Geological Time Scale

Below is a geological time scale.
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 conditions).


Graphics from Wikimedia Commons

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 greater detail).

The next eon, Phanerozoic, is divided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Paleozoic Era started with Cambrian Period 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 animals.

The rest of Paleozoic saw the first land plants (Ordovician), the first vascular plants (Silurian), the first insects (Devonian), the first reptiles (Carboniferous), and another ice age and mass extinction (Permian).

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, Cretaceous 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, the first monkeys (Eocene epoch); the first apes (Oligocene Epoch); Homo Habilis, Homo Erectus (Pliocene epoch); Neanderthals and Homo Sapiens (Pleistocene epoch).

Supercontinent Rodinia existed between 1100 and 830 million years ago.

Supercontinent Pangaea formed during Permian period and Gondwana continent existed between 520 and 180 million years ago.


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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 like algae 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.

The Great Barrier Reef formed about 11pm on 31 December.

Aboriginal People arrived in Australia about 11.54pm and captain James Cook arrived at one second to midnight.