AbstractThe globally invasive mosquito subspecies Aedes aegypti aegypti is a highly effective vector of human arboviruses because it specializes in biting humans and breeding in human habitats. Recent work suggests that specialization first arose as an adaptation to long, hot dry seasons in the West African Sahel, where Ae. aegypti is forced to rely on human-stored water for breeding. However, rainfall patterns in this region have changed dramatically over the past 10-20 thousand years, and we do not yet know exactly when specialization occurred. Here we use whole-genome cross-coalescent analysis to date the emergence of human specialist populations in the Sahel and thus further probe the climate hypothesis. Importantly, we take advantage of the known migration of human-specialist populations out of Africa during the Atlantic Slave Trade to calibrate the coalescent clock and thus obtain a more precise estimate of the older evolutionary event than would otherwise be possible. We find that human-specialist mosquitoes diverged rapidly from ecological generalists approximately 5,000 years ago, which corresponds to the end of the African Humid Period—a time when the Sahara dried and water stored by humans became a uniquely stable, aquatic niche in the Sahel. We also use population genomic analyses to date a previously observed influx of human-specialist alleles into major West African cities, where mosquitoes tend to be more attracted to humans than in nearby rural populations regardless of climate. In this case, the characteristic length of tracts of human-specialist ancestry present on a generalist genetic background in Kumasi, Ghana and Ouagadougou, Burkina Faso suggests the change in behavior occurred during rapid urbanization over the last 20-40 years. Taken together, we show that the timing and ecological context of two previously observed shifts towards human biting in Ae. aegypti differ; climate was likely the original driver, but urbanization has become increasingly important in recent decades. Understanding the changing relationship between mosquitoes and humans over time is critical for predicting and managing burdens of mosquito-borne disease.