Geological time periods

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.

Began:3.8 billion years ago

Ended:2.5 billion years ago

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.

Began:850 million years ago

Ended:635 million years ago

Known also as the Vendian, the 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. As soft bodies don't fossilise very well, remains from this period are rare. 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.

Began:635 million years ago

Ended:545 million years ago

The Cambrian is famed for its explosion of abundant and diverse life forms. Life had diversified into many forms and many ways of living: 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. However, sometimes geologists get lucky and find beautiful fossils of soft and squishy creatures - as at the Burgess Shale site. 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.

Began:545 million years ago

Ended:495 million years ago

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.

Began:495 million years ago

Ended:Ordovician-Silurian mass extinction
443 million years ago

The Silurian period 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. As the Ordovician ice ages ended, sea levels rose, making the Silurian a period of extensive seas. 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.

Began:Ordovician-Silurian mass extinction
443 million years ago

Ended:417 million years ago

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.

Began:417 million years ago

Ended:Late Devonian mass extinction
354 million years ago

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 period was originally called the Coal Measures after its proliferation of coal-bearing rocks. Though the Carboniferous started off warm - hence its lush coal forests - 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.

Began:Late Devonian mass extinction
354 million years ago

Ended:290 million years ago

The Permian started with an ice age and ended with the most devastating mass extinction the Earth has ever experienced. In fact, at least two mass extinctions occurred during this time. 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%.

Began:290 million years ago

Ended:Permian mass extinction
248 million years ago

The Triassic began after the worst mass extinction ever, at the end of the Permian. Life on Earth took a while to recover and diversify. The Triassic was characterised by heat, vast deserts and warm seas. Even the polar regions were warm, so lush forests grew there. However, the lack of other life, coupled with the period's particular environmental conditions, opened up some evolutionary opportunities. As a result, the very first mammals and dinosaurs evolved. During this time, the giant supercontinent of Pangaea began to break apart. The period ended as it had begun, with an extinction event that wiped out many species.

Began:Permian mass extinction
248 million years ago

Ended:Triassic-Jurassic mass extinction
205 million years ago

The Jurassic began after the mass extinction event that ended the Triassic. Life, however, was quick to recover from this blow and the Jurassic eventually became host to the most diverse range of organisms that Earth had yet seen. Amongst them were the first birds and some of the dinosaurs. Continental break-up during this time gave rise to the sea that would eventually widen to become the Atlantic Ocean. The ocean floor that formed at this time is the oldest surviving on the planet - all older ones having now been 'recycled' through plate tectonics.

Began:Triassic-Jurassic mass extinction
205 million years ago

Ended:142 million years ago

The Cretaceous ended with the most famous mass extinction in history - the one that killed the dinosaurs. Prior to that, it was a warm period with no ice caps at the poles. Much of what we now know as dry land - such as southern England and the midwest of the USA - was underwater, since sea levels reached their highest ever during this time. The Atlantic Ocean grew much wider as North and South America drew apart from Europe and Africa. The Indian Ocean was formed at this time, and the island that was India began its journey north towards Asia.

Began:142 million years ago

Ended:Cretaceous-Tertiary mass extinction
65 million years ago

The Paleocene epoch was a time of dense forests and evolutionary experiments. The extinction of the dinosaurs and other giant reptiles at the end of the Cretaceous paved the way for mammals and birds to evolve to fill those empty niches, so many new creatures appeared. During the Paleocene the island continent of India moved north and collided with Asia. At the end of the epoch, an abrupt rise in temperature across the planet made the climate much wetter and caused a sea level rise.

Began:Cretaceous-Tertiary mass extinction
65 million years ago

Ended:54.8 million years ago

The Eocene began as a time of global warming, with temperatures across the planet soaring. Forests thrived and trees grew even in polar regions. Eventually, the Eocene became cooler and drier. As India continued to drift northwards, pushing against the Eurasian continent, the mass of rocks thrust up between them formed the Himalayas. At this time Africa was an island, not yet joined to the Middle East and Europe, but its own journey north was to trigger the formation of the Alps. Many species of grass evolved in the Eocene epoch, but were very limited in extent - today's grassy plains were still far in the future.

Began:54.8 million years ago

Ended:33.7 million years ago

Over 30 million years ago, the Oligocene epoch saw the start of the global cooling that would eventually shift the Earth's climate to one where glaciers were present and ice ages were possible. Worldwide, this was the time when grasslands began to expand and forests - especially tropical ones - shrank correspondingly. Animals evolved to fit the new, open landscape and many fast-running prey and predator species arose as a result.

Began:33.7 million years ago

Ended:23.8 million years ago

The apes arose and diversified during the Miocene epoch, becoming widespread in the Old World. In fact, by the end of this epoch, the ancestors of humans had split away from the ancestors of the chimpanzees to follow their own evolutionary path. As in the Oligocene before it, grasslands continued to expand and forests to dwindle in extent. In the Miocene seas, kelp forests made their first appearance and soon became one of Earth's most productive ecosystems.

Began:23.8 million years ago

Ended:5.3 million years ago

The Pliocene world looked very similar to Earth today as North and South America had been drifting ever closer and the gap between them was sealed in this epoch. At the start of the Pliocene, over 5 million years ago, the north polar ice cap came and went with the seasons and with fluctuations in climate. However, as the world cooled in the late Pliocene, ice at the North Pole became permanent and grassland and tundra thrived. The human lineage split away from the chimpanzees' early on in the epoch.

Began:5.3 million years ago

Ended:2.6 million years ago

During the Pleistocene, glaciers came and went, resulting in a series of ice ages punctuated by warmer periods. There were at least 20 cycles of this advance and retreat. During the ice ages, global temperatures were 5 degrees centigrade cooler than today and it was much drier, since much of the world's water was locked up in massive ice sheets. The expansion of the deserts and the action of glaciers grinding up rocks meant that dust storms would have been a lot more common in the Pleistocene than they are now. Our species evolved during this epoch.

Began:2.6 million years ago

Ended:11.7 thousand years ago

The Holocene (or Recent) is the current geological epoch which started some 11,500 years ago when the glaciers began to retreat. This retreat marked the end of the glacial phase of the most recent ice age. Its character was set by the spread of forests as the ice retreated and then by their shrinkage as mankind's demand for timber and agricultural land grew. Although we think of the Holocene as a warm time for the planet, we are still in an ice age. This is indicated by the presence of ice caps at the poles - the planet as a whole is just in an interglacial phase.

Began:11.7 thousand years ago

Ended:Present day