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Friday, March 02, 2007

DNA Lactose Surprise Follow-up

The study referenced in Monday's post, DNA Lactose Surprise? Not to Me., has continued to cause comment around the globe.

First, let me give the abstract of the article, Absence of the lactase-persistence-associated allele in early Neolithic Europeans, , by J. Burger , M. Kirchner , B. Bramanti , W. Haak , and M. G. Thomas, from the Proceedings of the National Academy of Sciences site, Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0607187104.

Lactase persistence (LP), the dominant Mendelian trait conferring the ability to digest the milk sugar lactose in adults, has risen to high frequency in central and northern Europeans in the last 20,000 years. This trait is likely to have conferred a selective advantage in individuals who consume appreciable amounts of unfermented milk. Some have argued for the "culture-historical hypothesis," whereby LP alleles were rare until the advent of dairying early in the Neolithic but then rose rapidly in frequency under natural selection. Others favor the "reverse cause hypothesis," whereby dairying was adopted in populations with preadaptive high LP allele frequencies. Analysis based on the conservation of lactase gene haplotypes indicates a recent origin and high selection coefficients for LP, although it has not been possible to say whether early Neolithic European populations were lactase persistent at appreciable frequencies. We developed a stepwise strategy for obtaining reliable nuclear ancient DNA from ancient skeletons, based on (i) the selection of skeletons from archaeological sites that showed excellent biomolecular preservation, (ii) obtaining highly reproducible human mitochondrial DNA sequences, and (iii) reliable short tandem repeat (STR) genotypes from the same specimens. By applying this experimental strategy, we have obtained high-confidence LP-associated genotypes from eight Neolithic and one Mesolithic human remains, using a range of strict criteria for ancient DNA work. We did not observe the allele most commonly associated with LP in Europeans, thus providing evidence for the culture-historical hypothesis, and indicating that LP was rare in early European farmers.


I thought this was well known, but apparently not. The Daily Telegraph in an article by Science Editor Roger Highfield reported:
A study of DNA from skeletons suggest that all European adults living between 6,000 BC and 5,000 BC were unable to absorb lactose – a sugar found in milk.

Their findings back the idea that the ability to digest milk spread only after the introduction of cattle farming in Europe 20,000 years ago.

The rival idea, that dairy farming was pioneered by a small group of Neolithic farmers who were able to tolerate milk, is overturned by the genetic study by a team from University College London and Mainz University, Germany.

Instead, the Neolithic descendants of Palaeolithic (Stone Age) people evolved their tolerance of milk within the last 8,000 years due to exposure to dairy products, making this "the most rapidly evolved European trait of the past 30,000 years," according to Dr Mark Thomas of UCL. "Lactose tolerance is very much a Neolithic invention."

I've never heard of this rival idea. That's the surprise to me. In fact, a search for "reverse cause hypothesis" comes up empty outside of this abstract both on Google and on Google Scholar. MedlinePlus.gov does give hits, but only to irrelevant topics.

More details as I get them.

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