Now let’s look at a human parasite, Plasmodium falciparum, the single-celled eukaryote that causes malaria. It had been suggested that P. falciparum’s closest relative is another Plasmodium species, P. reichenowi, found in chimpanzees, our closest living relative. Were P. falciparum and P. reichenowi the products of co-speciation? When the ancestral population split millions of years ago to give rise to human and chimpanzee lineages, that population’s parasitic Plasmodium population could also have been split, ultimately yielding P. falciparum and P. reichenowi.
Recent studies, however, have disproved this hypothesis. We now know that P. falciparum was introduced to humans relatively recently from gorillas. Why doesn’t the evolutionary history of Plasmodium follow the classical host–parasite co-speciation pattern? We know that this history is complex and is still being unraveled. However, we also know that the mosquito-borne phase of its life cycle facilitates transfer to new hosts. Malaria parasites are thus not as inextricably tied to their hosts as the pocket gopher lice in Fig. 22.10 and so their evolutionary history is not as parallel to their hosts.