Plagues and Peoples

by William H. McNeill

  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 left by 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 language, systematic teaching of the arts of life to others became possible, while those arts themselves became susceptible to extraordinary elaboration, 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 drastic debility. 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 discovered new techniques allowing fresh exploitation of hitherto inaccessible resources, thereby renewing or intensifying damage to other forms of life. Looked at from the point of view of other organisms, humankind therefore resembles an acute epidemic disease, whose occasional lapses into less virulent forms of behavior have never yet sufficed to permit any really stable, chronic relationship to establish itself.

  That the first fully human hunters became the dominant predators in the savanna lands of Africa (and perhaps in similar regions of Asia) was only a modest presage of what was to come. No doubt, at the time, it was drastic enough, catapulting what had been one of the less conspicuous forms of primate life all the way to the apex of the food chain. As skillful and formidable hunters, humankind soon had little to fear from any animal rival. Our earliest fully human ancestors thus escaped one of the elemental checks upon population growth. Slaughter of man by man probably took over an equivalent demographic role, at least from the time when all suitable territory within the favorable savanna lands had been preempted by human hunting ba
nds and they began to rival one another. Other social controls on population growth may also have come into operation, e.g., abandonment of unwanted infants. At any rate, present-day hunters and gatherers have customary ways of keeping their numbers within the limits of available food supplies; and such customs are likely to be very old.9

  Within the African cradleland itself, human hunters established a relatively stable relation to the environment. Biggame hunting by humans started in Africa something like half a million years ago, although the full force of human bands, armed with weapons of stone and wood, may not have been felt much before 100,000 B.C. Despite occasional crises such as must have accompanied the extinction of a number of valuable species of game during ensuing millennia, human hunting bands continued to share the landscape with rich and varied populations of other forms of life.10 Indeed, even when agriculture later led to an enormous increase in the number of people and precipitated a drastic environmental change, many parts of Africa remained wild and uncultivated. Hunting bands, relegated in recent millennia to marginal regions un-suited for agriculture, continue to pursue traditional styles of life in parts of that continent even to this day.

  In other words, compensatory adjustments by other forms of life hemmed in human communities in such a tough and complex way that even after fully human skills had been achieved, the new efficiency attainable through cultural evolution was not sufficient to overpower and revolutionize the ecological system within which humanity had evolved. Probably the most significant factor in blunting the initial impact of humanity upon other forms of life was the peculiar richness and elaboration of African infestations and infections—an elaboration of parasitism that evolved along with humanity itself and tended to intensify as human numbers increased.11

  Many of the parasitic worms and protozoa that abound in Africa do not provoke immune reactions, i.e., the formation of antibodies in the bloodstream. This allows a sensitive and quite automatic ecological balance to assert itself, whereby if human numbers increase, the rate of infection also increases. Opportunities for transfer from one host to another multiply with increased human density, so that, if and when a critical threshold is surpassed, infection can suddenly develop into runaway hyperinfection. Such epidemic situations seriously interfere with normal activity. Chronic symptoms of lassitude, internal pains, or the like, may, if they become nearly universal in a human community, seriously hamper food-getting, or childbearing and rearing. This in turn can soon reduce a population until the local density sinks safely below the threshold necessary for hyperinfection. Then, as more individuals escape the debilitating effects of parasitic infection, human vigor can begin to pick up. Food-getting and other activities return to normal until some other form of infection asserts its power, or the population density again transgresses the point at which hyperinfection can recur.

  All such ecological disturbances of course have affected human prey as well as human parasites. Hunters who became too numerous would also find it harder and harder to discover suitable game. Undernourishment might therefore conspire with parasitic hyperinfestation to reduce human energies and child-rearing capacities until a more nearly stable balance again established itself.

  Moreover, all the interdependent forms of life simultaneously have responded to fluctuations in climate and other changes in the physical environment. Drought, grass fires, torrential rains, and other emergency conditions set limits for all forms of life; and these limits stood generally far below population possibilities at other, more favorable times. The ecological system, in other words, maintained a loose, fluctuating balance which, despite local and temporary departures from the norm, effectively resisted radical alteration. The establishment of human hunters at the top of the food chain, preying upon others, but too formidable to be preyed upon by large-bodied rivals, did not, in and of itself, do much to alter these age-old ecological relations. In triumphantly claiming a new niche, humanity did not, therefore, transform the system as a whole.

  The interactions that produced and sustained such fluctuating balances were (and remain) extremely complex. Despite several generations of scientific observation, the interrelationships of disease, food supply, human densities, habit patterns, not to mention insect vectors of disease and the number and distribution of alternate hosts for disease-causing organisms, are not fully understood in Africa, or anywhere else for that matter. Moreover, conditions in contemporary Africa do not exactly match the patterns of infection and infestation that must have existed when all men were hunters and before human agriculture intruded upon older natural balances.

  Yet the multiplicity of life forms in tropical Africa is an undeniable fact; and the toughness with which the biological balances of that continent have resisted efforts to import methods of agricultural production that worked in temperate zones is also a matter of record. In fact, until relatively recent times (say five thousand years ago), human communities in Africa played a comparatively modest role amid the abundance of other life forms. Humans were the chief predators, to be sure, but remained relatively rare in the balance of nature, like lions and other large-bodied beasts of prey with which human hunters had to compete for their food.

  Anything else would, in fact, be surprising. If, as seems probable, humankind originated in Africa, there was time while humanoids slowly evolved into fully human populations, for surrounding life forms to adjust themselves to the risks and possibilities that human activities presented. Conversely, the extraordinary variety of human parasites that exist in Africa suggests that Africa was the principal cradle for humankind, for nowhere else did the adjustment between human and nonhuman forms of life achieve anything like the same biological elaboration.

  What about the rest of the world, beyond the African rain forest and grasslands? Formidable humanoid hunters certainly existed in far scattered regions of the Old World, beginning perhaps as much as one and a half million years ago. Finds in China, Java, Germany attest substantial skeletal differentiation; but discoveries are too few to allow any definite connections to be made with the more abundant human and prehuman remains that have been found in Africa. Parallel evolution from some sort of ancestral primate stock in different regions of southern and southeastern Asia is possible, since the rewards for an enlarged brain, erect posture and tool-using hands were substantial even in environments less well endowed with big-game animals than the African savanna lands.

  Arguing from insufficient evidence can be misleading. Archaeological study of the vast areas concerned is still sketchy, and discovery of even a single new site, like Olduvai Gorge in Africa, might alter the over-all picture profoundly. Nevertheless, what little is known seems to indicate that prehuman and proto-human populations in Eurasia lagged behind the African efflorescence of humanoid populations. This remained the situation until the rather abrupt appearance of fully modern types of human beings sharply altered preexisting ecological balances over the entire earth beginning sometime between 100,000 and 50,000 years ago.

  Evidence remains too scanty to pinpoint exactly where Homo sapiens first evolved. Bone fragments, whose classification as exemplifying Homo sapiens is debatable, date back in East Africa about 100,000 years. Elsewhere traces of fully modern types of humankind remain subsequent to 50,000 B.C. Moreover, as modern forms of Homo sapiens appeared, pre-existing populations, like the well-known Neanderthal of western Europe, disappeared, leaving little or no trace.12

  Within Africa, the appearance of this eminently successful type of human population did not cause alterations as drastic as were to occur elsewhere. All the same, the extinction of a number of large-game animals and of rival humanoid forms, if correctly attributable to Homo sapiens, demonstrates what human hunters were capable of. Far more spectacular results occurred when humanity learned how to keep warm in cold climates by domesticating fire and putting other creatures’ skins and furs on their own backs.

  The grand invention of clothes allowed bands of hunters to attack animal populations of northern grassland
s and forests. The consequences were analogous to what happened when our primal ancestors first came down from the trees. That is, a new, or rather a series of new, ecological niches opened before the newcomers; and as they learned to exploit the new food possibilities that their skill opened up to them, a very rapid, global transformation of ecological relationships ensued. Between about 40,000 B.C. and 10,000 B.C., human hunting groups occupied all the main land masses of the earth, except for Antarctica. Hunting bands entered Australia between 40,000 and 30,000 years ago; some 5,000 to 15,000 years later other bands managed to cross the Bering Strait from Asia and entered the Americas. Within a few thousand years, human populations expanded across the entire climatic spectrum of North and South America, reaching Tierra del Fuego about 8000 B.C.

  Never before had a dominant, large-bodied species been able to spread all around the globe. Humans could accomplish this feat because they learned how to create micro- environments suitable to the survival of a tropical creature under widely varying conditions. Invention of different sorts of clothing and housing did the trick, insulating the human body from extremes of climate and assuring survival despite freezing temperatures. In other words, cultural adaptation and invention diminished the need for biological adjustment to diverse environments, thereby introducing a fundamentally disruptive, persistently changeable factor into ecological balances throughout the land masses of the entire earth.