previous eras if humans do not change their actions.
Sabre-tooth cat
Homo erectus
Wooly Mammoth
Flowering Plant
(Angiosperm)