The Evolutionary Origin of Plasmodium falciparum

Among species of Plasmodium, P. falciparum is an enigma. It is unusually pathogenic and is not characterized by the relapsing episodes characteristic of other species. Its primary vector is a species of mosquito that is highly anthropophilic (Anopheles gambiae), and which shows evidence of rapid genetic change apparently coincident with the rise of human agriculture. Plasmodium falciparum has no alternative vertebrate reservoir hosts, and if the number of human infections drops below a threshold level, the species is unable to maintain itself. All of these observations suggested a novel hypothesis to Boyd (1949, cited in Waters et al. [ 1991]), who proposed that P. falciparum is a relatively new species that originated when the advent of agriculture brought humans and certain types of mosquitoes into contact on a regular basis.