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Evolution: It Does a Body Good
26 February 2007 (All day)
Mammals are born to drink milk. But as they grow up, many animals and certain people develop an inability to digest it. In Europe 8000 years ago, pretty much everyone fell into this category, or so suggest new data from old DNA. The study provides the first evidence from ancient DNA that modern Europeans' ability to digest milk after early childhood arose relatively recently.
The milk sugar lactose is broken down by the enzyme lactase. In most cases, mammals stop producing lactase after weaning, but a nucleotide switch in their DNA can keep lactase flowing into adulthood, a trait called lactase persistence. Lactase persistence is far from a universal condition (half of modern humans cannot digest lactose), and there are at least two competing theories as to how it became the norm for European populations. The culture-historical hypothesis is that, shortly after the domestication of livestock, a few lucky farmers with a genetic anomaly hit white gold: nourishment via milk. Then, according to this theory, natural selection took over and these lactase-persistent folks proceeded to populate much of Europe with their milk-guzzling offspring. A competing hypothesis argues that ancient Europeans domesticated milk-bearing livestock because lactase persistence was already quite common in certain populations. In the current study, published online this week in the Proceedings of the National Academy of Sciences, researchers checked DNA from ancient European skeletons to see just how common lactase persistence was back then.
In all, the researchers tested nine ancient skeletons from five different archaeological sites scattered throughout central, northeast, and southeast Europe. Eight of the skeletons ranged in age from 5800 B.C.E. to 5200 B.C.E., and one of them was dated around 1800 B.C.E. The team focused on a genetic variant most commonly associated with lactase persistence in European populations. Strikingly, not a single ancient skeleton bore the mark of lactase persistence. The absence of this particular genetic switch means that most ancient Europeans could not digest milk, which supports the culture-historical hypothesis. "The key point here is that there was something that was very, very rare 8000 years ago but common today," says Mark Thomas, an evolutionary geneticist at University College London and an author of the study. "That's really strong natural selection."
If the data are true, "then I think it does lend credence to the [culture-historical] hypothesis," says Hendrik Poinar, an evolutionary geneticist at McMaster University in Ontario, Canada. But he says he would like to see other labs replicate the DNA results in these and additional skeletons before he fully accepts the findings.