It is a column made up of stratigraphic divisions based on rock sequences.
There's of course no rock
that shows all the time sequences in the Earth's history - geologists
have constructed that timeline which is put together from many
different data from many different rocks.
It is divided into units such as eons, eras, periods, epochs, ages and chrons.
the longest ones, lasting at least a half a million years, while chrons
are the shortest ones. Each division has been named, usually after the
place on the Earth where the first rocks from that time were found.
geological time scale is also constantly updated as new discoveries are
made. Only quite recently have we got a half descent idea about what
happened before Cambrium - the period when the marine life "exploded"
and many fossils were suddenly left behind. We have a much more
detailed picture of what happened since. Stratigraphy helps geologists
to match the rock sequences. This, however, only gives a relative, not
definite age. It is only since the mid-1900s when the radioactive
dating was developed as we can be quite sure that the dates are fairly
Precambrian Eon 4600 -
oceans formed, and the first single-celled organisms appeared. Towards
the end of Precambrian, simple multi-celled animals evolved. However,
they did not have any calcaceous body parts such as vertebrate or
shells, to leave fossils behind.
Cambrian Period 545 - 495
"cambrian explosion" of life - a lot of small marine animals formed,
including the first animals with shells, which could easily be
preserved as fossils.
Ordovician Peiod 495 -
warm period that caused large polar ice caps to melt. First land plants
and first jawless fish appeared. The period ended with a mass
Silurian Period 443 - 417
Caledonian Orogeny formed mountains in North America and northern
Europe. Animals started to invade land. First vascular plants.
Devonian Period 417 - 354
First insects, and first seed-bearing plants.
Extensive coal deposits formed. First reptiles. Pangaea started to form.
Permian Period 290 - 248
Cold period, which ended with a huge mass extinction - 96% of species
were wiped out.
Triassic Period 248 - 205
First mammals, first dinosaurs.
Jurassic Period 205 - 142
Pangaea began to break up. Dinosaurs dominated.
Cretaceous Period 142 -
Dinosaurs still dominated although the first predatory mammals had
evolved. Dinosaurs went extinct in the end of the period.
Cenozoic Era 65 mya - now
had broken up. Grand Canyon and Himalayas formed. Because dinosaurs
were gone, mammals started to dominate. Large mammals appeared,
primates and humans developed. First flowering plants.