Plagues and Peoples by William H. McNeill

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    Humanity’s place within the balance of nature in tropical regions differed fundamentally from what developed in temperate and Arctic climatic zones. As we have just seen, in sub-Saharan Africa humankind continued to confront biological checks that remained powerfully effective even after human hunting skills had upset the older balances of nature among large-bodied creatures. But when human communities learned to survive and flourish in temperate climes, they faced a simpler biological situation. In general, lower temperatures meant less propitious conditions for life. As a result, the forms of plants and animals adapted to temperate and northern climates were fewer in number than those that pullulated in tropical climes. Consequently a less richly articulated web of life greeted human hunters when first they burst upon the scene. Moreover, temperate ecological balances proved to be much more easily disrupted by human agency. The initial absence or near-absence of organisms capable of living parasitically on or inside human bodies was a passing phenomenon. In time, biologically and demographically significant diseasesdeveloped among human communities of temperate climes too, as we shall soon see. But the vulnerability of ecological balances to human manipulation remained a permanent feature of the extra-tropical scene.
    Thus humankind’s biological dominion in temperate climes assumed a different order of magnitude from the start. As a stranger and newcomer to temperate ecological systems, humanity was in a situation like rabbits met when introduced into Australia. Lacking both natural predators and natural parasites in the new environment, and finding, at least to begin with, abundant food, the rabbit population of Australia grew enormously and soon began to interfere with human efforts to raise sheep. Similar swarmings of imported forms of life—pigs, cattle, horses, rats, together with a broad spectrum of plants—occurred in the Americas when Europeans first arrived as well. But these initial runaway population explosions soon created their own correctives. 14
    In a long enough time perspective, perhaps the same will be true of humankind’s expansion into the diverse and novel ecological environments of the temperate world. But on the sort of time scale we are accustomed to, in which centuries and millennia rather than eons matter, ordinary biological adjustments among diverse species have not been enough to check the multiplication of humankind. The reason is that cultural rather than biological adaptations generated and sustained the entire adventure, so that, as one particular pattern of human exploitation of the environment began to encounter difficulties, thanks to exhaustion of one or another key resource, human ingenuity found new ways to live, tapping new resources, and thereby expanded our dominion over animate and inanimate nature, time and again.
    Riches in the form of woolly mammoths, giant sloths, and other large and inexperienced animals that awaited human slaughter did not endure for long. Indeed, one calculation suggests that skilled and wasteful human hunters took a mere thousand years to exterminate most large-bodied game in North and South America. According to this vision of theAmerican past, hunters gathered in large, organized groups along a moving frontier where large-bodied game could be found. Within a few years they so depleted the herds that they had to move on, ever southward, until most American species of big game had been completely destroyed. 15 Such a catastrophic pattern could of course only arise when skilled hunters collided with totally inexperienced game animals. In the Old World no such dramatic confrontation ever occurred. There, hunting skills were applied more gradually to the large-bodied herds of the North, if only because with each advance northward, the hunters had to adjust to a harsher climate and more arduous winters. In the Americas, on the contrary, the movement