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PEOPLE HAVE continued to evolve since leaving the ancestral homeland in northeastern Africa 50,000 years ago, both through the random process known as genetic drift and through natural selection.
Recent research indicates we have spread globally and evolved locally the human. genome bears many fingerprints in places where natural selection has recently remoulded the human clay, researchers have found, as people on the various continents adapted to new diseases, climates, diets and, perhaps, behavioural demands.
A striking feature of the changes: they are local. The genes found in one continent-based population or race vary from those that occur in others.
These genes so far make up a small fraction of all human genes. The concept of race having a biological basis is controversial, and most geneticists are reluctant to describe it that way.
But a notable instance of recent natural selection is the emergence of lactose tolerance - the ability to digest lactose in adulthood - among the cattle-herding people of northern Europe 5,000 years ago. Lactase is usually switched off after weaning.
Last year, Sarah Tishkoff of the University of Maryland tested 43 ethnic groups in East Africa and found three mutations, all different from the European one, that keep the lactase gene switched on in adulthood.
Researchers studying single genes have found evidence for recent evolutionary change in genes that mediate conditions like skin color, resistance to malaria and salt retention. Variants of two genes involved in hearing have become universal, one in Chinese, the other in Europeans.
Last year, researchers at the University of Chicago searched for genes under natural selection in Africans, Europeans and East Asians. In each race, 200 genes showed signals of selection suggesting that the populations were adapting to local challenges. A study led by S. Williamson of Cornell University in PLoS Genetics found 100 genes under selection in Chinese, African-Americans and European-Americans.
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