aboriginal connectedness between us and other kinds of host.
Malaria exemplifies this. Within the Plasmodium family tree, as revealed by molecular phylogenetics over the last two decades, the four human-afflicting kinds don’t cluster on a single branch. They are each more closely related to other kinds of Plasmodium, infecting nonhuman hosts, than to one another. In the lingo of taxonomists, they are polyphyletic. What that suggests, besides the diversity of their genus, is that each of them must have made the leap to humans independently. Among the questions that continue to occupy malaria researchers are: Which other animals did they leap from, and when?
Falciparum malaria, because its global impact in death and misery is so high, has received particular attention. Early molecular research suggested that P. falciparum shares a close common ancestor with two different kinds of avian plasmodia, and that the parasite must therefore have crossed into humans from birds. A corollary to that idea, based on sensible deduction but not much evidence, is that the transfer probably happened just five or six thousand years ago, coincident with the invention of agriculture, which allowed for sedentary settlement—crop fields and villages—constituting the first sizable and dense aggregations of humans. Such gatherings of people would have been necessary to sustain the new infection, because malaria (like measles, but for different reasons) has a critical community size and tends to die out locally if the hosts are too few. Simple irrigation works, such as ditches and impoundments, may have increased the likelihood of transfer by offering good breeding habitat for Anopheles mosquitoes. Domestication of the chicken, about eight thousand years ago in Southeast Asia, may have been another contributing factor, since one of the two forms of bird plasmodia in question is Plasmodium gallinaceum, known for infecting poultry.
That view of falciparum malaria’s avian origins was propounded in 1991, a relatively long time ago in this field, and lately it doesn’t look so persuasive. A more recent study suggested that the closest known relative of P. falciparum is P. reichenowi, a malarial parasite that infects chimpanzees.
Plasmodium reichenowi has been found in wild and (wild-born) captive chimps in both Cameroon and Côte d’Ivoire, suggesting that it’s widespread across chimpanzee habitat in Central and West Africa. It contains a fair degree of genetic variation—more than P. falciparum worldwide—suggesting that it may be an old organism, or anyway older than P. falciparum. Furthermore, all known variants of P. falciparum seem to be twigs within the P. reichenowi branch of the Plasmodium family tree. These insights emerge from data gathered by a team of researchers led by Stephen M. Rich, of the University of Massachusetts, who proposed that P. falciparum has descended from P. reichenowi after spilling over from chimps into humans. According to Rich and his group, the spillover probably occurred just once, as early as 3 million years ago or as recently as ten thousand years ago. Some mosquito bit a chimpanzee (the insect becoming thereby infected with P. reichenowi gametocytes) and then also bit a human (delivering sporozoites). The transplanted strain of P. reichenowi, despite finding itself in an unfamiliar sort of host, managed to survive and proliferate. It passed from sporozoites into merozoites into gametocytes again, filled the bloodstream of that first human victim, and then caught itself another mosquito ride. From that insect it traveled onward, further vector-borne, to other humans as they foraged in the forest. Along the way it was changed by mutation and adaptation: P. reichenowi became P. falciparum.
This scenario implies that largish agricultural settlements weren’t necessary for the disease to take hold among humans, since no such settlements existed in those areas of Africa ten thousand (let alone 3 million) years ago. Rich’s group evidently considered the agricultural factor unnecessary. The genetic evidence they offered was compelling. Among Rich’s coauthors were a handful of luminaries in the fields of anthropology, evolution, and disease. Their paper appeared in 2009. But it wasn’t the last word.
Another group, led by a French anthropologist named Sabrina Krief and the malaria geneticist Ananias A. Escalante, published an alternative view in 2010. Yes, they agreed, P. falciparum may be more closely related to P. reichenowi than to any other known plasmodium. And yes, it seems to have spilled into humans within the relatively recent past. But look here, they said, we’ve located another host of P. falciparum itself—a host in which that parasite seems to have evolved before spilling into humans: the bonobo.
The bonobo (Pan paniscus) is sometimes known as the pygmy chimpanzee. It’s an