This passage describes a scientific finding about human evolution.

Lexile Level: 1140L

Categories: Sports & Health Science & Technology

In 2006 a team of genetic researchers discovered evidence of recent human evolution. The discovery relates to the ability of humans to digest lactose, the main sugar found in milk. Lactose is broken down in the body by an enzyme called lactase. Throughout most of human history, the genes that control lactase production were switched off when a child was weaned because lactase was no longer needed. When cattle were first domesticated about 9,000 years ago, humans continued to consume milk into adulthood, however. As a consequence, natural selection favored genetic mutations that kept lactase production switched on in adults. Such mutations are known to have occurred among people in Europe who raised cattle. Now researchers have identified similar mutations among people in East Africa. These mutations are believed to have taken place as recently as 3,000 years ago. This date corresponds with the evidence that suggests cattle-raising tribes arrived in parts of East Africa about 4,500 years ago.

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