When humans left Africa some 50,000 to 60,000 years ago, some were already infected with the world's most deadly malaria parasites, a new study says. The findings contradict the view, held by some scientists, that malaria only began afflicting us about 10,000 years ago, around the time agriculture was invented.
Transmitted by Anopheles mosquitoes, the Plasmodium falciparum parasite infects hundreds of millions of people every year and may kill more than a million. Researchers are still debating when this scourge first began afflicting us. Some have argued that a predecessor to the parasite must have infected the last common ancestor to humans and chimps, 6 million or 7 million years ago, and then co-evolved when Homo sapiens stepped onto the world stage in a process called cospeciation. In this scenario, a related species, P. reichenowi, co-evolved with chimpanzees.
But others, such as famed evolutionary biologist Francisco Ayala of the University of California, Irvine, argue that P. reichenowi jumped the species barrier from chimps to humans quite recently and then became P. falciparum. Ayala believes that this may have happened as recently as 10,000 years ago, when humans settled down and started farming; at that time, irrigation and huts would have created ideal breeding grounds for mosquitoes to help the parasites spread. Genetic studies allowing scientists to draw the Plasmodium family tree should be able to settle the issue, but so far, there's no consensus.
The new paper, published yesterday in Current Biology, doesn't quite solve the question of whether cospeciation occurred, but it claims that malaria couldn't possibly have arisen with the advent of agriculture. The authors, from 13 institutes on four continents, reasoned that if humans were suffering from malaria when they left the African continent, it should be evident in the genetic makeup of parasite populations in different parts of the world; the theory predicts that parasites farther away from Africa should be less diverse, just as is the case in humans.
So the team analyzed hundreds of malaria samples from seven countries, sequencing two genes to determine genetic variability within each local population, all the way from West Africa to Indonesia and Oceania. (The Americas, where malaria is believed to have been introduced just several hundred years ago during the slave trade, were left out of the main analysis.)
The researchers found that genetic diversity did indeed decrease at greater distances from Africa. The correlation is very strong, says lead author Francois Balloux of the MRC Centre for Outbreak Analysis and Modelling in London, and the pattern matches human migration out of Africa, which scientists believe started some 50,000 to 60,000 years ago.
"It makes sense," says Austin Hughes of the University of South Carolina, Columbia, who recently published another analysis showing that malaria has been with humans for a very long time. "It's consistent with everything we have tried to say for a long time."