Over the last half century, researchers have assembled bits and pieces of evidence that suggest humans arrived in the western hemisphere by crossing a land bridge between modern-day Siberia and Alaska—a explanation known as the Berengia Theory. Proving that theory, however, has been slow going, with just a handful of archeological sites and artifacts discovered to illustrate the southward expansion.
Using new genomic techniques, however, researchers patched together the history of human expansion down the west coast of North America, tracking the populations' migration. The DNA also captured the marked decline of populations throughout the Americas roughly 500 years ago when European settlers and their diseases led to a collapse of the many groups, from the Incas to North American tribes.
“[The study] is confirming a picture that has been emerging” about how and when the first people arrived in the Americas, Jon Erlandson, an archaeologist at the University of Oregon tells Lizzie Wade at Science. “It challenges archaeologists to catch up with the genomics people, because they’re creating models for us that need to be tested.”
Using samples extracted from 92 skeletons and mummies aged 500 to 8,600 years, mainly from Bolivia, Chile and Peru, researchers looked at the mitochondrial DNA of the early Americans, which is passed directly from mother to child, allowing them to track 84 lineages. They also looked at the rate of mutations in the DNA, a method that helps the scientists calculate when groups of humans split off from one another.
The study, published in the journal Science Advances, suggests that the earliest Americans split off from populations in Siberia about 23,000 years ago. A group of roughly 10,000 people then spent about 6,000 years in isolation in Berengia—a landmass that once stretched between Russia and Alaska that was blocked by ice sheets.
Then, around 16,000 years ago, the DNA shows that the population boomed, likely because the ice sheets began to recede, allowing these pioneers to spread down the west coast of the Americas and access a new host of resources. It only took about 1,500 years for humans to span the entire western hemisphere, from Alaska to southern Chile. The quick colonization means early settlers likely used boats to travel the coast.
The analysis also suggests that most of the groups were content to stay where they were. Instead of traveling and mixing lineages, most populations created their own communities and bred among themselves. That remained the status quo until about 500 years ago.
“When Europeans arrived,” Bastien Llamas, a geneticist at the University of Adelaide and an author of the study explains, “some of those populations were wiped out completely.”
In fact, none of the 84 sample lineages have been found in living people, though researchers admit South American populations have not been sampled very well and there is a chance some of the mitochondrial DNA lives on.
“We knew that Native Americans living today have a relatively low genetic diversity,” Llamas tells Eva Botkin-Kowacki at The Christian Science Monitor. “Meaning it is highly likely that some time in the past, they lost some of their genetic diversity in what we call a bottleneck. Was it because of Europeans? Or was it because of the very early events that led to the peopling of the Americas?”
Llamas suggests that the answer is likely a combination of the two. Researchers estimate that 50 to 90 percent of the indigenous population of the Americas died off in the centuries following contact with European explorers, decimated by diseases like smallpox, which spread rapidly across the hemisphere and was sometimes used to infect native populations on purpose.
While the paper sheds new light on the history of people in the Americas, it’s also important for showing how genetics and archeology can work hand in hand. “I think that DNA in archeology is just going to get stronger as time goes on and I would hope that more archeologists would begin thinking and sampling in such ways that we can explore the genetic evidence that’s around us all the time,” Dennis Jenkins, an archeologist at the University of Oregon tells Botkin-Kowakcki. “It’s going to become a really great tool for archeology.”