Wade 2007edited - Humans Have Spread Globally, and Evolved...

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Humans Have Spread Globally, and Evolved Locally (N.B. See associated figure in the accompanying human evolution figure.pdf) By NICHOLAS WADE Published: June 26, 2007 The New York Times Historians often assume that they need pay no attention to human evolution because the process ground to a halt in the distant past. That assumption is looking less and less secure in light of new findings based on decoding human DNA. People have continued to evolve since leaving the ancestral homeland in northeastern Africa some 50,000 years ago, both through the random process known as genetic drift and through natural selection. The genome bears many fingerprints in places where natural selection has recently remolded the human clay, researchers have found, as people in the various continents adapted to new diseases, climates and diets. A striking feature of many of these changes is that they are local. The genes under selective pressure found in one continent-based population or race are mostly different from those that occur in the others. These genes so far make up a small fraction of all human genes. 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 some 5,000 years ago. Lactase, the enzyme that digests the principal sugar of milk, is usually switched off after weaning. But because of the great nutritional benefit for cattle herders of being able to digest lactose in adulthood, a genetic change that keeps the lactase gene switched on spread through the population. Lactose tolerance is not confined to Europeans. Last year, Sarah Tishkoff of the University of Maryland and colleagues tested 43 ethnic groups in East Africa and found three separate mutations, all different from the European one, that keep the lactase gene switched on in adulthood. One of the mutations, found in peoples of Kenya and
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This note was uploaded on 12/06/2009 for the course BIO 225 taught by Professor Pavgi during the Spring '08 term at University of Michigan.

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Wade 2007edited - Humans Have Spread Globally, and Evolved...

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