There is one major center of the S allele in Central Africa and one in West Africa, apparently arising from separate mutations. In India and Southeast Asia, Hemoglobin E and a thallasemia predominate. Click here to see World Wide Distribution of Hb S Click here to see African Distribution of Hb S Cultural Factors Spread of swidden agriculture into previous Tropical Rainforest regions 2-3,000 years ago. Clearing of the forest and high levels of rainfall erodes the soils which are lateritic and relatively impermeable, making for formation of pools. During the rainy season pools of fresh water, exposed to sunlight because of the forest clearing, form, which along with high humidity creates an ideal breeding habitat for A. gambiae.
Agricultural expansion supports more people, living in sedentary villages.