December 11, 2007

Think Humans Have Stopped Evolving? Think Again.

As usual, the paper mentioned in the following article hasn't actually appeared in the PNAS Early Edition yet. However, those wanting a preprint copy can request one here. Before going on to the press release, perhaps it would be nice to at least see the abstract for this paper, which appears at Dr. Hawks' blog by clicking the link above. The title of the paper is Recent acceleration of human adaptive evolution.

OK, on with excerpts from the press release:

Genome Study Places Modern Humans In Evolutionary Fast Lane

Countering a common theory that human evolution has slowed to a crawl or even stopped in modern humans, a new study examining data from an international genomics project describes the past 40,000 years as a time of supercharged evolutionary change, driven by exponential population growth and cultural shifts.

In a study published in the Dec. 10 issue of the Proceedings of the National Academy of Sciences (PNAS), a team led by University of Wisconsin-Madison anthropologist John Hawks estimates that positive selection just in the past 5,000 years alone -- around the period of the Stone Age -- has occurred at a rate roughly 100 times higher than any other period of human evolution. Many of the new genetic adjustments are occurring around changes in the human diet brought on by the advent of agriculture, and resistance to epidemic diseases that became major killers after the growth of human civilizations...

While the correlation between population size and natural selection is nothing new -- it was a core premise of Charles Darwin, Hawks says -- the ability to bring quantifiable evidence to the table is a new and exciting outgrowth of the Human Genome Project...

The researchers identify recent genetic change by finding long blocks of DNA base pairs that are connected. Because human DNA is constantly being reshuffled through recombination, a long, uninterrupted segment of LD is usually evidence of positive selection. Linkage disequilibrium decays quickly as recombination occurs across many generations, so finding these uninterrupted segments is strong evidence of recent adaptation, Hawks says.

Employing this test, the researchers found evidence of recent selection on approximately 1,800 genes, or 7 percent of all human genes.

This finding runs counter to conventional wisdom in many ways, Hawks says. For example, there's a strong record of skeletal changes that clearly show people became physically smaller, and their brains and teeth are also smaller. This is generally seen as a sign of relaxed selection -- that size and strength are no longer key to survival.

But other pathways for evolution have opened, Hawks says, and genetic changes are now being driven by major changes in human culture. One good example is lactase, the gene that helps people digest milk. This gene normally declines and stops activity about the time one becomes a teenager, Hawks says. But northern Europeans developed a variation of the gene that allowed them to drink milk their whole lives -- a relatively new adaptation that is directly tied to the advance of domestic farming and use of milk as an agricultural product.

The biggest new pathway for selection relates to disease resistance, Hawks says. As people starting living in much larger groups and settling in one place roughly 10,000 years ago, epidemic diseases such as malaria, smallpox and cholera began to dramatically shift mortality patterns in people. Malaria is one of the clearest examples, Hawks says, given that there are now more than two dozen identified genetic adaptations that relate to malaria resistance, including an entirely new blood type known as the Duffy blood type...

The recent changes are especially striking, he says. "Five thousand years is such a small sliver of time -- it's 100 to 200 generations ago," he says. "That's how long it's been since some of these genes originated, and today they are in 30 or 40 percent of people because they've had such an advantage. It's like 'invasion of the body snatchers..."
I've written to Dr. Hawks to request a preprint; this sounds like some very interesting information. More later, perhaps, when I've had a chance to see the actual paper. Science by press release sucks.

It's nice to know that we furless bipeds haven't quite hit an evolutionary brick wall yet. Well, most of us, anyhow.

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