Timeline of the evolutionary history of life

The timeline of the evolutionary history of life represents the current scientific theory outlining the major events during the development of life on planet Earth. Dates in this article are consensus estimates based on scientific evidence, mainly fossils.

In biology, evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organization, from kingdoms to species, and individual organisms and molecules, such as DNA and proteins. The similarities between all present day organisms imply a common ancestor from which all known species, living and extinct, have diverged. More than 99 percent of all species that ever lived (over five billion) are estimated to be extinct. Estimates on the number of Earth's current species range from 10 million to 14 million, with about 1.2 million or 14% documented, the rest not yet described. However, a 2016 report estimates an additional 1 trillion microbial species, with only 0.001% described.

There has been controversy between more traditional views of steadily increasing biodiversity, and a newer view of cycles of annihilation and diversification, so that certain past times, such as the Cambrian explosion, experienced maximums of diversity followed by sharp winnowing.

Extinction

Species go extinct constantly as environments change, as organisms compete for environmental niches, and as genetic mutation leads to the rise of new species from older ones. At long irregular intervals, Earth's biosphere suffers a catastrophic die-off, a mass extinction, often comprising an accumulation of smaller extinction events over a relatively brief period.

The first known mass extinction was the Great Oxidation Event 2.4 billion years ago, which killed most of the planet's obligate anaerobes. Researchers have identified five other major extinction events in Earth's history, with estimated losses below:

• End Ordovician: 440 million years ago, 86% of all species lost, including graptolites
• Late Devonian: 375 million years ago, 75% of species lost, including most trilobites
• End Permian, The Great Dying: 251 million years ago, 96% of species lost, including tabulate corals, and most trees and synapsids
• End Triassic: 200 million years ago, 80% of species lost, including all conodonts
• End Cretaceous: 66 million years ago, 76% of species lost, including all ammonites, mosasaurs, plesiosaurs, pterosaurs, and nonavian dinosaurs

Smaller extinction events have occurred in the periods between, with some dividing geologic time periods and epochs. The Holocene extinction event is currently under way.

Factors in mass extinctions include continental drift, changes in atmospheric and marine chemistry, volcanism and other aspects of mountain formation, changes in glaciation, changes in sea level, and impact events.

Detailed timeline

In this timeline, Ma (for megaannum) means "million years ago," ka (for kiloannum) means "thousand years ago," and ya means "years ago."

Hadean Eon

4540 Ma – 4000 Ma

Archean Eon

4000 Ma – 2500 Ma

Proterozoic Eon

2500 Ma – 539 Ma. Contains the Palaeoproterozoic, Mesoproterozoic and Neoproterozoic eras.

Phanerozoic Eon

539 Ma – present

The Phanerozoic Eon (Greek: period of well-displayed life) marks the appearance in the fossil record of abundant, shell-forming and/or trace-making organisms. It is subdivided into three eras, the Paleozoic, Mesozoic and Cenozoic, with major mass extinctions at division points.

Palaeozoic Era

538.8 Ma – 251.9 Ma and contains the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods.

Mesozoic Era

From 251.9 Ma to 66 Ma and containing the Triassic, Jurassic and Cretaceous periods.

Cenozoic Era

66 Ma – present

See also

References

Bibliography

Further reading

• Dawkins, Richard (2004). The Ancestor's Tale: A Pilgrimage to the Dawn of Life. Boston: Houghton Mifflin Company. ISBN 978-0-618-00583-3. LCCN 2004059864. OCLC 56617123.

External links

• "Understanding Evolution: your one-stop resource for information on evolution". University of California, Berkeley. Retrieved 2015-03-18.
• "Life on Earth". Tree of Life Web Project. University of Arizona. January 1, 1997. Retrieved 2015-03-18. Explore complete phylogenetic tree interactively
• Brandt, Niel. "Evolutionary and Geological Timelines". TalkOrigins Archive. Houston, TX: The TalkOrigins Foundation, Inc. Retrieved 2015-03-18.
• "Palaeos: Life Through Deep Time". Palaeos. Retrieved 2015-03-18.
• Kyrk, John. "Evolution". Cell Biology Animation. Archived from the original (SWF) on 2012-10-22. Retrieved 2015-03-18. Interactive timeline from Big Bang to present
• "Plant Evolution". Plant and Animal Evolution. University of Waikato. Archived from the original on 2012-07-28. Retrieved 2015-03-18. Sequence of Plant Evolution
• "The History of Animal Evolution". Plant and Animal Evolution. University of Waikato. Archived from the original on 2016-06-27. Retrieved 2015-03-18. Sequence of Animal Evolution
• Yeo, Dannel; Drage, Thomas (2006). "History of Life on Earth". Archived from the original on 2015-03-15. Retrieved 2015-03-19.
• Exploring Time. The Science Channel. 2007. Retrieved 2015-03-19.
• Roberts, Ben. "Plant evolution timeline". University of Cambridge. Archived from the original on 2015-03-13. Retrieved 2015-03-19.
• Art of the Nature Timelines on Wikipedia