Extinction is a fact of nature. All of the species of animal that live on earth will, at some point, become extinct. Some, through the process of evolution, may give rise to descendents—new forms to exploit different niches—while others may disappear, leaving no line of descent.
Ever since animals made their first appearance in the story of life on earth, billions of species have disappeared. Some of these have fallen to some huge, cataclysmic events, of which there have been many in the last few hundred million years, while others have been outcompeted by other organisms or were unable to react to small changes in their environment. In 1982, scientists proposed that in the last 500 million years—a window of time in which animals have evolved to exploit the vast majority of habitats on earth—there have been around six mass extinction events. It's fascinating to think how life on earth has been pushed to the edge on a number of occasions, only to spring back with renewed vigor when conditions have become more favorable.
These mass extinctions happened such a long time ago that the evidence for what caused them is not immediately obvious, and for some of them, the evidence may have been worn away completely. Scientists have attributed these extinctions to meteorite impacts, massive volcanic eruptions, and movement of the solar system through a galactic gas cloud, to name but a few explanations. Regardless of the cause, some of these events saw the disappearance of huge numbers of species. The largest of these mass extinctions, which occurred 250 million years ago, resulted in the disappearance of 96 percent of all marine life and around 70 percent of all terrestrial life. During this time, animal life must have been pushed to the very edge, reduced to a shadow of its former glory—perhaps a few species clinging on to life in what had become a very harsh world indeed. We may only be able to guess at the causes of these extinctions, but the fossil record gives us a glimpse of these times. To those who can decipher it, the fossil record from around these periods shows an unprecedented die-off of species, with many disappearing completely. The fossil record is a story in stone, shell, and bone of life on earth. It enables us to picture the lives of long-dead creatures and shows how cataclysmic events have ravaged life on earth on numerous occasions.
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