Plasmodium falciparum is the agent of the deadliest form of human malaria. A survey of Plasmodium diversity in African apes reveals that western gorillas are the reservoir species for this parasite. See Article p. 420
When it comes to disease, chimpanzees get a bad press. We have known for more than a decade that chimpanzees are the source of HIV-1, the major cause of AIDS, and probably acquired the virus themselves by eating infected monkeys1. More recently, chimpanzees have been proposed as the reservoir of human Plasmodium falciparum2, a parasite that causes the most severe type of malaria. On page 420 of this issue3, however, Liu et al. show that another of our great-ape cousins — the gorilla — is in fact a more likely progenitor species for this form of human malaria.
Common chimpanzees (Pan troglodytes) carry their own type of malaria parasite, Plasmodium reichenowi. For some time, this was represented by a single isolate, which seemed to be the closest relative of human P. falciparum. Such an intimate phylogenetic relationship quite reasonably led to the idea that these two Plasmodium species diverged at the same time as humans and chimpanzees separated, between 5 million and 7 million years ago4. This theory of host–parasite co-divergence was accepted with little challenge until recent studies revealed that African primates carry a far greater diversity of Plasmodium than previously realized2,5,6, leading to a re-evaluation of the likely ancestry of human malaria. Of most note, it has been proposed that the genetically homogeneous P. falciparum of humans is simply a strain of the more diverse P. reichenowi that jumped from chimpanzees, and far more recently than implied by co-divergence2.
Liu et al.3 made their discovery through two key extensions to the sampling of Plasmodium biodiversity in non-human primates. First, by using ape faecal material, obtained non-invasively, as a source of Plasmodium DNA, they were able to examine more than 2,500 retrospectively collected samples from chimpanzees, bonobos (Pan paniscus) and both eastern gorillas (Gorilla beringei) and western gorillas (Gorilla gorilla) from central Africa. Second, they employed a genome-amplification technique that enabled them to screen for animals that had been infected with multiple Plasmodium lineages.
The results of their survey are striking: whereas no Plasmodium infection was evident in either bonobos or eastern gorillas, 32–48% of the chimpanzee and western gorilla individuals sampled carried Plasmodium parasites, with mixed infection being commonplace. In addition, the Plasmodium lineages found in western gorillas were clearly the closest relatives of human P. falciparum, strongly suggesting that this parasite jumped into humans from western gorillas, not chimpanzees, and that this cross-species transmission event occurred just once (Fig. 1).
Studies of pathogen biodiversity are in vogue, and are often set within the idea that they will allow the next human pandemic to be predicted, and hence prevented7. It is probably simplistic to think that describing what is out there in nature will permit disease emergence to be forecast with any accuracy, particularly because successful emergence also depends on aspects of pathogen genetics and epidemiology. But field studies of pathogen biodiversity undoubtedly provide a valuable genetic catalogue that will greatly assist efforts to understand the origins of human disease. Although the screen of Plasmodium biodiversity undertaken by Liu et al.3 is by far the largest of its kind, it seems certain that more intensive samplings of non-human primates will uncover even more diverse lineages of the malaria parasite, some of which may have the potential to emerge in humans. Enhanced sampling will also help to determine whether the apparent absence of Plasmodium in eastern gorillas and bonobos is because it has never infected these species, or has become extinct in them, or because it is simply at very low prevalence.
The study of Liu et al. sheds new light on the origins of malaria, but it unfortunately complicates our understanding of the timescale over which this emergence event took place. The beauty of the idea that P. reichenowi and P. falciparum co-diverged with their primate hosts was that it provided a ready-made time-frame for the evolutionary history of malaria. If these two parasites diverged with their hosts between 5 million and 7 million years ago, then estimating the time span of the remainder of the Plasmodium family tree, or of more recent diversification within human P. falciparum, becomes a relatively straightforward scaling exercise.
However, no such simple calibrations are possible with cross-species transmission, and so the timescale of malaria's origin is now shrouded in mystery. For rapidly evolving RNA viruses such as HIV-1, it is possible to calibrate the 'molecular clock' and estimate divergence times simply by counting the number of mutations between isolates sampled at different times8. However, this approach is inappropriate for organisms such as Plasmodium, in which rates of evolutionary change are far lower.
The only remaining option is to use some other external calibration point to set the tick rate of the molecular clock, perhaps invoking ancient geological events such as continental drift, or more recent evolutionary phenomena such as the movement of archaic human populations out of Africa9. Understandably, however, these assumptions can prove contentious. Resolving the timescale of malaria's origin provides added impetus for the more widespread sampling of Plasmodium genetic diversity in a diverse array of mammalian species.
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Proceedings of the National Academy of Sciences (2011)