Phanerozoic

This is because it was once believed that life began in the Cambrian, the first period of this eon, due to the lack of Precambrian fossil record back then.

[1] In the 19th century, the boundary was set at time of appearance of the first abundant animal (metazoan) fossils, but trace fossils of several hundred groups (taxa) of complex soft-bodied metazoa from the preceding Ediacaran period of the Proterozoic eon, known as the Avalon Explosion, have been identified since the systematic study of those forms started in the 1950s.

The Paleozoic features the evolution of the three most prominent animal phyla, arthropods, molluscs and chordates, the last of which includes fish, amphibians and the fully terrestrial amniotes (synapsids and sauropsids).

The Cenozoic begins with the extinction of all non-avian dinosaurs, pterosaurs and marine reptiles, and features the great diversification in birds and mammals.

Complex algae evolved, and the fauna was dominated by armoured arthropods (such as trilobites and radiodontids) and to a lesser extent shelled cephalopods (such as orthocones).

The Ordovician was a time in Earth's history in which many groups still prevalent today evolved or diversified, such as primitive nautiloids, vertebrates (then only jawless fish) and corals.

A group of freshwater green algae, the streptophytes, also survived being washed ashore and began to colonize the flood plains and riparian zones, giving rise to primitive land plants.

By the end of the Ordovician, Gondwana had moved from the equator to the South Pole, and Laurentia had collided with Baltica, closing the Iapetus Ocean.

Tetrapods also diversified during the Carboniferous as semiaquatic amphibians such as the temnospondyls, and one lineage developed extraembryonic membranes that allowed their eggs to survive outside of the water.

The Earth was relatively dry compared to the Carboniferous, with harsh seasons, as the climate of the interior of Pangaea was not moderated by large bodies of water.

Amniotes still flourished and diversified in the new dry climate, particularly synapsids such as Dimetrodon, Edaphosaurus and therapsids, which gave rise to the ancestors of modern mammals.

Many tetrapods during this epoch represented a disaster fauna, a group of survivor animals with low diversity and cosmopolitanism (wide geographic ranges).

[33] This changed late in the Carnian period with a 2 million years-long wet season which transformed the arid continental interior into lush alluvial forests.

[39] The climate was much more humid than during the Triassic, and as a result, the world was warm and partially tropical,[40][41] though possibly with short colder intervals.

[45][43] True mammals were present during the Jurassic[46] but remained small, with average body masses of less than 10 kilograms (22 lb) until the end of the Cretaceous.

[53] Dinosaurs continued to be abundant, with groups such as tyrannosauroids, avialans (birds), marginocephalians, and ornithopods seeing early glimpses of later success.

Whether or not pterosaurs went into a decline as birds radiated is debated; however, many families survived until the end of the Cretaceous, alongside new forms such as the gigantic Quetzalcoatlus.

[55][56] The Cenozoic featured the rise of mammals and birds as the dominant class of animals, as the end of the Age of Dinosaurs left significant open niches.

This epoch featured a general warming trend that peaked at the Paleocene-Eocene Thermal Maximum, and the earliest modern jungles expanded, eventually reaching the poles.

The late Eocene Epoch saw the rebirth of seasons, which caused the expansion of savanna-like areas with the earliest substantial grasslands.

The Isthmus of Panama formed, and animals migrated between North and South America, wreaking havoc on the local ecology.

[61][62] The Quaternary spans from 2.58 million years ago to present day, and is the shortest geological period in the Phanerozoic Eon.

This epoch was marked by a series of glacial periods (ice ages) as a result of the cooling trend that started in the mid-Eocene.

There were numerous separate glaciation periods marked by the advance of ice caps as far south as 40 degrees N latitude in mountainous areas.

Mammoths, giant ground sloths, dire wolves, sabre-toothed cats and archaic humans such as Homo erectus were common and widespread during the Pleistocene.

A more anatomically modern human, Homo sapiens, began migrating out of East Africa in at least two waves, the first being as early as 270,000 years ago.

After a supervolcano eruption in Sumatra 74,000 years ago caused a global population bottleneck of humans, a second wave of Homo sapiens migration successfully repopulated every continents except Antarctica.

All the continents were affected, but Africa was impacted to a lesser extent and retained many large animals such as elephants, rhinoceros and hippopotamus.

[67] The character of biodiversity growth in the Phanerozoic Eon can be similarly accounted for by a feedback between the diversity and community structure complexity.

It has been suggested that the similarity between the curves of biodiversity and human population probably comes from the fact that both are derived from the superposition on the hyperbolic trend of cyclical and random dynamics.