How Climate Change Played a Role in Ice Age Journey

By Elspeth Dehnert, E&E reporter
3/13/2014 2:06:24 PM

The climate 25,000 years ago was cold, very cold. It was the height of the last ice age, and survival required desperate measures -- especially from those in Eurasia, where food and wood fuel ran low. Some chose to migrate, but distances and directions among groups varied.

One faction ended up on the now-submerged Bering land bridge, a place where they would live in isolation for 10,000 years. They were the first Native Americans, according to a recent column in the journal Science.

Using genetic and paleoecological evidence, University of Utah anthropological geneticist Dennis O'Rourke, University of Colorado archaeologist John Hoffecker and University of London paleoecologist Scott Elias were able to bolster a long-standing theory known as the "Beringian Standstill." The hypothesis took hold in 2007, when a team from the University of Tartu in Estonia identified a set of mutations in a large sample of Native American mitochondrial DNA.

While it had long been known that the group originated from Asia, the variation in their genetic blueprint indicated they must have been isolated from their ancestors for an extended period of time before spreading throughout the Americas. The researchers identified the land mass connecting Asia with North America, the Bering land bridge, as the most likely location. But no one could explain how or why -- until now.

A 'good' place to live

About 25,000 years ago, the Earth entered what is known as the Last Glacial Maximum (LGM), a period of climate change that caused temperatures to drop and glacial masses to expand, O'Rourke said. It also drastically diminished sea levels, which, in turn, allowed the Bering land bridge to rise out of the ocean.

"People may well have simply been expanding throughout the region, and some would have found themselves on the forefront of that movement and would have been in central Beringia, on the land bridge," he said.

The reason they were able to stay there, isolated, for 10,000 years had to do with a bit of luck, a lack of options and an increasingly frigid climate.

The journey across the Bering Strait for the Native Americans who first populated North America, as pieced together from genetic data, took 10,000 years. They found refuge from the last ice age there, according to scientists. Map courtesy of Erika Tamm and Wikipedia.

"They became isolated," O'Rourke said. "There wasn't any way, without traversing long distances of terrain that were much less hospitable than where they already were, for them to remain in contact with people that were moving further and further away from them as the climate deteriorated."

While most of the region endured dry, freezing weather, pockets of "refugia," or vegetated areas not affected by climate change, remained scattered along the Bering land bridge throughout the LGM period. These unaffected areas provided the Native American founder population with moist, mild temperatures as well as adequate amounts of shrubs, trees and animals.

"The refugia were good places to live," he added. "Good foraging, good hunting and wood availability."

But their delayed entrance into the Americas was not by their own will. They were blocked during the LGM due to the coalescence of ice sheets from the northern Rockies (the Cordilleran Ice Sheet) and north-central Canada (the Laurentide Ice Sheet), which cut off access to both the coast and interior of northwest North America, Hoffecker said.

Once the LGM came to a close, however, the climate began to warm, the sea level rose and ice masses started melting away, allowing the Native American founder population to enter into North America nearly 15,000 years ago, according to the genetic record.

Bringing the theory to life

While the genetic evidence found in 2007 was "pretty robust," according to O'Rourke, many scientists disregarded the theory due to a lack of archaeological evidence, most of which would have presumably disappeared underwater after sea levels rose.

In recent years, however, several paleoecologists have collected sediment cores from areas in and around Beringia that contain plant, insect and pollen fossils. They revealed the presence of refugia -- the theory's missing link, of sorts -- which strengthened the notion that the now-submerged land mass could have been the Native American founder population's 10,000-year home.

O'Rourke, Hoffecker and Elias considered the new evidence a springboard from which to solidify the theory. So they combined forces.

"I had begun looking at a variety of different kinds of things including genetic data on different organisms, what they might tell us about what was going on in Beringia, and I was aware of other data on refugia," O'Rourke said. "John recognized that if there are these refugia and they're clustered in the lowlands of Beringia, those would be the logical places where a population could actually survive rather well" during the LGM.

Meanwhile, Elias and others discovered the presence of certain beetle species that live only in specific temperature zones, suggesting that the temperatures on the land bridge during that time would have been relatively mild compared to other regions.

By joining the old and new evidence together, researchers said the untapped archaeological proof may no longer present an obstacle for the "Beringian Standstill" theory.

"Now the genetic evidence, paleoecological and environmental evidence seem to be converging in ways that make the absence of the archaeological record, perhaps, not so surprising or problematic," O'Rourke said.

Still, they are hopeful that possible traces of refugia existing on modern-day land masses like Alaska and the Chukotka Peninsula in Russia will finally provide the missing archaeological component.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.

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