Thursday, September 17, 2009

Study finds humans still evolving, and quickly

The pace has been increasing since people started spreading through Europe, Asia and Africa 40,000 years ago.
Plaster casts of skulls, from left to right, of the earliest fossil finds of humanoids to modern man at the National Museum of Kenya in Nairobi
The pace of human evolution has been increasing at a stunning rate since our ancestors began spreading through Europe, Asia and Africa 40,000 years ago, quickening to 100 times historical levels after agriculture became widespread, according to a study published today.

By examining more than 3 million variants of DNA in 269 people, researchers identified about 1,800 genes that have been widely adopted in relatively recent times because they offer some evolutionary benefit.

Until recently, anthropologists believed that evolutionary pressure on humans eased after the transition to a more stable agrarian lifestyle. But in the last few years, they realized the opposite was true -- diseases swept through societies in which large groups lived in close quarters for a long time.

Altogether, the recent genetic changes account for 7% of the human genome, according to the study published in the Proceedings of the National Academy of Sciences.

The advantage of all but about 100 of the genes remains a mystery, said University of Wisconsin-Madison anthropologist John Hawks, who led the study. But the research team was able to conclude that infectious diseases and the introduction of new foods were the primary reasons that some genes swept through populations with such speed.

"If there were not a mismatch between the population and the environment, there wouldn't be any selection," Hawks said. "Dietary changes, disease changes -- those create circumstances where selection can happen."

One of the most famous examples is the spread of a gene that allows adults to digest milk.

Though children were able to drink milk, they typically developed lactose intolerance as they grew up. But after cattle and goats were domesticated in Europe and yaks and mares were domesticated in Asia, adults with a mutation that allowed them to digest milk had a nutritional advantage over those without.

As a result, they were more likely to have healthy offspring, prompting the mutation to spread, Hawks said.

The mechanism also explains why genetic resistance to malaria has spread among Africans -- who live where disease-carrying mosquitoes are prevalent -- but not among Europeans or Asians.,0,5882337.story

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