Popular theory on how humans populated North America can't be right, study shows

A popular theory about how the first North Americans moved from Alaska and Yukon into the U.S. and Central and South America can't be right, suggests evidence from lakes in B.C. and Alberta.

For decades, anthropologists had suggested that people entered North America from Siberia via the Bering land bridge, then spread south into the U.S. and Mexico via a corridor that opened up between the melting ice sheets in what is now Alberta and B.C. about 13,000 years ago.

But a new study by Danish, Canadian and American scientists shows that would have been impossible, as there wasn't enough food and vegetation growing in the corridor to support humans until long after people were living south of the ice sheets.

"The ice-free corridor was long considered the principal entry route for the first Americans," said University of Copenhagen PhD student Mikkel Pedersen, lead author of the study, in a news release. "Our results reveal that it simply opened up too late for that to have been possible."

The study was published today in the journal Nature.

For a long time, anthropologists considered the Clovis people to be the first culture to populate North America. They used distinctive stone tools that first appeared about 13,000 years ago and have been found from Nova Scotia to Mexico.

Because that timeline coincides with when the ice-free corridor is thought to have opened up through B.C. and Alberta, a theory was proposed in the 1960s that the Clovis people spread south through that corridor into the rest of North America at the end of the last ice age, said Charles Schweger, a University of Alberta researcher who co-authored the new study.

"There just seems to have been a good narrative, a good story," he said. "But there was very, very little data."

Evidence has been growing that the story was wrong, and some archeologists have argued it's more likely that early North Americans travelled south via the Pacific coast.

Human archeological sites as old as 14,600 years old have been found south of the ice sheet, in Oregon, Florida, Texas and even as far south as Chile.

Meanwhile, no Clovis sites have been found in Alaska or Yukon, although one dating to around 13,000 years ago has been found in Charlie Lake, B.C.. That raised the possibility that it could have belonged to people who came through the ice-free corridor.

But Schweger noted that researchers had no idea what the environment was like in the corridor when it first opened up, and whether it could even have supported humans when the ice was rapidly melting and pooling into proglacial lakes that had nowhere to drain to at that time.

'Miserable' conditions

"It must have been miserable," he suggested. "You had two major ice sheets on either side of you, you had proglacial lakes that blocked you at every turn. How did plants and animals get in there?"

Years ago, one of his graduate students tried to answer that question by analyzing ancient plant pollen from the time that the corridor opened up. That pollen is now trapped in layers of sediment at the bottom of lakes in the corridor. The analysis found that plant life was very sparse at the time when the corridor opened up.

Then Eske Willerslev, an evolutionary geneticist from the Centre for GeoGenetics at the University of Copenhagen and Pedersen's supervisor, proposed analyzing not just pollen but DNA from the ancient lake sediments.

The Danish and Canadian researchers collaborated to collect sediment cores from Charlie Lake and Spring Lake in the Peace River drainage basin straddling eastern B.C. and western Alberta — the last part of the ice free corridor to open up.

The Canadians analyzed plant material and pollen in the samples, while the Danish team focused on the DNA. The results showed that even though the corridor may have opened up 13,000 years ago, there wasn't much vegetation until 12,600 years ago, when grasses and grazing animals such as bison and woolly mammoths started to appear, along with smaller animals like jackrabbits and voles.

About a thousand years later, it was colonized by trees, moose, elk and bald eagles.

That's when humans could finally move in, Schweger suggests.

"Having a tree suddenly changed the game, because now we have fuel, we have shelter, we have a raw material that could be used for making tools."

Eventually, around 10,000 years ago, the corridor was gradually taken over by a boreal forest dominated by spruce and pine trees.

But this all happens later than the first Clovis remains in B.C. south of the ice sheet, suggesting that the Clovis people likely came from the south, not the north, perhaps following wild animals such as bison — recent evidence has shown that some bison did move north through the ice-free corridor.

Meanwhile, Schweger said, "People that used the corridor may never have come from the north."

The researchers suggest the new findings add to evidence that the first North Americans moved south along the Pacific Coast rather than inland through the ice-free corridor.