Paper short abstract:

It has been proposed that the human sickle cell mutation co-evolved with intensive agriculture in Upper West Africa. Re-analysis of sickle cell S patterns, and new findings relating to the fitness of the sickle cell C allele suggests a different interpretation. West African sickle cell gene frequencies, it will be argued, are better understood in relation to differential patterns of involvement in the intra-regional slave trade.

Paper long abstract:

In a classic paper, Livingstone proposed that two West African forest enclaves with low frquencies of sickle cell S allele could be explained - in a region of endemic malaria - by the late arrival of intensive rice agriculture. This was read by Durham as a clear instance of biological and cultural co-evolution. But a later paper by Livingstone extends analysis comparatively to West Africa and India, and argues for the diffusion of the S allele from a Middle Eastern centre of origin. Isolation rather than agriculture explains the Upper Western African low frequency regions. But isolation from what? This question can be answered, it is suggested, by paying attention to sickle cell C as well as sickle cell S. Sickle cell C is endemic to the Voltaic region of West Africa. Recent research establishes that sickle cell C has much higher fitness than sickle cell S. It is suggested that the puzzle of why C remains confined within a narrow region, while the less-fit S is widely distributed in West Africa, can be resolved by paying attention to the evolution of the intra-West African slave trade. It is consistent with this hypothesis that the low sickle cell S enclaves and the high sickle cell C exclave all have a cultural history of resistance to involvement in the slave trade. Cultural introversion, not diffusion of agriculture, blocked inward geneflow of the S allele and prevented outward geneflow of sickle cell C, it is suggested.