阅读理解　　　 Many prehistoric people subsisted as hunters and gatherers. Undoubtedly, game 　　　 animals, including some very large species, provided major components of human diets. 　　　 An important controversy centering on the question of human effects on prehistoric wildlife 　　　 concerns the sudden disappearance of so many species of large animals at or near the end (5) of the Pleistocene epoch. Most paleontologists suspect that abrupt changes in climate led 　　　 to the mass extinctions. Others, however, have concluded that prehistoric people drove 　　　 many of those species to extinction through over-hunting. In their "Pleistocene overkill 　　　 hypothesis," they cite what seems to be a remarkable coincidence between the arrival of 　　　 prehistoric peoples in North and South America and the time during which mammoths, (10) giant ground sloths, the giant bison, and numerous other large mammals became extinct. 　　　 Perhaps the human species was driving others to extinction long before the dawn of history. 　　　 Hunter-gatherers may have contributed to Pleistocene extinctions in more indirect 　　　 ways. Besides over-hunting, at least three other kinds of effects have been suggested: 　　　 direct competition, imbalances between competing species of game animals, and early (15) agricultural practices. Direct competition may have brought about the demise of large 　　　 carnivores such as the saber-toothed cats. These animals simply may have been unable 　　　 to compete with the increasingly sophisticated hunting skills of Pleistocene people. 　　　 Human hunters could have caused imbalances among game animals, leading to the 　　　 extinctions of species less able to compete. When other predators such as the gray wolf (20) prey upon large mammals, they generally take high proportions of each year s crop of 　　　 young. Some human hunters, in contrast, tend to take the various age-groups of large animals 　　　 in proportion to their actual occurrence. If such hunters first competed with the larger 　　　 predators and then replaced them. they may have allowed more young to survive each year, 　　　 gradually increasing the populations of favored species As these populations expanded, (25) they in turn may have competed with other game species for the same environmental niche, 　　　 forcing the less hunted species into extinction. This theory, suggests that human hunters 　　　 played an indirect role in Pleistocene extinctions by hunting one species more than another.