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FIGURE 1. Phylogeny of 12 Plasmodium species inferred from Csp gene sequences. P. falciparum, malariae, and vivax are human parasites; berghei and yoelii are rodent, and all others are primate parasites. The numbers refer to different strains. Bootstrap values above branches assess the reliability of the branch clusters; values above 70 are considered statistically reliable. Reprinted with permission from Ayala et al. 1999.

years ago, which is roughly consistent with the time of divergence between the two host species, human and chimpanzee. A parsimonious interpretation of this state of affairs is that P. falciparum is an ancient human parasite, associated with our ancestors since the divergence of the hominids from the great apes. Fig. 1 shows that P. malariae, a human parasite, is genetically indistinguishable from Plasmodium brasilianum, a parasite of New World monkeys; similarly, human P. vivax is genetically indistinguishable from Plasmodium simium, also a parasite of New World monkeys. It follows that lateral transfer between hosts has occurred in recent times, at least in these two cases. Whether the transfer has been from humans to monkeys or vice versa is a moot question (for discussion, see Ayala et al., 1999).

TABLE 1. Time (in million years) of divergence, between Plasmodium species, based on genetic distances at two gene loci (see Fig. 1 and Fig. 2; adapted from Escalante et al. [1995], and Escalante and Ayala [1994])

Plasmodium

rRNA

CSP

falciparum vs. reichenowi

11.2 ± 2.5

8.9 ± 0.4

vivax vs. monkey*

20.9 ± 3.8

25.2 ± 2.1

vivax vs. malariae

75.7 ± 8.8

103.5 ± 0.6

falciparum vs. vivax/malariae

75.7 ± 8.8

165.4 ± 1.6

* brasilianum not included.



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