SPOTLIGHT ON MASS EXTINCTIONS
Mass extinctions are episodes in the history of life on Earth during which unusually large numbers of species die off. They stand in contrast to the background rate of extinction, which occurs even when the diversity of life is increasing.
Scientists recognize five major mass extinctions in the Earth’s history. The extinctions are measured in terms of large groups of related species, called families.
The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors.
The sixth mass extinction appears to be in progress now, and the main cause is environmental change brought about by human activities.
| Millions of Years Ago |
Name of Extinction Episode |
Percent of Families That Died Out |
Note |
| 440 |
Late Ordovician |
20 - 50 % |
Tropical areas are hit hardest. No terrestrial vertebrates have evolved yet. |
360 |
Late Devonian |
20 - 30 % |
Many fish and marine invertebrates die out. |
250 |
End of Permian |
50 % |
Greatest mass extinction ever. |
| 213 |
Late Triassic |
20 - 35 % |
Dinosaurs and mammals had just recently evolved; both live through this extinction. |
| 66 |
Late Cretaceous |
15 % |
Dinosaurs become extinct, possibly caused by a metorite colliding with the Earth. Mammals and fishes begin to proliferate. 65 million of years after this mass extinction, early humans evolve.
|
| NOW |
Holocene |
? |
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