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infections and infestations probably flared up into fatal complications whenever serious injury or some other severe stress (famine, for instance) upset the host’s internal physiological balances. In the absence of some such serious disturbance, however, a tolerable state of health can be supposed, such as exists among wild primates of the forest today.
    As long as the biological evolution of humankind’s ancestors kept pace with the evolution of their parasites, predators, and prey, no very important alteration in this sort of tightly woven web of life could occur. Evolutionary development, proceeding through genetic variation and selection, was so slow that any change in one partner was compensated for by changes in the other partner’s respective genetic and/or behavioral patternings. When humankind began to respond to another sort of evolution, however, elaborating learned behavior into cultural traditions and systems of symbolic meaning, these age-old biological balances began to confront new sorts of disturbances. Cultural evolution began to put unprecedented strains upon older patterns of biological evolution. Newly acquired skills made humanity increasingly capable of transforming the balance of nature in unforeseen and far-reaching ways. Accordingly, the disease liability of emerging humankind also began to change dramatically.
    The first discernible upheaval of this kind resulted from the development of skills and weapons suitable for killing the sorts of large-bodied herbivores that abounded on the grasslands of the African savanna (and perhaps in similar landscapes in Asia). No definite date can be offered for this transition: it may have begun as much as four million years ago.
    The first pre-human primates who came down from the trees and started to prey upon the antelope and related species probably could catch only the weak or very young. They may have had to compete with hyenas and vultures for carrion leftby more efficient predators like lions. Among such pre-human primate populations hovering around the fringes of a concentrated food resource like that offered today by the vast herds of herbivores on the African savanna, any genetic change that improved hunting efficiency was sure to pay off handsomely. 6 Enormous reward awaited any group possessing muscular and mental skills that permitted more effective cooperation in the hunt. Emergent humanity reaped these rewards by developing patterns of communication that allowed increasingly effective mutual support in moments of crisis, and by elaborating tools and weapons to augment an unimpressive musculature and puny teeth and claws. In such circumstances, new traits that paid off cumulated rapidly—rapidly, that is, by the spacious standards of biological evolution. Any fresh variation, permitting more of what had begun to work well already, enlarged the food supply and increased chances for survival.
    This sort of evolutionary spurt is known among biologists as “orthogenic,” and is often associated with a transition to a new ecological niche. 7 No one can expect to disentangle all the genetic changes that this process provoked among the pre-human populations. When variations could be so extravagantly successful, however, displacement of one humanoid population by another even more effective group of hunters must have occurred frequently. Survival was more likely for the more formidable in battle as well as for the more efficient in the hunt.
    A major landmark in the resulting evolutionary development was the elaboration of language. Genetic changes governing the formation of brain, tongue, and throat were necessary to open the way for articulated language; language in turn allowed vastly improved social co-ordination. Talking things over and thereby enacting and re-enacting roles allowed human beings to practice and perfect skills ahead of time, so as to achieve otherwise unattainable precision in hunting and in other co-operative activities. With