The species alive today are only a tiny fraction of all those that have ever lived. The vast majority of species in the history of the planet have become extinct. The fossil record indicates that the average "lifespan" of a species, from its origin to its extinction, is between 1 and 5 million years. The process of fossilization itself, however, suggests that this is probably a substantial overestimate, and the lifespan of many species is much shorter, on the order of thousands of years. This means that in the grand sweep of the history of life, extinction is occurring all the time. Extinctions affecting one or a few species and occurring in one locality rather than globally belong to a pattern known as background extinction.
The fossil record, however, describes sudden, global extinctions that affect many species. These dramatic events are known as mass extinctions. Some populations seem more vulnerable than others: tropical forms of life have been drastically affected, while land plants tend to be highly resistant to mass extinctions. Ammonites and trilobites were vulnerable, but snails seem to pass through mass extinction events relatively unscathed.
There have been at least five mass extinctions, and maybe many more, but the fossil record is unclear. The two biggest extinctions were at the end of the Permian Period, about 250 million years ago, and at the end of the Cretaceous, some 65 million years ago. The Permian extinction saw the loss of 80 to 96 percent of all marine species. In the Cretaceous event, perhaps 60 to 75 percent of marine species disappeared.
What caused these immense die-offs? No one process has been implicated, but there are many suspects, including the most dramatic: asteroids crashing into Earth. This was first suggested in 1980 when a layer of iridium was found in rocks laid down at the Cretaceous/Tertiary, or K/T, boundary. Greeted with skepticism at first, the impact hypothesis has gained strength as more evidence supporting it has emerged. A crater of the right size and age, known as the Chixulub crater, was found off the coast of Yucatan, along with rocks known as shocked quartzes and tektites, which suggest a high-velocity collision. An impact big enough to create the Chixulub would certainly have had dramatic effects, throwing up global dust clouds, perhaps enough to render the planet dark and sunless for some time. Other possible consequences have also been suggested, including acid rain, massive volcanic eruptions, and climatic heating.
It is now widely accepted that an impact occurred at the time of the K/T extinction, but whether it was the primary cause of the extinction event is still the subject of debate. Some fossil evidence suggests that many organisms, including dinosaurs, were declining in numbers well before the K/T event. Other factors, less dramatic and sudden than an asteroid impact but potentially just as devastating in their effects, were operating, too. Sea levels were dropping, causing changes to climate and vegetation that would severely affect large herbivores, including many dinosaurs.
Changes in sea level, in fact, accompanied each of the five mass extinctions, while only the K/T event has been persuasively linked to an asteroid impact. Among the causes that have been suggested for the other mass extinctions are volcanism, climate changes (mostly cooling), changes in the shorelines of continents as crustal plates shifted and coalesced, and changes in the circulation and chemistry of the oceans. The question of mass extinctions and their causes is an area of vigorous research.