Malaria, which affects some 500 million people a year worldwide, was first transmitted to humans by chimpanzees, according to a US study.
The origins of mosquito-carried malaria has long been unclear, and has lead scientists to come up with several inconclusive theories.
Researchers knew that chimpanzees carry a parasite -- Plasmodium reichenowi -- that is similar to the human malaria parasite, Plasmodium falciparum, but they were not sure how the two strains were related.
One hypothesis suggested that they had evolved from a common strain over millions of years in their respective host species.
Another theory held that the parasite first emerged in humans and was transmitted to chimpanzees, where it gradually evolved into a separate strain.
But the authors of the study, published in Proceedings of the National Academy of Science, examined a third possibility, using samples taken from wild and wild-born chimpanzees in Cameroon and the Ivory Coast.
They identified several new parasites among the chimpanzees they sampled, indicating that the malaria parasite first jumped from chimpanzees to humans.
The study, led by Francisco Ayala of the Department of Ecology and Evolutionary Biology at the University of California, Irvine, suggests the malaria strain probably jumped species in "a single host transfer."
The leap could have happened as early as two to three million years ago, or as recently as 10,000 years ago, in a process similar to the way the AIDS and SARS viruses jumped from animals to humans, the study said.
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