The first Americans 'island hopped' down the west coast past the glaciers

Two new studies released this week help to tell the story of the first Americans.

One looks at geological evidence to reveal the possible path they took — threading their way down the west coast of British Columbia between water and massive glacial ice sheets, perhaps as early as 17,000 years ago, and the second looks at the genetic legacy of those first people: showing how they diversified and spread through North and South America to become the ancestors of modern indigenous people on those two continents.

Part 1: A coastal pathway to the Americas

Towards the end of the last ice age —  around 20 thousand years ago — a hardy group of people were migrating across the Bering Land Bridge that, at the time, joined Siberia and Alaska.

• Research paper in Science Advances

They were on their way to become the first people in the Americas, but they encountered a major obstacle. Massive ice sheets stretched across northern Canada blocking their way south. 

We know they made it pas the ice sheets.  Archaeologists have found evidence that by at least 13,000 years ago, the descendents of these people had spread across large parts of North and South America, well south of the ice. That leaves a major mystery - how did they get past those ice sheets?  

Many modern archaeologists think the best bet is that they took a coastal route — dodging the ice by island-hopping along the Pacific Coast. And recently, a team led by Alia Lesnek, a Ph.D. candidate from the Department of Geology at the University at Buffalo, found geological evidence in favour of this coastal migration theory, which shows that the West coast was a viable route for the first settlers to reach the south.

"We found glaciers covering this area starting around 20,000 years ago started to retreat 17,000 years ago," says Lesnek. "This is really important because previously, we had not known if there was any open land surface along the coastal route by the time people would've been travelling through."

Her research took her to the Alexander Archipelogo in Alaska —  a group of islands off the west coast of Canada.

They collected rock samples from four islands in the region —  Dall Island, Suemez Island, Baker Island and Warren Island — and used Beryllium-10 dating on the rock surfaces.  

Beryllium-10 is an isotope that's only produced when rock surfaces are exposed to the sun — the longer the exposure, the more beryllium-10 exists inside the rocks.

"By going out and measuring how much Beryllium-10 is in the rock surface," explains Lesnek, "we know how long it's been exposed and that tells us when the glaciers retreated."

Beyond geological evidence, earlier findings of ancient ringed seal bones found in caves in the region also dated back to about the same time period, which suggests that they could've been a source of food for travellers when they passed by.

The researchers are careful to say that this does not mean that early settlers definitely took the coastal route to spread into the Americas, and added they only looked at one section of the coast.

However, their work does suggest that the coastal theory of migration is viable.  Lesnek hopes to study other locations along the coastline and collaborate with archeologists to build on this study.  

"Our study is an important step towards finding potential evidence of human occupation," says Lesnek. "Archeologists can use the information we provide to guide them in finding potential sampling sites."  

Part 2: Using genetics to trace the spread of a population

In the future, archaeologists will perhaps be able to provide more information about how humans migrated down from Alaska and spread out into the Americas. But that's not the only tool that scientists have to understand early migration.

• Research paper in Science

A second study released this week uses genetics to add to the story that was told by geology and archaeology about what happened after they came down Alaska.

Dr. Genevieve Dewar is an Assistant Professor in the Department of Anthropology at the University of Toronto's Scarborough Campus, and co-author of the new study.

"The earliest migration actually involves what would become two different lineages," says Dewar. "They migrated across the Bering land bridge and down the coast into North America, and then split into two different ancestries."  

The takeaway here is that there's a lot more interaction and dynamism involved in human migrations than we had picked up on before.- Genevieve Dewar

That separation has been known for a long time. One group is referred to as the northern branch — they migrated eastward to the Great Lakes region after the split, while the other is referred to as the southern branch — they continued southward along the coast, eventually reaching deep into South America.

What Dewar's study found is that those two populations didn't stay apart after the separation. She thinks the two groups were separated for a few thousand years, during which their genomes became distinct through the process known as genetic drift.  But her study found that somehow the populations reconverged as the southern branch travelled south along the coast into Central and South America.

They think the initial separation probably happened between 18,000 and 15,000 years ago, but can't say where the reconvergence happened, or the number of times it happened.

In a sample of contemporary Central and South American populations that was analyzed, researchers found a significant proportion of "northern" genes in their genome, ranging between 42 percent to as high as 71 percent.

"The takeaway here is that there's a lot more interaction and dynamism involved in human migrations than we had picked up on before," says Dewar.

Dewar's study also supports the coastal migration theory as some of the ancient genomes that were studied came from sites in California before 13,000 years ago. This suggests it's plausible that early Americans took the coastal route down from Alaska as they spread across the Americas.

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