Plagues and Peoples by William H. McNeill

reconstruct all the circumstances that may have allowed a new disease pattern to assert itself.
    There are, however, some exceptions. In western Africa, for instance, when agriculture began to spread into rain forest environments, slash-and-burn methods of cultivation clearly put new strains on older ecological balances. An unexpected result was to give malaria a new, epidemic intensity. What seems to have happened is this: clearings multiplied breeding places for a kind of mosquito,
Anopheles gambiae
, that feeds by preference on human blood. Indeed,
Anopheles gambine
can properly be described as a weed species that proliferates enormously in the gashes human agriculture creates in the African rain forest. With the advance of agriculture, it supplants other mosquito species accustomed to feeding on creatures other than man. As a result, the man-mosquito malarialcycle attains an unexampled intensity, affecting practically every human being who ventures into these forest clearings. 18
    African cultivators were nevertheless able to persist in their effort to tame the rain forest for agriculture; not, however, without genetic adaptation whereby the frequency of a gene that produces sickle-shaped red corpuscles in heterozygous individuals increased markedly. Such cells are less hospitable to the malarial plasmodium than normal red blood cells. Consequently, the debilitating effects of malarial infection are reduced in individuals who have this kind of red corpuscle.
    But the cost of such protection was very high. Individuals who inherit the sickling gene from both parents die young. Resulting heavy child mortality is further increased by the fact that those born entirely without the sickling gene are liable to lethal malarial infection. Indeed, in the most intensively malarial regions of West Africa, half the infants born among populations bearing the sickle-cell trait are biologically vulnerable. Since the agricultural penetration of the rain forest is still in process, the contemporary distribution of malaria,
Anopheles gambiae
and the sickling trait permit a plausible reconstruction of the unusually drastic consequences the alteration of older ecological patterns entailed, and continues to entail, in that environment. 19
    In central and eastern Africa, events in the nineteenth and twentieth centuries connected with ill-conceived efforts by European colonial administrators to alter traditional patterns of herding and cultivation also illustrate the unexpected side effects that sometimes arise from agricultural expansions into new regions. These efforts, in fact, precipitated veritable epidemics of sleeping sickness in parts of Uganda, the Belgian Congo, Tanganyika, Rhodesia, and Nigeria; and the end result, as colonial regimes came to an end, was a land more thickly infested with death-dealing tsetse flies than before government policy set out to utilize what looked like good agricultural land more effectively. 20
    Obviously human attempts to shorten the food chain within the toughest and most variegated of all natural ecosystemsof the earth, the tropical rain forests and adjacent savanna regions of Africa, are still imperfectly successful, and continue to involve exceptionally high costs in the form of exposure to disease. That, more than anything else, is why Africa remained backward in the development of civilization when compared to temperate lands (or tropical zones like those of the Americas), where prevailing ecosystems were less elaborated and correspondingly less inimical to simplification by human action.
    Ecosystems in the regions of the earth where early and historically important agricultural societies first developed were all intrinsically less resistant to human alteration than in tropical Africa. In temperate zones, fewer and less formidable parasites lay in wait to take advantage of any notable increase in human numbers. But because the major breakthrough and principal alterations of natural balances took place