An international team of researchers reports that the malaria parasite that causes the worst form of the disease in humans originally jumped the species barrier from an ancestor of chimpanzees. Scientists say the discovery could lead to better treatments for malaria.
Both humans and chimpanzees, our closest primate relative, can become infected with a mosquito-borne parasite that causes malaria.
The human parasite known as plasmodium falciparum causes malignant malaria, the deadliest form of the disease, which sickens an estimated 500 million people globally each year, killing more than one million individuals, most of them children.
For years, evolutionary biologists believed that malignant malaria evolved millions of years ago in humans and that plasmodium reichenowi, a relatively benign form of the pathogen in chimps, evolved separately in primates.
But an international team of scientists, conducting a genetic analysis of reichenowi, has concluded that the chimp parasite - carried aloft by infected mosquitoes in equatorial Africa - jumped to humans and became falciparum, possibly as recently as ten thousand years ago.
Stephen Rich, head of the Department of Plant, Soil and Insect Sciences at the University of Massachusetts in Amherst, led the study that determined the origin of the deadly human malaria parasite.
"For scientists, that's an exciting opportunity to figure out exactly which genes or which characteristics are the ones that make falciparum - the human parasite - so deadly and the chimp parasite more or less benign," he said.
The researchers conducted a DNA analysis of malaria parasites taken from the tissue of ninety-four wild-born chimps in Cameroon and Ivory Coast, and compared it to falciparum.
The scientists determined that deadly falciparum is distantly related to three other parasites that cause malaria in humans, but is most closely related to reichenowi, which jumped from the chimpanzee lineage to our common human ancestor.
Evolutionary biologist and study co-author Francisco Ayala at the University of California Irvine doesn't find that surprising, given that genetically, humans and chimpanzees are ninety-eight percent identical.
"No doubt the fact that we are closely related to chimpanzees makes the possibility of transmission more likely because we are genetically very similar," said Ayala.
Researchers say our human ancestors were originally resistant to the ravages of falciparum but the pathogen became deadly, over time, as a result of genetic mutation.
They suspect the enhanced virulence of malaria occurred as early man turned from a hunter-gatherer existence to settled agrarian societies, in which mosquitoes were able to spread vast numbers of parasites to human hosts.
Stephen Rich of the University of Massachusetts says the fact that falciparum jumped the species barrier means that scientists are going to have to be vigilant about other primate parasites doing the same and causing serious new illnesses in humans.
"We know from our study that this has happened in the case of falciparum arising from an ancestor of reichenowi in the not too distant past and so, yes, that does present the opportunity that these things could be recurring all the time," he said. "And without much further sampling, we're just not going to be able to figure out how often that happens."
Rich predicts his team's finding will spark surveillance efforts for the emergence of new human pandemics from wildlife.
The study on the origin of malignant malaria is published this week in Proceedings of the National Academy of Sciences.