A discovery by an international team led by University of Maryland researcher Sarah Tishkoff* identifies, for the first time, genetic mutations in East Africans that are associated with the ability to digest milk as adults.
Tishkoff's study of DNA, described in the journal Nature Genetics [1], found that the mutations evolved at the time in history when some Africans were beginning to raise cattle, and they evolved independently from the mutation that regulates milk digestion in Europeans.
The findings are not only evidence of how genes and culture co-evolve, says Tishkoff, associate professor of biology at Maryland, "they reveal one of the most striking genetic footprints of natural selection ever observed in humans."
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When humans were hunter-gatherers, before some began to domesticate cattle, they were able to digest milk only until they were about four years old. Lactase-phlorizin hydrolase (LPH), the enzyme that lets humans digest lactose, the main carbohydrate in milk, works in the small intestine to help the body absorb the sugars found in lactose. But LPH levels decrease rapidly after weaning and are at low levels in adults
When milk became a human food source, evolution went to work. Says Tishkoff, "The ability to digest milk as adults, called lactase persistence, appears to be an excellent example of gene-culture co-evolution. In this case, it is the development of technology - raising cattle - and the genetic mutation - the ability to digest milk - that becomes common."
