「考古学、言語を、遺伝的手法で解析する」 当たり前といえば当たり前ですが、人類は一方的にアフリカから 出て行ったのでなく、行ったり、戻ったりしながらアフリカを 出たのですね それにまずははっとしました nature/news/african-genes-tracked-back-1.13607 ┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏┏ 遺伝学手法で考古学的、言語学的データを補うことにより、初期人類のアフリ カへの回帰が明らかに。 African genes tracked back Method extends archaeological and linguistic data by tracing early hum an migration. Erika Check Hayden 27 August 2013 Article toolsPrint Email Download PDF Rights & Permissions Share/bookmark Facebook Twitter Delicious Digg Google+ LinkedIn StumbleUpon Reddit The first humans left Africa some 200,000 years ago, dispersing to pop ulate the rest of the world. But this was not a one-way trip: some peo ple came back. Scientists say that they have traced a reverse migratio n that, in two steps, carried genes from the rest of the world back to southern Africa, long before European colonizers arrived. The findings are part of a flurry of research enabled by better tools to survey African genomes. For the first time, population geneticists can examine the complex history of human migration in Africa effective ly, a field long dominated by the analysis of bones, artefacts and lan guages. “Up until now this was mostly done based on linguistics and archaeolo gy, and now we can use genetics to test ideas,” says Carina Schlebusc h, a geneticist at Uppsala University in Sweden. “It’s a really exci ting time for African genetics.” ExpandGenetic signs of reverse migrations had been spotted before. Ov er the past decade, evidence has accrued that people returned to Afric a after the original ‘out of Africa’ event. And more recently, DNA d ata suggest that a small group of east Africans migrated to southern A frica to mix with the hunter-gatherers who lived there 2,000 years ago . A new analysis connects these two migrations. In a paper posted on the preprint server arXiv on 30 July, population geneticists at Harvard M edical School in Boston, Massachusetts, report their use of a method t o identify traces of this two-step migration within DNA of the minorit y Khoe-San ethnic groups of southern Africa (J. K. Pickrell et al. Pre print at arxiv.org/abs/1307.8014; 2013). The researchers examined more than half a million sites of genetic var iation in the genomes of around 1,000 people, including some 200 south ern Africans representing 22 African groups. Turning to the Khoe-San, the team looked for regions of DNA in which individuals seemed to have inherited a chromosome from a different ethnic group at some time in the past. These DNA regions contain blocks of genetic variants that are linked t ogether in patterns similar to those seen in non-Khoe-San groups. The blocks are shorter and more scattered in the Khoe-San than in the pres umed donor group, because the genetic recombination that occurs with e ach generation has, over time, fragmented the donated chromosome. Related stories Human history writ large in a single genome Out of southern Africa Africa yields two full human genomes More related stories By measuring the extent of the fragmentation, the researchers could es timate how many generations ago the chromosome had been introduced int o Khoe-San genomes. They saw signs reflecting two waves of migration: one about 3,000 years ago, of non-Africans entering east Africa, and a second one 900?1,800 years ago, as east Africans migrated to souther n Africa and brought non-African genes along with them (see ‘Out of A frica ? and back in’). Because of this two-step migration, some Khoe-San groups who were thou ght to have been genetically quite isolated actually carry 1?5% non-A frican DNA, report the study’s authors, led by David Reich and postdo ctoral fellow Joseph Pickrell. Geneticists had already been able to detect past instances of ‘admixt ure’, or interbreeding, but this was the first time that multiple adm ixtures had been detected within a single group’s genetic profile. “ The most important finding is that this migration from east to south w as a carrier for non-African genes,” says Luca Pagani, a geneticist a t the Wellcome Trust Sanger Institute near Cambridge, UK, who has stud ied reverse migration to Ethiopia. The finding makes sense in light of previous archaeological and lingui stic studies, says Sarah Tishkoff, a genetic anthropologist at the Uni versity of Pennsylvania in Philadelphia. For instance, southern Africa n speakers of Khoe-Kwadi, a family of languages that traces its origin s to east Africans who brought herding techniques to the south, had th e highest proportion of non-African genes in the new study. Also, 2,00 0-year-old cattle and sheep bones and pottery characteristic of east A frican herding cultures have been found across southern Africa. “Perh aps this method could be applied to other cases where there isn’t goo d archaeological data,” says Tishkoff. But the finding also raises questions about the identity of the easter n Africans who met the non-Africans, and of the non-Africans themselve s. The non-African genes in the Khoe-San most closely resemble those f ound in modern southern Europeans. But the authors think it more likel y that the carriers of those genes at the time actually crossed into A frica from the Middle East or the Arabian peninsula. Advance on a chip The research underscores the use of improved methods and richer data s ets to explore the complex genetic history of Africa. Studies suggest that Africa is the most genetically diverse continent in the world and that the Khoe-San people have the world’s oldest genetic lineages. T hey are thought to be descendants of the first people to split from th e other modern humans who began the journey out of Africa. Despite the region’s outsizeimportance in human history, only in the past year has a significant amount of data on groups from all parts of Africa become available. Genomes of just a handful of the 2,000 Afric an ethnic groups had previously been sequenced, partly because collect ing DNA from remote African ethnic groups is an ethical and logistical challenge, and partly because most funding for genetic studies goes t o medical, not anthropological, applications. Compounding the problem, DNA micro?arrays designed to survey human ge netic diversity were initially made to sample the variation seen in Eu ropean and European-American genomes, and did not capture African dive rsity. That changed in 2011, when Reich and other geneticists worked w ith the company Affymetrix, based in Santa Clara, California, to desig n a gene chip ? the Human Origins Array ? that targets sites of vari ation seen in more diverse samples. Pickrell and Reich used the chip i n their analysis, as have other surveys of African diversity. That represents a welcome sea change, says Pagani. “Until now, we hav e been applying tools designed specifically for non-African people to African people.” Journal name: Nature Volume: 500, Pages: 514 Date published: (29 August 2013) DOI: doi:10.1038/500514a ┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛┛
Posted on: Sat, 31 Aug 2013 22:35:34 +0000
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