西伯利亚人可能是美洲印第安人后裔

The genome sequence of a 24,000-year-old Siberian individual has provided a key piece of the puzzle in the quest for Native American origins. The ancient Siberian demonstrates genomic signatures that are basal to present-day western Eurasians and close to modern Native Americans. This surprising finding has great consequences for our understanding of how and from where ancestral Native Americans descended, and also of the genetic landscape of Eurasia 24,000 years ago. The breakthrough is reported in this week's Nature (Advance Online Publication) by an international team of scientists, led by the Centre for GeoGenetics at the Natural History Museum of Denmark (University of Copenhagen).

The search for Native American ancestors has been focused in northeastern Eurasia. In late 2009, researchers sampled at the Hermitage Museum, St. Petersburg the remains of a juvenile individual (MA-1) from the Upper Palaeolithic site of Mal'ta in south-central Siberia. The MA-1 individual dated to approximately 24,000 years ago. Now, the team reports genomic results from the MA-1 individual which unravel the origins of the First Americans -- ancestors of modern-day Native Americans.

"Representing the oldest anatomically modern human genome reported thus far, the MA-1 individual has provided us with a unique window into the genetic landscape of Siberia some 24,000 years ago," says Dr. Maanasa Raghavan from the Centre for GeoGenetics and one of the lead authors of the study. "Interestingly, the MA-1 individual shows little to no genetic affinity（类同） to modern populations from the region from where he originated -- south Siberia."

Instead, both the mitochondrial and nuclear genomes of MA-1 indicate that he was related to modern-day western Eurasians. This result paints a picture of Eurasia 24,000 years ago which is quite different from the present-day context. The genome of MA-1 indicates that prehistoric populations related to modern western Eurasians occupied a wider geographical range into northeast Eurasia than they do today.

Dual ancestry of Native Americans

The most significant finding that the MA-1 genome reveals is its relation to modern Native Americans. This relative of present-day western Eurasians shows close affinity to modern Native Americans, but surprisingly not to East Asians who are regarded as being genetically closely related to Native Americans.

Furthermore, the team finds evidence that this genetic affinity between MA-1 and Native Americans is mediated by a gene flow event from MA-1 into the First Americans, which can explain between 14-38% of the ancestry of modern Native Americans, with the remainder of the ancestry being derived from East Asians. Supported by numerous reasons against these signatures being caused by contamination from modern DNA sources or from post-Columbian admixture (post 1492 AD), the study concludes that two distinct Old World populations led to the formation of the First American gene pool: one related to modern-day East Asians, and the other a Siberian Upper Palaeolithic population related to modern-day western Eurasians.

"The result came as a complete surprise to us. Who would have thought that present-day Native Americans, who we learned in school derive from East Asians, share recent evolutionary history with contemporary western Eurasians? Even more intriguingly, this happened by gene flow from an ancient population that is so far represented only by the MA-1 individual living some 24,000 years ago," says Professor Eske Willerslev from the Centre for GeoGenetics who led the study.