Ancient DNA solves milk mystery
Analysis of fossilized bones suggests milk-drinking mutations emerged after dairy herding.
When did ancient populations learn that drinking milk 'does a body good'? A team of scientists in Germany has tried to answer this question by studying ancient DNA extracted from skeletons thousands of years old.
Many adult humans can drink cow's milk — a rare feat among mammals, which usually lose the ability to digest the sugar in milk after they are weaned. Scientists have found the genetic mutations that allow many Europeans and some Africans to digest milk (see 'Human evolution: How Africa learned to love the cow'). Geneticists have estimated that these mutations first spread 3,000 to 7,000 years ago in eastern Africa, and slightly earlier than that in Europe.
But some researchers have posed a 'chicken-and-egg' question about milk drinking: was dairy herding adopted only by those populations who could already drink milk? Or did the invention of dairy herding favour those people who had the mutation, so that the mutation quickly spread throughout the population?
Joachim Burger of the University of Mainz, Germany, and colleagues worked with Mark Thomas of University College London, UK, to address this riddle by studying DNA from skeletons scattered throughout Europe. The team examined ten skeletons ranging in age from 3,800 to nearly 6,000 years old. The skeletons were discovered at archaeological sites in Germany, Hungary, Poland and Lithuania.
Bones offer support
The team drilled into bone and tooth samples from the skeletons, and extracted DNA from the samples. They then checked the DNA for the most common milk-drinking mutation found in Europeans today. The team did not find any evidence of this mutation, it reports in the Proceedings of the National Academy of Sciences today1.
Burger and his colleagues say this supports the dominant theory on how milk drinking evolved — that milk-drinking mutations were uncommon before the practice of dairying began. Then, when humans learned to herd cattle, the milk-drinking mutations spread rapidly, because they conferred a huge advantage on those who had them — perhaps due to the extra protein and fats available in cow's milk, the team speculates.
Burger says this is the first time that scientists have tried to use very old DNA to answer questions about milk drinking: "While the technique is common in modern-day population genetics, this is the first time that it is published for really ancient DNA," he says.
Other scientists cautioned that with so few skeletons analysed, from different areas, the results are not necessarily definitive. But, they said, the method adds a new way to look at milk drinking and other traits.
"The study does provide an excellent example of cross-checking the age estimates based solely on modern variations with datable ancient DNA," says Diane Gifford-Gonzalez, an anthropologist who studies the origins of dairying at the University of California, Santa Cruz.
- Burger J., et al. Proc. Natl. Acad. Sci. , doi/10.1073/pnas.0607187104 published online (2007).