This chapter discusses extinction—its causes, the challenges it poses, and the implications it leaves for studies into the history of life. Any history of life, after all, inevitably stresses first appearances, evolutionary relationships, and continuity. But the fossil record bears widespread evidence, not only of appearances, but also of disappearances; not only of continuity but also of a particular kind of discontinuity: extinction. Extinction represents the disappearance of a species or subspecies, and its fossil record helps us to pinpoint the end point of its existence. Some of its causes can be traced back to human actions—indeed, biologists have been concerned that human activity accelerates the rate of extinction. Yet still other forms of extinction have occurred without human intervention; particularly such things as changing ecological conditions, dwindling food supply, climate change, and competition from other species.
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