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Migrants settled New World in tandem

The first Americans may have traveled across a land bridge and south from Alaska in two separate groups at about the same time

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11:58am, January 8, 2009
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Diversity ruled among the first American settlers. Within a relatively short time span, at least two groups of people trekked across a land bridge from Asia to Alaska and then went their separate ways, one down the Pacific Coast and the other into the heart of North America, a new genetic study suggests.

A team led by geneticist Antonio Torroni of the University of Pavia in Italy estimates that these separate migrations into the New World occurred between 17,000 and 15,000 years ago.

Even more populations with distinct genetic signatures and languages may have crossed a now-submerged strip of land, known as Beringia, that connected northeastern Asia to North America within that relatively narrow window of time, the scientists also contend in a paper published online January 8 and in the Jan. 13 Current Biology.

“Whereas some recent investigators had thought that a single major population expansion explained all mitochondrial DNA variation among Native Americans, this new report revives earlier ideas about multiple expansions into the New World,” comments molecular anthropologist Theodore Schurr of the University of Pennsylvania in Philadelphia.

Torroni’s team analyzed entire genomic sequences of mitochondrial DNA, the genetic material in cells’ energy-generating units that gets passed from mothers to children. Genetic data came from Native American groups in North, Central and South America that had already provided blood samples for study. The researchers focused on the disparate geographic distributions of two rare mitochondrial DNA haplogroups — which are characterized by a distinctive DNA sequence derived from a common maternal ancestor — that still appear in Native Americans.

“Our study presents a novel scenario of two almost concomitant paths of migration, both from Beringia about 15,000 to 17,000 years ago, that led to the dispersal of the first Americans,” Torroni says.

If that hypothesis holds up, he adds, it suggests that separate groups of New World migrants founded prehistoric Native American tool traditions independently in eastern and western North America. The new findings also raise the possibility that the first Americans spoke languages from more than one language family, in Torroni’s view. Linguists have debated for decades whether late–Stone Age migrants to the Americas spoke tongues from a single language family that would have provided a foundation for many later Native American languages.

Despite the new evidence, scientific consensus on how and when the New World was settled remains elusive.

“Peopling of the Americas is a hard problem,” remarks geneticist Jody Hey of Rutgers University in Piscataway, N.J. “My guess is that it will be a couple more years before we have a good picture of what happened.”

Hey takes a skeptical view of the new study. Different present-day Native American populations display signature mitochondrial DNA patterns, so it’s not surprising that rare haplogroups would be unique to separate regions, he says. But Torroni’s analysis doesn’t explicitly address whether their genetic data more closely reflect a single migration, a pair of simultaneous migrations or some other pattern of population movements, in Hey’s view.

Some climate reconstructions suggest that an ice-free corridor from Alaska into North America wasn’t passable until around 12,000 years ago, Schurr says. If so, that creates a major roadblock for Torroni’s scenario of an inland migration at least 15,000 years ago.

Investigators also differ on how best to study ancient population movements using genetic data.

Two approaches currently dominate DNA-based attempts to explain the population movements and evolution of people and other animals, Hey notes. Some researchers track today’s geographic distribution of different haplogroups and generate tree diagrams that portray patterns of ancestry, as Torroni’s group did. Other investigators, such as Hey, use statistical methods to test whether genetic data fit simple models of how populations might have been structured.

In 2005, Hey took the model-based approach to examine mitochondrial DNA from northeastern Asians and Native Americans. He concluded that a single group of New World settlers, consisting of perhaps 70 fertile adults, crossed Beringia no more than 14,000 years ago (SN: 5/28/05, p. 339).

In this study, Torroni and his colleagues got different results by searching a large genetic database for mitochondrial DNA. The team found 55 unrelated individuals who displayed either of two rare Native American haplogroups, called D4h3 and X2a, identifying 44 instances of haplogroup D4h3 and 11 instances of haplogroup X2a.

Further analyses indicated that the D4h3 haplogroup spread into the Americas along the Pacific Coast, rapidly reaching the southern tip of South America. Estimated ages of D4h3 sequences from ChiIe are nearly as old as the estimated time of the Beringia crossing.

In contrast, haplogroup X2a crossed Beringia and spread through an ice-free corridor in what’s now western Canada, eventually clustering in the Great Lakes area, the new study suggests.

Examination of an additional 276 mitochondrial DNA sequences, all from unrelated people, representing the six haplogroups common in Native Americans indicated that those genetic types entered the Americas at about the same time as the two rare haplogroups did.

“Within a rather short period of time, there may have been several entries into the Americas from a dynamically changing Beringian source,” Torroni says.

Extensive mitochondrial DNA data have yet to be obtained for many Native American populations, Schurr cautions. Hence, precise age estimates don’t exist yet for the major New World haplogroups and their sub-branches. Such estimates are needed to check the veracity of competing scenarios of ancient migration to the Americas, including Torroni’s.

Such scenarios also include recent mitochondrial DNA studies that argued for a single founding group of New World migrants. One research team concluded that northeastern Asians reached Beringia as early as 40,000 years ago but had to wait for ice sheets to melt before entering the Americas at least 20,000 years later (SN: 2/16/08, p. 102).

A separate investigation, published online September 17 in PLoS ONE, concluded that migrants from northeastern Asia stayed in Beringia only a few thousand years before crossing into the New World about 16,000 years ago.

“The more we learn about this story, the more we realize that there is much more to understand about this segment of human history and migration,” Schurr says.

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