Deadly Malaria May Have Arisen With Spread of Agriculture
The Independent (London)
|June 25, 2001|
Malaria became the biggest killer in human history starting in about 8000 B.C., after the birth and expansion of agriculture, according to scientists who have found evidence to suggest that the rise of the disease can be attributed to the spread of farming.
The scientists believe malaria was not a major problem before the introduction of agricultural techniques that led to new mosquito-breeding sites and a rise in human population density.
A study of the human genes that confer limited immunity against the malaria parasite has found that a mild form of the disease arose in tropical Africa about 10,000 years ago. But it was only after the introduction of farming in the Middle East when the disease took a more lethal turn.
Malaria, caused by a microscopic blood parasite transmitted in a mosquito's bite, infects about 500 million people each year, killing about 2 million.
Helped Shape History
The disease has helped to shape human history by decimating invading armies and making certain places difficult to colonize (see sidebar). West Africa became known as the "white man's grave" at the time of the British Empire because of malaria.
A research team led by Sarah Tishkoff of the University of Maryland analyzed mutations in a human "housekeeping" gene, called G6PD, that is usually involved in metabolizing glucose but which, when mutated, can confer limited protection against infection with Plasmodium, the malaria parasite.
Although such mutations can cause fairly serious problems for people in modern society, their high prevalence within an ethnic group suggests they must have provided some advantage in the pastnotably, malaria resistance.
Tishkoff's study, published in the June 22 issue of the journal Science, found that two mutations in the G6PD gene almost certainly arose as a result of the emergence of malaria.
Tishkoff said: "We looked at variations of the mutation that have appeared in several areas where the incidence of malaria is high. In each region, the mutations in G6PD appear to have arisen at about the same time that malaria became prevalent."
Dual Patterns of Emergence
One of the mutations arose in Africa sometime from 3,840 to 11,760 years ago and the other arose in the Mediterranean region 1,600 to 6,640 years ago, she noted.
About 7,000 to 12,000 years ago, a dramatic change in the climate of Africa increased temperatures and humidity, creating sunlit pools of water in which mosquitos could breed.
At about the same time, agriculture in the Middle East and northeast Africa led to forest clearing in the Mediterranean region and the creation of mosquito-breeding pools. With a growing population of people living in villages, the conditions were ripe for a more lethal form of malaria to spread fast.
Tishkoff believes the second mutation arose in the Mediterranean area as a result of the emergence of a deadlier form of malaria, which may have been introduced by Greek farmers and spread throughout Asia by the army of Alexander the Great.
"By studying how nature copes with malaria, we may be able to design more effective treatments," she said.
(C) 2001 The Independent (London)
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