In reply, two main scenarios have been mooted. Option A: a relatively small number of people from Siberia trekked over to America across a Bering Strait land bridge some 12,000 years ago. Option B: the ancestors of today’s Native Americans come from other parts of Asia or Polynesia, arriving multiple times at several places on the two continents, by sea as well as by land, in successive migrations that began as early as 30,000 years ago. Which was it? An international team of geneticists and anthropologists, from US, British, Canadian, Swiss, and Central and South American universities, has produced new genetic evidence that is likely to hearten proponents of the land bridge theory (our option A).
The researchers examined genetic variation in the DNA of present-day members of 29 Native American populations across North, Central and South America. They also analysed data from two Siberian groups. Their work shows that genetic diversity, as well as genetic similarity to the Siberian groups, decreases the farther a native population is from the Bering Strait. This adds to the existing archaeological and genetic evidence that the ancestors of Native North and South Americans came by the north-west route. The analysis also shows that a unique genetic variant is widespread in Native Americans across the American continents – suggesting that the first humans in the Americas came in a single migration or multiple waves from a single source, not in waves of migrations from different sources. This variant has not been found in genetic studies of people elsewhere in the world except in eastern Siberia.
The scientists say that the variant probably occurred shortly prior to migration to the Americas, or immediately afterwards. ‘We have reasonably clear genetic evidence that the most likely candidate for the source of Native American populations is in fact somewhere in east Asia,’ explained Dr Noah A. Rosenberg of the University of Michigan Medical School and the University of Michigan Life Sciences Institute. ‘If there were a large number of migrations, and most of the source groups didn’t have the variant, then we would not see the widespread presence of the mutation in the Americas,’ he added.
Moreover, the pattern that the research uncovered – that as the founding populations moved south from the Bering Strait, their genetic diversity declined – is what would be expected when migration is relatively recent, said team member Mattias Jakobsson. This is because there has not yet been time for mutations, which typically occur over longer periods, to diversify the gene pool. In addition, the study’s findings hint at supporting evidence for scholars who believe early inhabitants followed the coasts to spread south into South America, rather than moving in waves across the interior. ‘Assuming a migration route along the coast provides a slightly better fit with the pattern we see in genetic diversity.’ Rosenberg concluded. The group’s study, published by the Public Library of Science (PloS) Genetics – see www.plosgenetics.org – is one of the most comprehensive analyses of genetic data to shed light on the settlement history of the Americas.
This article is an extract from the full article published in World Archaeology Issue 27. Click here to subscribe