Until now, the most deadly form of malaria in humans, Plasmodium falciparum, has been linked to the disease in chimpanzees, with the two similar strains thought to have diverged around the same time as humans and chimps did on the evolutionary timeline.
And that's been difficult to verify, in part because the malarial form in chimpanzees and people bears such little resemblance to the disease in other animals.
But a new phylogenetic analysis — based on the amplification of entire mitochondrial genomes of malarial parasites that feed on humans, rodents, birds and lizards -- suggests that malarial parasites in tree-dwelling rats share a close evolutionary relationship with the disease in humans and chimpanzees.
"This is the first time that a relationship has been found between human and rodent malaria," said Susan Perkins, assistant curator of Invertebrate Zoology at the American Museum of Natural History, where researchers at the Sackler Institute for Comparative Genomics completed the study. "In all past studies, P. falciparum seemed to not be closely related to anything else but the chimpanzee parasite. But this study places it in a sister group of parasites from rodents."
The analysis points to the African thicket rat as the carrier of similar malarial parasites, which is interesting to researchers because that particular parasite is also the most common laboratory model for human malarial research.
"The link between human malaria and rodent malaria is exciting because, if they really are that closely related, our laboratory models might be more powerful for helping to study how to fight the disease," Perkins said in a release accompanying the findings, which were published in the December online issue of the journal Mitochondrial DNA.
Perkins also believes there may be more to the malarial link. Still-unpublished data collected previously in her laboratory suggested a closely related form of Plasmodium in bats; the most virulent form of malaria could have jumped into humans from these other arboreal animals.
"Like Ebola and SARS, this could be another example of bat-human linkage," Perkins said. "Although the results of this study are unambiguous, they are nonetheless still based on just a very small portion of the parasite's entire DNA."