By the seventh century, the incidence of the sickle-cell gene would have built up to levels sufficient to provide the population [in the Guinea zone, West Africa] with considerable protection against malaria; initially the introduction of agricultural methods and ways of life would have increased the incidence of malaria. Mobile hunting bands of about twenty-five people make a much less fertile breeding ground for the establishment of any endemic disease than larger aggregates of settled agricultural populations.
In addition, in the case of falciparum malaria, the conditions produced by clearing areas in the forest for agriculture favoured the disease. This is because the mosquito Anopheles gambiae, the principal vector oí falciparum malaria, has few suitable natural breeding grounds in the primeval forest, since pools tend not to form on the leaf-covered humus of the forest floor, and where they do are too dark for the habits oí Anopheles gambiae, which likes, to lay its eggs in sunny or well-lit pools. On the other hand open waterholes and domestic rubbish (such as discarded calabashes) of an agricultural village provide ideal breeding grounds for mosquitoes; and the roofs and eaves of thatched huts furnish dark lurking grounds for them during the daytime.
We do not know precisely when or how the sickle-cell gene mutation took place. If a child receives it from both parents he will die of sickle-cell anaemia before reaching reproductive age; if he receives it from neither parent he runs a high risk of dying of malaria before reaching maturity; if, however, he receives it from only one parent, he will not die of sickle-cell anaemia and he is provided with a large measure of protection against malaria.
Where the incidence of sickle-cell is high in the population, this is always in areas of endemic malaria; it has been able to grow to these high levels in spite of its lethal effect when received from both parents because of the protection it affords against malaria. It has been calculated that it must have taken at least 1500 years to build up to the levels recorded in north-eastern Nigeria; the build-up is probably slower in less humid areas. There tends to be a gradient from south to north in West Africa, with the highest incidence near the coast and a northward diminution.
Thurstan Shaw (U.K.); author of numerous works on the prehistory of West Africa; Professor of Archaeology; Vice-President of the Pan-African Congress on Prehistory; President of the Prehistoric Society.
Source: General History of Africa Vol.III. [Editor: M. El Fasi, Assistant Editor: I. Hrbek] Africa from the Seventh to the Eleventh Century. Chapter 16. The Guinea zone: general