The timeline of life shows the best scientific idea about the main events that happened as life developed on Earth. The dates in this article are agreed-upon estimates based on scientific evidence, mostly from fossils.
In biology, evolution means any change in the traits passed down through generations in living groups. These changes lead to variety in all levels of life, from large groups like kingdoms to smaller ones like species, individual organisms, and even molecules like DNA and proteins. The similarities among all living things today suggest they all came from a single ancestor. More than 99% of all species that ever lived (over five billion) are believed to be extinct. Scientists estimate there are between 10 million and 14 million species on Earth today, but only about 1.2 million, or 14%, have been officially recorded. The rest are still unknown. A 2016 report also suggests there may be 1 trillion types of tiny microbes, but only 0.001% have been studied.
Some scientists believe biodiversity has always increased over time, while others think it has gone through cycles of growth and loss. For example, during the Cambrian explosion, many new life forms appeared quickly, but later many were lost.
Extinction
Species become extinct regularly as environments change, as living things compete for resources, and as genetic changes create new species from older ones. Occasionally, Earth's biosphere experiences a mass extinction, a time when many species disappear quickly, often due to several smaller extinction events happening close together.
The first known mass extinction was the Great Oxidation Event 2.4 billion years ago, which caused the death of most organisms that could not survive in oxygen-rich environments. Scientists have identified five other major extinction events in Earth's history, with the following estimated losses:
- End Ordovician: 440 million years ago, 86% of species were lost, including most graptolites.
- Late Devonian: 375 million years ago, 75% of species were lost, including most trilobites.
- End Permian, The Great Dying: 251 million years ago, 96% of species were lost, including tabulate corals, most trees, and synapsids.
- End Triassic: 200 million years ago, 80% of species were lost, including all conodonts.
- End Cretaceous: 66 million years ago, 76% of species were lost, including all ammonites, mosasaurs, plesiosaurs, pterosaurs, and non-avian dinosaurs.
Smaller extinction events have occurred between these major events, sometimes marking the boundaries of geological time periods. The Holocene extinction event is currently happening.
Factors that contribute to mass extinctions include continental drift, changes in air and ocean chemistry, volcanic activity and mountain-building processes, changes in ice coverage, changes in sea level, and impact events.
Detailed timeline
In this timeline, Ma stands for million years ago, ka stands for thousand years ago, and ya means years ago.
2500 Ma – 539 Ma. This time includes the Palaeoproterozoic, Mesoproterozoic, and Neoproterozoic eras.
The Phanerozoic Eon (Greek: period of well-displayed life) is a time when many types of living things that left behind shells or traces appeared in the fossil record. It is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. Major mass extinctions happened at the times when these eras began.
538.8 Ma – 251.9 Ma. This time includes the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
From 251.9 Ma to 66 Ma. This time includes the Triassic, Jurassic, and Cretaceous periods.