EXTINCTION

in biology, the dying out or termination of a race or species. Extinction occurs when a species can no longer reproduce at replacement levels. Most extinctions are thought to have resulted from environmental changes that affected the species in either of two ways. The doomed species might not have been able to adapt to the changed environment and thus perished without descendants; or it may have adapted but, in the process, may have evolved into a distinctly new species. The effect of humans on the environment, through hunting, collecting, and habitat destruction, has become a significant factor in plant and animal extinctions. Although extinction is an ongoing feature of the Earth's flora and fauna (the vast majority of species ever to have lived are extinct), the fossil record reveals the occurrence of a number of mass extinctions, each involving the demise of vast numbers of species. One such mass extinction occurred at the end of the Cretaceous period, some 66,000,000 years ago, when the dinosaurs and much of the marine life of the day perished. Evidence points to the impact of an asteroid hitting the Earth as the cause of the Cretaceous extinctions. It is suspected that catastrophic eventssuch as an asteroid impactmay have triggered other mass extinctions as well. In fact, mass extinctions appear to have taken place approximately every 26,000,000 years, which has led some paleontologists to propose that a cyclical cosmic event causes these periodic die-offs.