New DNA evidence from ancient skeletal remains suggests that the indigenous groups of southern Alaska and the west coast of British Columbia, Canada are descendants of the first humans to settle in northwest North America more than 10,000 years ago.
The finding is based on the remains of an Alaskan individual known as Shuká Káa, who's estimated to have lived around 10,300 years ago, and contributes to a growing body of evidence that reveals the complex population history of these early American peoples.
"Our analysis suggests that this is the same population living in this part of the world over time, so we have genetic continuity from 10,000 years ago to the present," says one of the team, Ripan Malhi, an anthropologist from the University of Illinois.
The research builds on previous studies that have analysed the mitochondrial DNA of North America's most ancient peoples, but fills in some major gaps by analysing the nuclear genome instead - the 3,200,000,000 nucleotides of DNA contained in our chromosomes.
While mitochondrial DNA survives twice as long in ancient specimens as nuclear DNA, it's got one big limitation - it's only passed down from the mother to her offspring.
That means information from the male lineage is skipped over in mitochondrial DNA studies, and Malhi and his team wanted to see what the beginnings of North American settlement would look like with information from the whole population.
"Mitochondrial DNA just traces the maternal line - your mother's mother's lineage - so, you're missing information about all of these other ancestors," says one of the team, John Lindo, from the University of Chicago.
"We wanted to analyse the nuclear genome so we could get a better assessment of the population history of this region."
With permission from the current Tlingit and Haida populations of Alaska, and a number of tribes farther to the south in British Columbia, the team extracted nuclear DNA from Shuká Káa, whose remains were uncovered in the On Your Knees Cave of southeastern Alaska in 1996.
Nuclear DNA was also extracted from the remains of three other ancient humans from the coast of British Columbia, which have been dated from 6,075 to 1,750 years ago.
The oldest of these three - called 939 - died 6,075 years ago on the coast of British Columbia. Then there's a 2,500-year-old skeleton from Prince Rupert harbor in British Columbia called 302, and a 1,750-year-old skeleton from the same area called 443.
With the genetic information in hand, the researchers compared it to 156 current indigenous groups from around the world, and found a direct link between the three most recent remains and a number of existing indigenous groups on the Pacific Northwest Coast, including the Tsimshian, Tlingit, and Haida nations.
The 939 specimen in particular demonstrates the genetic continuity of the populations, with ties to both the current indigenous groups of the region, and the 10,300-year-old Shuká Káa.
"[W]e observe a trend of genetic continuity through time, which is exemplified by individual 939, who displays affinities with both the more recent Northwest Coast ancient individuals and Shuká Káa," the team concludes.
Shuká Káa himself could be what links these peoples to the populations that existed prior to North American settlement, because while he appears to be a genetic ancestor of the younger skeletons and existing populations of the area, his DNA is more closely related to ancient groups elsewhere in the world.
"Shuká Káa ... appears more closely related to groups living in South and Central America today, such as the Karitiana, Suruí, and Ticuna of Brazil's Amazon," Ann Gibbons reports for Science magazine.
"But the signal is not statistically strong, and it may simply be a sign that the tribes all share DNA from the same ancient ancestors in Asia or Beringia, where humans lived before they entered the Americas."
While the evidence also reveals for the first time that at least two different groups of people might have populated North America more than 10,000 years ago, further study is needed to confirm this, and to strengthen the case of Shuká Káa being directly linked to the current populations.
But as Rosita Worl, director of the Sealaska Heritage Institute in Alaska and a member of the Tlingit nation, told Gibbons, the evidence from the three more recent skeletons fits with Tlingit and Tsimshian oral traditions that speak of the Tsimshian people migrating west along British Columbia's Naas River to the coast, before spreading out to the north and south.
"We supported DNA testing of Shuká Káa because we believed science ultimately would agree with what our oral traditions have always said - that we have lived in southeast Alaska since time immemorial," she says in a press statement.
"The initial analysis showed the young man was native, and now further studies are showing that our ancestral lineage stems from the first initial peopling of the region. Science is corroborating our oral histories."
The research has been published in Proceedings of the National Academy of Sciences.