There is evidence to support the changes in life forms over Earth's history (additions and extinctions) are often accompanied by changes in environmental conditions on Earth. Impacts, climate changes, and volcanism can produce sudden and rapid changes to environmental conditions. Many organisms cannot adapt to these sudden and rapid changes resulting in the extinction of those species. When these events are global in nature then mass extinctions occur.
Extinction
A species is extinct if no members of that species are living. Most organisms that have ever lived on Earth are now extinct.
Mass Extinctions Have Shaped Earth's History
- A mass extinction is usually defined as a loss of about three quarters of all species in existence across the entire Earth over a “short” geological period of time. Given the vast amount of time since life first evolved on the planet, “short” is defined as anything less than 2.8 million years.
- Of all species that have existed on Earth, 99.9 percent are now extinct. Many of them perished during five catastrophic events.
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Topic Tips
- A species is extinct if no members are living.
- Most organisms that have lived on Earth are now extinct.
- Mass extinctions have shaped Earth's geologic and biological history.