Plagues and Peoples (6 page)

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 grasslands 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.

Decisive as cultural adaptation to differing natural environments was for humankind’s extraordinary expansion between 40,000 and 10,000
B.C
., there was another factor of considerable importance. In leaving tropical environments behind, our ancestors also escaped many of the parasites and disease organisms to which their predecessors and tropical contemporaries were accustomed. Health and vigor improved accordingly, and multiplication of human numbers assumed a hitherto unparalleled scale.
¹³

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 diseases
developed 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 the
American 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 was from north to south, from severity to mildness. The result was a far more sudden and widespread extermination of large-bodied game than occurred in the Old World.

Subsequent discoveries of new techniques allowed people to re-enact this frontier phenomenon of easy exploitation and rapid depletion of resources over and over again. Current oil shortages outside the Middle East are only the most recent example of humanity’s spendthrift ways. Yet as a result of the Stone Age occupancy of the temperate and sub-Arctic parts of the globe, humankind also entered into a far more enduring new pattern of co-existence with other forms of life—a pattern that was to play an important role in later history. Human distribution across diverse climatic zones created what may be called a parasitic gradient among the different communities that resulted. The general thinning out of the variety of life forms that took place as climates became colder and/or drier implies, after all, a diminution in the number and variety of parasitic organisms capable of afflicting human beings. Conditions for successful transfer from host to host became more difficult as temperatures (and humidity) dropped and as the seasons of warmth and sunlight shortened. The effect was to create a gradient of infection and infestation such that populations from warmer, wetter climes could travel to cooler
and/or drier regions with little likelihood of encountering unfamiliar parasites, whereas infections and infestations lurking in southern and warmer or wetter lands constituted a standing threat to intruders from the cooler North or drier desert.

The gradient may be described conversely as follows: the farther human populations penetrated into cold and/or dry climates, the more directly their survival depended on their ecological relations with large-bodied plants and animals. Balances with minute parasitic organisms, so important in the tropics, became comparatively insignificant.

This difference has an important corollary. Nearly all microparasites are too small to be seen with the unaided human eye, and this meant that until the invention of the microscope and other elaborate aids to human powers of observation, no one was able to understand or do much to control encounters with such organisms. Despite the intelligence which served humankind so well in dealing with things it could see and experiment with, relations with microparasites remained until the nineteenth century largely biological, that is, beyond or beneath human capacity for conscious control.

In places where microparasites were less pervasive and significant, however, intelligence could play freely upon the parameters of human life that mattered most. As long as men and women could see food and foe, they could invent new ways to cope with both; and by so doing eventually became no longer the rare predator that a hunting mode of life alone allowed. Instead human numbers proliferated into millions in landscapes where only a few thousand hunters had been able to exist. Escape from the tropical cradleland, therefore, had far-reaching implications for humanity’s subsequent role within the balance of nature, giving a much wider scope to cultural invention than had been attainable within the tighter web of life from which naked humanity had originally emerged.