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language, systematic teaching of the arts of life to others became possible, while those arts themselves became susceptible to extraordinaryelaboration, since words could be used to classify things, order them, and define appropriate reactions to all sorts of circumstances. Language, in short, made hunters fully human for the first time, inaugurating a new dimension of social-cultural evolution which soon put vast and hitherto unmatched strains upon the ecological balance within which humanity arose.
    What of disease amid this relatively rapid evolution? Clearly any change of habitat, such as that involved in coming down from the trees to walk and run in open grasslands, implies a substantial alteration in the sort of infections one is likely to encounter. To be sure, some infections presumably remained almost unaffected. This would be the case for those transmitted by close bodily contact, as is true, for instance, of most of the intestinal bacteria. Other parasites, such as those requiring moist conditions for successful transfer from one host to another, must have become less abundant, finding conditions on the savanna far less propitious. As the rain forest types of infestation and infection thinned out, however, new parasites, and fresh diseases, especially those contracted from association with the herds of the savanna, must have begun to affect the bodies of burgeoning humankind.
    We cannot say just what these infestations and infections may have been. Various kinds of worms, for instance, that infest herbivores today may transfer their parasitism to humans when, in eating meat, we inadvertently consume the eggs or some encysted form of the parasite. This must have happened anciently, too.
    A more important exposure was to the trypanosome that today causes sleeping sickness in many parts of Africa. This organism dwells as a “normal” parasite in many species of antelope and is transferred from one host to another by the tsetse fly. It produces no noticeable signs of sickness in the fly or in the host animal, and is, therefore, an example of a stable, well-adjusted, and presumably very ancient parasitism. Injected into a human body, this same organism provokes drasticdebility. Indeed, one species of this trypanosome is usually lethal to its human host within a few weeks.
    It is, in fact, mainly because sleeping sickness was and remains so devastating to human populations that the ungulate herds of the African savanna have survived to the present. Without modern prophylaxis, humans simply cannot live in regions where the tsetse fly abounds. Hence, until very recently, the vast herds of these regions remained the prey of lions and of other well-adapted predators, but were spared more than casual contact with that far more destructive newcomer among the beasts of prey: humankind. If, as seems almost certain, the trypanosome of sleeping sickness existed among the ungulate herds before our ancestors left their trees, the presence of this parasite must have set sharp limits upon the zones within which earliest humankind was able to take advantage of the abundance of game available on African grasslands. Conversely, within the tsetse’s range, something resembling a pre-human ecological balance survives to the present. 8
    Incidentally, it is not absurd to class the ecological role of humankind in its relationship to other life forms as a disease. Ever since language allowed human cultural evolution to impinge upon age-old processes of biological evolution, humankind has been in a position to upset older balances of nature in quite the same fashion that disease upsets the natural balance within a host’s body. Time and again, a temporary approach to stabilization of new relationships occurred as natural limits to the ravages of humankind upon other life forms manifested themselves. Yet sooner or later, and always within a span of time that remained minuscule in comparison with the standards of biological evolution, humanity