Our present estimate is that the Earth is 4600 million years old.
Geologists have divided the earth's history to different eons and eras,
where a start of a new era means a major change in events (and often
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Precambrian eon is by far the longest, it covers 90% of the Earth's
history, but we know quite little about it.
We can, of course, date
precambrian rocks and we do know that the only life forms during
Precambrian were cells and bacteria; and that the eon ended in long ice
ages and mass extinctions (we just know about the later periods in
The next eon, Phanerozoic, is divided
eras: Paleozoic, Mesozoic, and Cenozoic.
Era started with Cambrian
and the Cambrian Explosion where an amazing amount of animal species
evolved during only a few millions of years.
They were far more
complicated than the Precambrian life, but still quite primitive marine
Mesozoic era started with the Triassic
period when the first dinosaurs, and about
10 million years later, the first mammals evolved. The next, Jurassic
Period, saw the first birds, and the last,
period, saw the first flowering plants
and ended in a mass extinction (of dinosaurs).
Cenozoic era is the era of mammals. During the
Mesozoic, dinosaurs were so successful that mammals never got a chance
to evolve to as many species as we see today.
So the Cenozoic era
started with the development of the highest forms of mammals in the
following order: the first primates, the first whales,
monkeys (Eocene epoch); the first apes (Oligocene Epoch); Homo Habilis,
Homo Erectus (Pliocene epoch); Neanderthals and Homo Sapiens
Supercontinent Rodinia existed between 1100 and
830 million years ago.
Supercontinent Pangaea formed during Permian
period and Gondwana
between 520 and 180 million years
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A classic way to comprehend the magnitude of geological time scale is
to compress 4600 million years into one year.
On that scale the oldest
preserved rocks formed in mid-March. The simplest life forms
and bacteria evolved in May.
Plants and land animals evolved in the end
of November. Dinosaurs
lived from 10
to 26 December, when Australia
separated from Antarctica and started drifting northwards.
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