Scientists say they have genetic¹ proof malaria² spread by mosquitoes jumped species from chimpanzees to humans.

科学家称他们已经找到遗传证据证明通过蚊子传播的疟疾可以从黑猩猩跳跃物种到人身上。

chimp³ from the Mfou National Park in Cameroon" width="226" border="0" src="/upimg/090804/4_071935_1.jpg" />
A chimp from the Mfou National Park in Cameroon

By looking at blood samples, a US team discovered all world strains of the human malaria（疟疾） parasite⁴（寄生虫，食客） falciparum stem from a malaria parasite in chimps⁵.

They tell Proceedings⁶ of the National Academy of Sciences how the species shift probably happened 10,000 years ago when humans turned to agriculture.

Man's encroachment⁷（侵蚀，侵犯） upon the natural forest habitat of chimps is blamed.

It brought the two species into close contact and the deforestation（采伐森林） created pools of stagnant⁸（不流动的，不景气的） water and other conditions favourable⁹ for mosquito breeding.

"Today, human encroachment into the last forest habitats has further extended, leading to a higher risk of transfer of new pathogens（病原体）, including new malaria parasites¹⁰," the researchers warn.

Species jump

Previously¹¹, malaria's origin in humans had been unclear.

But this latest work suggests malaria, like HIV, has jumped species from one of our closest relatives.

Although chimps were known to harbour a parasite - Plasmodium reichenowi - that is closely related to the most common of the human malaria parasites, Plasmodium falciparum（恶性疟原虫）, many scientists had assumed that the two had co-existed separately.

But blood tests on 94 wild and captive chimpanzees in Cameroon and the Ivory Coast suggest falciparum evolved from reichenowi.

Francisco Ayala, of the University of California, Irvine, and colleagues found eight new strains of reichenowi that had striking similarities to falciparum and were genetic precursors¹²（先驱者，前导） to the human disease.

The leap could have happened as early as two to three million years ago, but most likely to our Neolithic¹³ ancestors as recently as 10,000 years ago.

The scientists hope their discovery will help others looking for new drugs and vaccines¹⁴（疫苗） to stop human malaria.

Professor Brian Greenwood, a malaria expert at the London School of Hygiene¹⁵ and Tropical Medicine, said: "This is interesting work.

"There has been dispute about how long falciparum has been around for and as genetic techniques get better we can get a more accurate idea."