Why the Pleistocene Extinctions Occurred

The Cenozoic era is characterized by the diversification of mammal species, but it has also been punctuated by mass extinctions. Its best-known extinctions are probably those that occurred at the end of the Pleistocene epoch. The Pleistocene extinctions appear dramatic because of the extinction of the megafauna (large animals).

There is much debate about the cause of the Pleistocene extinctions. The main extinctions occurred at the end of the last glacial period, between 13,000 and 11,500 years ago. Surprisingly enough, animals appear to be more vulnerable to extinction when the climate changes from glacial to interglacial (a period with warmer temperatures and less extensive glaciers) rather than the other way around, probably because warming occurs faster. Thus climatic change would be an obvious explanation. However, many scientists have noted that it is only the last glacial period—rather than any of the previous ones—that brought extinctions of such magnitude. This observation suggests that part, if not all, of the blame for megafauna extinctions should be placed on the spread of modern humans and modern hunting techniques, which were concurrent with that time period.

Many scientists today insist that human activity, rather than climatic change, must be the root cause of the Pleistocene extinctions. This is the overkill hypothesis, and the survival of mammoths until only a few thousand years ago on human-free Wrangel Island, off the coast of Siberia, appears to support this view. However, Steve Wroe and Colleagues point out that our actual knowledge about the effect of humans on mammal extinctions is drawn from historical examples of island animals. Hunting and/or habitat disturbance may have been the cause of the comparatively recent (within the past few thousand years) extinctions of the giant lemurs (monkey-like animals) on the island of Madagascar or the moas (giant herbivorous birds) on the islands of New Zealand, but there are problems with extending such scenarios to larger landmasses, such as North America or Australia.

Other researchers argue for climate change being the key factor in megafaunal extinctions. The extinctions in North America did not follow a north-to-south pattern, as would be expected with the invasion of humans from Beringia, an ancient land bridge that connected areas today known as Siberia and Alaska. Furthermore, horses in Alaska underwent a rapid decrease in body size shortly before becoming extinct—and before human arrival—and that pattern is consistent with the hypothesis of climate change being the extinction agent. Human hunters and climate change are not mutually exclusive hypotheses, and hunting pressure may have provided the final factor that drove already unstable populations to extinction.

About 30 percent of mammals became extinct at the end of the Pleistocene. That is approximately the magnitude of the other two major Cenozoic extinctions, which occurred during the late Eocene and late Miocene epochs. However, the preceding two extinctions differ in several critical ways from that of the Pleistocene. The late Eocene extinctions were associated with the dramatic fall in higher-latitude temperatures. Higher-latitude forests turned to temperate woodlands, with the accompanying disappearance of mammals adapted to these tropical-like forests. This included not only a diversity of archaic mammals but also some early, more modern types, such as higher-latitude primates and early horses.

The late Miocene extinctions were associated again not only with falling higher-latitude temperatures but also with global drying. The major extinctions were of browsing mammals (animals that feed on twigs and leaves rather than grass), including a variety of large browsing horses, which suffered habitat loss as the savanna woodlands turned into open grasslands and prairie. North America was especially hard hit by the climatic events of the late Miocene because of its relatively high latitudinal position and the fact that animals could not migrate to more tropical areas in South America before the formation of the Isthmus of Panama, which now connects North and South America, in the Pliocene.

Most significantly for the overkill hypothesis, mammals of all body sizes (not just large ones) were affected in both the Eocene and Miocene. Other organisms, both terrestrial and marine, also experienced profound extinctions. The late Pleistocene extinction affected primarily large mammals and birds, which are the species most likely to be human hunters' prey or competitors.