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. "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. "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. Estimates are needed to check the veracity of competing scenarios of ancient migration to the Americas, including Torroni's. "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.

Source: ScienceNews (8 January 2009)