C.
Prehistoric peoples subsisted for the most part as hunters and gatherers; they were
dependent on what the ecosystem could provide naturally and probably did little or nothing,
apart from devising more effective methods of hunting, to modify that output. However, this
should not be taken as meaning they had no impact on their environment. Undoubtedly the
animals they shared the plains and forests with, including some very large and impressive
species, comprised an important part of the human diet. Assuming our ancestors were more
than just scavengers; that they actively sought and killed their prey, they would have had at
least as great an effect on the populations of those animals as did any other predator. In
actuality, the impact was probable much greater given the relatively advanced capacity of
humans to learn from mistakes; to modify their behavior based on observed results, and to
pass on the accumulating lessons from generation to generation.
Among paleontologists there is an important controversy centering on the question of
just how profound the effect of human activity was on prehistoric wildlife; in particular, how
much human hunters contributed to the sudden disappearance of so many species of large
animals from the North American continent between 10,000 and 12,000 years ago, at or near
the end of the Pleistocene Epoch. One school of thought attributes the extinctions to climatic
change too abrupt to allow adaptation by larger species. Another group, however, points to a
different possibility. They support the view that the prehistoric Homo sapiens drove many of
those species to extinction, mainly through over hunting. This has come to be known as the
“Pleistocene Overkill Hypothesis.”
This theory helps explain what, in its absence, could only be regarded as a remarkable
coincidence between the arrival of prehistoric peoples in the Western Hemisphere and the
disappearance of mammoths, giant ground sloths, the giant bison, and numerous other large
land mammals. Maybe, as the theory’s proponent’s claim, human-caused extinction is not
exclusively a modern phenomenon; perhaps the human species has been driving other
species to extinction since long before the dawn of history, and in complex ways that go
beyond the immediate result of killing individual animals for food.
The reduction of animal populations through direct predation might be the most
obvious mechanism of influence, but it may not have been the only one. Human
hunter-gatherers could have contributed to the Pleistocene extinctions in more indirect ways.
At least three other mechanisms have been suggested: direct competition, human-included
imbalances between competing species of game animals, and early agricultural practices.
Direct competition might have brought about the demise of large carnivores; animals
such as the saber-toothed cats. Though probably too formidable to have been the objects of
human hunting activity themselves, these animals would have preyed on many of the same
species humans hunted. Their strength and speed would have proved inadequate when faced
with human competition. Consequently, they would have suffered as the numbers of
potential prey shrank.
Human activity could have created competitive imbalances among prey species as well.
It is a generally recognized characteristic of predation that when non-human hunters, even
those that hunt cooperatively, like wolves for example, prey on animals larger than
themselves, they generally take higher proportions of each year’s population of young.
This is simply opportunism combined with risk reduction. The young are the most vulnerable
and the least likely to cause injury to the predator during the kill. In contrast, human hunters,
benefiting from their intelligence, ingenuity and adaptability, suffer fewer limitations and
risks. As a result, when hunting larger animals, they tend to take the various age-groups in
closer proportion to their actual occurrence.
If human hunters first competed with the larger predators and then eventually replaced
them, they may have allowed more of the young of certain prey species to survive with each
successive year, thus allowing the populations of these favored species to increase. As these
populations expanded, they would have competed with other game species for the same
inventory of environmental resources. Ultimately and somewhat ironically, this could have
led to the extinction of the species less often hunted by humans. 【題組】28. According to the Pleistocene Overkill Hypothesis, all of the following contributed to the
Pleistocene extinctions EXCEPT __________
(A) the hunting practices of humans.
(B) competition between species of prey animals.
(C) competition between predators.
(D) climate change.
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