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4月20日付、「Science Magazine」より。
「アメリカ自然人類学会」が３月26日から31日までペンシルベニア州フィラデルフィアで開催され、ヨーロッパ人の皮膚の色が現在のように白くなったのは、比較的最近で、およそ６千年から１万２千年前だろうとの最新の研究結果が報告された。
http://www.sciencemag.org/cgi/content/summary/316/5823/364a
Science 20 April 2007:
Vol. 316. no. 5823, p. 364
News Focus
AMERICAN ASSOCIATION OF PHYSICAL ANTHROPOLOGISTS MEETING:
European Skin Turned Pale Only Recently, Gene Suggests
Ann Gibbons
PHILADELPHIA, PENNSYLVANIA--At the American Association of Physical Anthropologists meeting, held here from 28 to 31 March, a new report on the evolution of a gene for skin color suggested that Europeans acquired pale skin quite recently, perhaps only 6000 to 12,000 years ago. (Read more.)
あとは購読者以外には読めないようになっているが、下記に上記の記事内容が一部、書かれている。
http://foreigndispatches.typepad.com/dispatches/2007/04/when_europeans_.html
April 20, 2007
When Europeans Became White
Some interesting news for y'all, hot from the pages of Science magazine:
Researchers have disagreed for decades about an issue that is only skin-deep: How quickly did the first modern humans who swept into Europe acquire pale skin? Now a new report on the evolution of a gene for skin color suggests that Europeans lightened up quite recently, perhaps only 6000 to 12,000 years ago. This contradicts a long-standing hypothesis that modern humans in Europe grew paler about 40,000 years ago, as soon as they migrated into northern latitudes. Under darker skies, pale skin absorbs more sunlight than dark skin, allowing ultraviolet rays to produce more vitamin D for bone growth and calcium absorption. "The [evolution of] light skin occurred long after the arrival of modern humans in Europe," molecular anthropologist Heather Norton of the University of Arizona, Tucson, said in her talk.
The genetic origin of the spectrum of human skin colors has been one of the big puzzles of biology. Researchers made a major breakthrough in 2005 by discovering a gene, SLC24A5, that apparently causes pale skin in many Europeans, but not in Asians. A team led by geneticist Keith Cheng of Pennsylvania State University (PSU) College of Medicine in Hershey found two variants of the gene that differed by just one amino acid. Nearly all Africans and East Asians had one allele, whereas 98% of the 120 Europeans they studied had the other (Science, 28 October 2005, p. 601).
Norton, who worked on the Cheng study as a graduate student, decided to find out when that mutation swept through Europeans. Working as a postdoc with geneticist Michael Hammer at the University of Arizona, she sequenced 9300 base pairs of DNA in the SLC24A5 gene in 41 Europeans, Africans, Asians, and American Indians.
Using variations in the gene that did not cause paling, she calculated the background mutation rate of SLC24A5 and thereby determined that 18,000 years had passed since the light-skin allele was fixed in Europeans. But the error margins were large, so she also analyzed variation in the DNA flanking the gene. She found that Europeans with the allele had a "striking lack of diversity" in this flanking DNA--a sign of very recent genetic change, because not enough time has passed for new mutations to arise. The data suggest that the selective sweep occurred 5300 to 6000 years ago, but given the imprecision of method, the real date could be as far back as 12,000 years ago, Norton said. She added that other, unknown, genes probably also cause paling in Europeans.
As there are several other genes which are involved in skin color, it's a bit of an exaggeration to say that Europeans "suddenly" turned white a few thousand years ago, at least until we're sure that the rest of the mutations responsible are of similarly recent vintage. Still, SLC24A5 accounts for enough of the color variation for us to be able to confidently say that until relatively recently Europeans didn't look distinguishable from the people of northern India or southern Arabia.
Either way, the implication is that our European ancestors were brown-skinned for tens of thousands of years--a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza. He argued that the early immigrants to Europe, who were hunter-gatherers, herders, and fishers, survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin.
The poor diet of the farmer is to blame! Whether this old hypothesis is correct or not is too early to determine, but this report certainly gives it new life. I imagine that the increased paleness of East Asians is also of relatively new vintage (as are all the other derived, prototypically "Asian" features), perhaps dating back no further than about 5,000 years, when one people* started farming rice somewhere in China and underwent a radical population explosion; similarly, I don't think you'll find any people who look like today's West Africans before ~10,000 years ago. All the external "racial" features people invest so much importance in are of surprisingly recent vintage.
*There's no reason to suppose said people were speakers of proto-Chinese, by the way, as the genetic evidence suggesting that all north-east Asians share very recent common roots also suggests - based on the distribution of languages in the region - that the shared ancestor of the Korean and Japanese languages has its roots in northeast China. The speakers of proto-Chinese likely came from the Himalayan region after rice-growing had become established, and made the original inhabitants of their new homeland drop their old languages in favor of the invaders'.
PS: Any subscribers to Molecular Biology and Evolution out there? From the abstract, we have the following:
Polymorphisms in 2 genes, ASIP and OCA2, may play a shared role in shaping light and dark pigmentation across the globe, whereas SLC24A5, MATP, and TYR have a predominant role in the evolution of light skin in Europeans but not in East Asians. These findings support a case for the recent convergent evolution of a lighter pigmentation phenotype in Europeans and East Asians.
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