History of life on Earth

Archean Age

3800 BCE

It was during the Archean era that life first arose on Earth. At this time there were no continents, just small islands in a shallow ocean. There was a vast amount of carbon dioxide in the atmosphere, but since the sun was much fainter back then, the combined effect did not raise Earth's temperature to an extreme. Such levels of carbon dioxide would be toxic to the majority of animals alive today - as would the low oxygen levels.

Cryogenian Period

850 BCE

A succession of incredibly harsh ice ages waxed and waned during the Cryogenian. It is nicknamed Snowball Earth as it's been suggested that the glaciation was so severe it may even have reached the equator. Life during the Cryogenian consisted of tiny organisms - the microscopic ancestors of fungi, plants, animals and kelps all evolved during this time.

Ediacarian Period

85 BCE

Ediacaran was the final stage of Pre-Cambrian time. All life in the Ediacaran was soft-bodied - there were no bones, shells, teeth or other hard parts. The world's first ever burrowing animals evolved in the Ediacaran, though we don't know what they looked like. The only fossils that have been found are of the burrows themselves, not the creatures that made them. This period gets its name from the Ediacara Hills in Australia, where famous fossils of this age were found.

Cambrian Period

54 BCE

The Cambrian is famed for its explosion of abundant and diverse life forms. Animals now swam, crawled, burrowed, hunted, defended themselves and hid away. Some creatures had evolved hard parts such as shells, which readily fossilised and left a clear record behind. In Cambrian times there was no life on land and little or none in freshwater - the sea was still very much the centre of living activity.

Ordovician Period

49 BCE

During the Ordovician, a few animals and plants began to explore the margins of the land, but nothing colonised beyond these beachheads, so the majority of life was still confined to the seas. The Ordovician began with shallow, warm seas but the end of the period experienced a 500,000 year long ice age, triggered by the drift of the supercontinent, Gondwana, to the south polar regions. The Ordovician ended with a mass extinction.

Silurian Period

44 BCE

Was the time when reefs got their act together, grew really big and created a completely new type of ecosystem for marine life. Silurian reefs weren't built by the same types of coral around now, but by a host of tabulate and rugose corals, crinoids and sponges. Bony fish made their first appearance. Meanwhile, on land, plants became more established, and grew in a zone along the edges of rivers and lakes to give Earth its first riverine and wetland habitats.

Devonian period

41 BCE

The Devonian is also known as the Age of Fishes, since several major fish lineages evolved at this time. Sea levels were high and the global climate was warm. Sea surface temperatures in the tropics averaged 30 Celsius, much like the warmer parts of the Pacific today. Growth rings from corals living during the Devonian period have provided evidence that there were more than 365 days in the year back then - about 404 at the start of the period, falling to 396 by the end.

Permian period

29 BCE

The Permian started with an ice age and ended with the most devastating mass extinction the Earth has ever experienced. It's also when all the continents of the world finally coalesced into one supercontinent, named Pangaea (meaning 'the entire Earth'). As the globe warmed up and the ice retreated, many areas of Pangaea became very arid. The oxygen level plummeted too, from a high of 35% of the total atmosphere to around 15%. For comparison, today's oxygen content is 21%.

Carboniferous period

35

The Carboniferous is famed for having the highest atmospheric oxygen levels the Earth has ever experienced and for the evolution of the first reptiles. Plants grew and died at such a great rate that they eventually became coal. The temperature began to drop and the polar regions were plunged into an ice age that lasted millions of years. In North America, the Carboniferous is divided into two epochs, the Mississippian and the Pennsylvanian.