Malaria

Among the human malarias, P. falciparum is the most virulent and associated with the largest number of deaths. Chimpanzees are hosts to a closely related pathogen (P. reichenowii) and can be experimentally infected with human P. falciparum without exhibiting any symptoms of severe disease. Conversely, an old study indicated that humans could not be infected by P. reichenowii. One study comparing the merozoite stage EBA-175 receptors of these two pathogens and their respective binding preferences to host target molecules on red blood cells indicates that human P. falciparum evolved to exploit the uniquely human sialic acid content of target attachment molecules - possibly explaining the difference in virulence of P. falciparum in humans and apes. More recent studies show that all extant P. falciparum strains are likely derived from a single "great ape" to human transfer event.  A likely explanation is that the loss of expression of the Neu5Gc sialic acid in hominin ancestors made them partially or completely resistant to the extant P. reichenowii, and that one strain later evolved to bind the Neu5Ac-rich erythrocytes of humans, eventually becoming P. falciparum