"One if by land, two if by sea." In Henry Wadsworth Longfellow's poem about Paul Revere's ride, a seminal event in the American Revolution, the number of lanterns signaled the route by which the British were coming. But for researchers studying the arrival of the first Americans, the question is still undecided: Did the immigrants follow a coastal route from their homelands in Asia and Alaska, or did they travel inland? A new study concludes that they did both.
The original colonizers of North and South America came from eastern Asia and migrated to the Americas after spending some time--perhaps several thousand years--in a region called Beringia, which included parts of Siberia and Alaska and the land bridge that once connected them. Recent archaeological and genetic evidence has convinced most researchers that the colonists had already arrived in the Americas at least 15,000 years ago. But scientists have long debated what routes they took. The traditional view is that they tramped through an "ice-free corridor" that opened up in modern-day Canada at the end of the last Ice Age. More recently, however, some archaeologists and geneticists have argued that the first Americans marched as far south as Chile along the Pacific coast, which was also free of ice at the time.
To find more clues to the puzzle, an international team led by population geneticist Antonio Torroni of the University of Pavia in Italy analyzed genetic markers in the mitochondrial DNA (mtDNA) of modern-day Native Americans, the descendants of those ancient travelers. Other teams have also analyzed such markers, called haplogroups, but they have mostly concentrated on the five most common ones, which have failed to resolve the issue. Torroni's team instead focused on two relatively rare haplogroups, called D4h3 and X2a. In 2007, researchers isolated D4h3 from a 10,000-year-old skeleton found at On Your Knees Cave in Alaska, suggesting that this haplogroup was indeed harbored by prehistoric people living in Beringia. X2a, although very rare overall in the Americas, is quite common today among the Ojibwas of northern Canada.
The group sequenced mtDNA genomes of 55 people with Native American heritage, 44 harboring the D4h3 haplogroup and 11 with the X2a haplogroup. The researchers found that nearly all of the D4h3 haplogroups were from people living in South America, whereas all of the X2a haplogroups came from the United States or Canada. An estimate of the age of both haplogroups suggested that they arose about 16,000 years ago. The team concluded that the two haplogroups are evidence of two migratory waves out of Beringia at about the same time, one that took the coastal route to South America and one that traveled the ice-free corridor to the Great Lakes and Great Plains. In their report, published online today in Current Biology, Torroni and his co-workers also suggest that the two groups, which must have been genetically separate even though they both inhabited Beringia at about the same time, spoke different languages.
Michael Waters, an archaeologist at Texas A&M University in College Station, says that the genetic study is "very stimulating" and consistent with the "solid archaeological evidence" of human occupation on the Chilean coast and in Wisconsin by about 15,000 years ago. And Sandro Bonatto, a geneticist at the Federal University of Rio Grande do Sul in Porto Alegre, Brazil, says that "the data and basic results are solid." But Bonatto, who favors the theory of a single coastal route for the peopling of the Americas, says that the team's conclusion that the X2a haplogroup represents a second, inland route is unconvincing because the distribution of this very rare genetic marker might be due to chance. Bonatto and other researchers are also not convinced that the two genetic markers corresponded to different language families. "This is too speculative," says geneticist Ripan Malhi of the University of Illinois, Urbana-Champaign.