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A deadly form of malaria has something in common with HIV-AIDS. Both appear to have jumped to humans from chimpanzees in Africa. That's according to a new genetic study of the origins of what's known as malignant malaria.

NPR's Jon Hamilton explains.

JON HAMILTON: Malaria kills more than a million people each year. It's spread by mosquitoes carrying parasites that cause the disease. And a parasite called Plasmodium falciparum causes the most dangerous form of malaria.

Professor FRANCISCO AYALA (University of California, Irvine): It's a very, very nasty disease.

HAMILTON: Francisco Ayala of the University of California, Irvine has spent the past 15 years trying to trace the origin of the falciparum parasite. He says people who grow up in areas where it is common develop some immunity. But everyone else is very vulnerable, including scientists like the famous geneticist William Hamilton. He was in Africa a few years ago doing research on the origin of HIV.

Prof. AYALA: The sad story is he got bitten. He got infected with malignant malaria. A few days later he was dead. That's why falciparum malaria is feared.

HAMILTON: But scientists couldn't figure out where the parasite came from.

Dan Hartl, a biologist at Harvard, says there was a lot of confusion.

Professor DANIEL HARTL (Biology, Harvard University): The earliest data suggested that human malaria might have come from birds.

HAMILTON: That was based on a genetic analysis that showed similarities between parasites carried by birds and those in people.

Prof. HARTL: Everybody was a little skeptical of that result. But that's the way the data were pointing at the time.

HAMILTON: Ayala was among the skeptics. So, he and his team began studying parasites carried by chimps. First, they discovered a parasite that was much closer to falciparum than the bird parasites. Then they tried to figure out whether this parasite had infected a common ancestor of humans and chimps millions of years ago or whether the chimp version had jumped into humans more recently. His team took samples of parasites from chimps and humans and studied regions of DNA in which mutations tend to accumulate at a known rate.

Prof. AYALA: Because they are timekeepers.

HAMILTON: The regions act as a sort of genetic clock: the longer a parasite has been around, the more mutations it acquires. Ayala says his team found that the chimp parasite called reichenowi, had a lot more mutations than the human parasite, falciparum.

Prof. AYALA: Indicating, thereby, clearly that the reichenowi is the ancestor of falciparum. At some point there was a transmission from a chimp to a human.

HAMILTON: Hartl says the finding offers a major revision of malaria's history.

Prof. HARTL: It suggests that there was a single transfer from chimps to humans, and that it surely occurred well within the last, let's say, 100,000 years.

HAMILTON: And perhaps quite recently. Scientists say it's only in the past 10,000 years or so that nomadic people in Africa began to settle down and cultivate the land. That meant denser populations and standing water, the conditions that make it easy for mosquitoes to transmit parasites from one person to the next. Hartl says Ayala's version of events makes sense to him.

Prof. HARTL: This brings the human malaria picture into consonance with what we know about other monkey and primate malarias.

HAMILTON: As well as what we know about diseases including HIV. And that should make it easier to come up with better medicines or a vaccine. The malaria research appears in the Proceedings of the National Academy of Sciences.

Jon Hamilton, NPR News.

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