It turns out that it's the tiny mosquito and Timothy Winegard has written a book about it titled, The Mosquito: A Human History of Our Deadliest Predator, which Joseph Bottum reviews at the Washington Free Beacon.
Winegard, in Bottum's telling of it, writes that the mosquito transmits a catalog of deadly diseases including yellow fever, dengue fever, West Nile virus, and many others, but the worst is malaria:
Malaria has probably killed, down through the millennia, more human beings than any other illness. The Mosquito offers the high estimate of 52 billion people killed by mosquitoes, half the people in the history of the world. The global death toll from mosquito-borne diseases is still 830,000 people a year.In fact, the mosquito is at least partly the reason that Africans became the chief victims of the slave trade:
Somewhere around 6,000 B.C., certain African populations along the Niger River acquired a genetic mutation that caused red blood cells to have a crescent-like sickle shape—and people with the mutation survived because sickle cells provided relative immunity to the malaria caused by mosquito bites.This fact raises a question, however, that the review doesn't answer, although Winegard's book might: How did indigenous populations of indians in the Caribbean survive these diseases for millennia before Europeans and Africans ever came to their shores? Did they also build up some sort of immunity?
That population also had the potential to suffer oxygen deprivation, because the same sickle shape that prevented malarial infections from taking hold in red blood cells also weakened the ability of red blood cells to carry oxygen to internal organs.
The result could be bad, especially at higher altitudes, and The Mosquito devotes a powerful section to the story of Ryan Clark, the safety for the Pittsburgh Steelers who collapsed after a 2007 football game at Mile High Stadium in Denver and nearly died from oxygen-deprivation damage to his spleen and gallbladder, caused by the sickle-cell trait in his blood.
Still, a harsh Darwinian biology in early human history favored the mutation: Fewer people of childbearing age died from sickle-cell anemia than died from mosquito-borne malaria. In the Niger Delta, mosquitoes were the killers, not altitude.
One terrible result is that decreased mortality from mosquito-borne disease also made those Africans valuable as slaves in the lowlands of the Caribbean, Spanish Main, and southern regions of North America. Many still died, but the captive workforce generally survived better than others the brutal outdoor life in swampy, lowland areas.
American slavery, Timothy Winegard argues, happened in part because of mosquitoes.
Anyway, it's certainly interesting to reflect that such a tiny pestiferous insect as the mosquito (and the flea which transmitted the bubonic plague which devastated Europe and Asia in the 14th through 17th centuries) can have such an enormous impact on the flow of human history and the actions of man.