" Furthermore, some researchers have suggested a strong association with the presence of the protective gene in populations who have historically farmed yams. To plant yams, farmers clear forests. Cleared forest means more standing water. More standing water means more mosquitoes. More mosquitoes means more malaria—so the idea goes. The emergence of the disease, and as a consequence the resistance gene, may well have been enabled, or at least nurtured, by yam farming. The persistence of sickle cell anemia is the cost of positive selection for resistance against the most destructive disease in our history. "

― Adam Rutherford , A Brief History of Everyone Who Ever Lived: The Stories in Our Genes