Sunday, October 30, 2011

Tracing the Paths of the First Americans

The last great peopling of the continents—when humans finally spread into North and South America—is among the most mysterious. This much is known: Perhaps about 20,000 years ago, hunter-gatherers in Asia traveled east into the Bering Strait land bridge, where they lingered for a while in the now largely submerged region known as Beringia. Then they spread rapidly into North and South America, reaching Chile by at least 14,000 years ago.
           But many of the details remain unclear. Was there only one wave of migration or many? How long did people stay in Beringia, and how did populations interact after they took up residence from the Yukon to Tierra del Fuego?
In a special issue of the American Journal of Physical Anthropology, six research teams used the genetics of living and ancient people to probe this momentous migration. The findings support earlier indications that the Paleoindians, the ancestors of today’s Native Americans, stem from a single Asian source population. But the data also suggest that this population may have become genetically quite diverse during thousands of years of Beringian occupation. Because of admixture with Europeans, it’s difficult to get a clear picture of Paleoindians from living Native Americans. So researchers have sought ancient DNA from skeletons.
More than 60 such studies are reviewed by a team led by anthropologists Jennifer Raff and Dennis O’Rourke of the University of Utah in Salt Lake City. Unlike the situation on the other continents, now including Australia, researchers in the Americas lack a full nuclear genome of a native; most of the studies are of mitochondrial DNA (mtDNA) from sites 6000 years old or younger. But four sites feature five individuals older than 8000 years, including the 14,000-year-old Paisley Cave in Oregon (Science, 4 April 2008, p. 37).
Surprisingly, each of these five ancient individuals featured a different genetic marker, called a haplotype, out of the 15 currently recognized “founder haplotypes” thought to represent the genetically diverse founder population. To some researchers, this result supports the so-called Beringian Incubation Model, which suggests that Asian migrants occupied Beringia for thousands of years before moving on to the Americas, and shows that their genomes diversified in this time. That diverse population eventually headed east and south to give rise to all later Paleoindians and Native Americans. A few years ago, University of Florida, Gainesville, anthropologist Connie Mulligan and her colleagues analyzed mtDNA and archaeological evidence to conclude
that these Beringian populations might have become genetically isolated from mainland Asians by 30,000 years ago—and then stayed in Beringia for as long as 15,000 more years.
The ancient DNA findings “do suggest that there was a fair amount of diversity in Beringia just before the peopling” of the Americas, says geneticist Ripan Malhi of the University of Illinois, Urbana- Champaign. But although most researchers now accept that there was a single migration from Asia, O’Rourke cautions that the evidence cannot entirely rule out the alternative scenario of multiple migrations.
However the first Americans arrived, the mtDNA evidence shows that by about 5000 years ago, the basic geographic distribution of their descendants was set. With only a few exceptions, the pattern of distribution of genetic markers in North and South America has been stable for all that time.
Indeed, once early Americans got settled, they didn’t move around too much, according to work by Sohini Ramachandran of Brown University and Noah Rosenberg of the University of Michigan, Ann Arbor. Their paper supports a hypothesis by University of California, Los Angeles, biologist Jared Diamond that people—and their technology and domestic animals—travel more quickly and easily along lines of latitude, where they experience similar climatic and environmental conditions, than along longitude lines, which would take them into diverse environments. Diamond argued that the east-west orientation of Eurasia fostered the spread of technology among peoples and helped Eurasians achieve technological dominance over Americans living along a north south continental axis. Ramachandran and Rosenberg compared genetic variation in 39 Eurasian populations and 29 Native American populations to test that theory. Such variation can serve as a proxy for the speed of past population movements, Ramachandran explains: Faster movements mean more genetic exchanges among groups and thus reduce variation, whereas slower movements foster isolation and more genetic variation. Ramachandra  and Rosenberg found much more genetic variation along the long axis of North and South America than along the east-west axis of Asia and Europe. The findings “seem consistent with Diamond’s hypothesis,” says anthropologist Brian Kemp of Washington State University, Pullman. Diamond agrees. “With this paper, human gene flow joins crop diffusion and state government spread as important phenomena affected by the differing axisorientations of Eurasia and the Americas,” he says.
Despite this recent progress, researchers agree that more Native American samples— especially ancient DNA from very early sites—are key to filling in the blanks in the story of the Americas. That means closer collaborations with sometimes reluctant Native American groups, researchers say (Science, 8 October 2010, p. 166). But the collaboration can work, Malhi says Canada’s Heiltsuk people agreed to let his lab sample 5000-year-old human remains from Namu, in British Columbia, shortly before the bones were ceremonially reburied earlier September.



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