February 28, 2014
Bering Land Bridge Was Home To Early Natives For 10,000 Years: Study
April Flowers for redOrbit.com - Your Universe Online
After the ancestors of modern day Native Americans left Asia, they spent approximately 10,000 years living in the shrubby lowlands of the Bering land bridge, according to genetic and environmental evidence. There is no available archaeological evidence, however, because it drowned beneath the Bering Sea when the sea levels rose about 18,000 years ago.
Their argument, published as a Science Perspectives column, seeks to reconcile existing genetic and paleoenvironmental evidence for human habitation on the Bering land bridge, also known as Beringia. However, this is a challenge with the absence of archaeological evidence.
The team notes that Science Perspectives columns do not feature research by the authors. They are meant to highlight and/or provide context for exciting new research in a field or across fields.
The team discuss the "Beringia Standstill," a that theory was first proposed in 1997 by two Latin American geneticists and refined by a team of researchers led by the University of Tartu in Estonia in 2007.
Hoffecker said that this theory was not popular outside of the genetic community, but the team's paper adds credence to the Beringia Standstill theory by linking the genetics to paleoecological evidence.
The cumulative evidence, according to O'Rourke, indicates the ancestors of Native Americans lived on Beringia "in the neighborhood of 10,000 years," from approximately 25,000 years ago until 15,000 years ago when they began to migrate into the Americas—facilitated by the melting glacial ice sheets and opened migration routes.
"Nobody disputes that the ancestors of Native American peoples came from Asia over the coast and interior of the land bridge" during an ice age called the "last glacial maximum," which lasted from 28,000 to at least 18,000 years ago, O'Rourke says.
During this ice age, the ice sheets extended south into what is the modern day Pacific Northwest, Wyoming, Wisconsin and Ohio. Large areas of Siberia and Beringia were cold, but without glacial formations. Beringia had a warm summer climate, however, caused by North Pacific circulation patterns that brought moist and relatively warm air to the region.
Part of the evidence for this climate system is garnered from a study by Elias and his colleagues where they analyzed certain beetle species that live in very specific temperature zones. These beetles can be used as tiny thermometers. The findings indicated that temperatures there were relatively mild during last glacial maximum that ran from about 27,000 years to 20,000 years ago, or perhaps only slightly cooler in some regions than temperatures in the region today.
"Archaeologists have not given much credence to the idea there was a population that lived on the Bering land bridge for thousands of years," said O'Rourke, adding that this was probably because of the lack of archaeological sites and the inhospitable nature of open, treeless tundra steppes.
ARCHAEOLOGY AND GENETICS
Sediment cores have been drilled from the Bering Sea and Alaskan bogs in recent years by paleoecologists—scientists who study ancient environments. The pollen, plant and insect fossils found in the cores suggest that the Bering land bridge wasn't just barren, grassy tundra steppe. Instead, it was dotted by "refugia"—refuges where there were brushy shrubs and even trees such as spruce, birch, willow and alder.
"We're putting it together with the archaeology and genetics that speak to American origins and saying, look, there was an environment with trees and shrubs that was very different than the open, grassy steppe. It was an area where people could have had resources, lived and persisted through the last glacial maximum in Beringia," O'Rourke says. "That may have been critical for the people to subsist because they would have had wood for construction and for fires. Otherwise, they would have had to use bone, which is difficult to burn."
The authors theorize that a population of hundreds to possibly thousands of people stayed in the area, possibly because of the presence of wood in some places to use as a fuel to supplement bone, which burns hot and fast. At least some wood is needed to make bone a practical fuel source.
"A number of supporting pieces have fallen in place during the last decade, including new evidence that central Beringia supported a shrub tundra region with some trees during the last glacial maximum and was characterized by surprisingly mild temperatures, given the high latitude," said Hoffecker, of CU-Boulder's Institute of Arctic and Alpine Research.
Thick glacial ice sheets extended south into what now is the northern United States during the last glacial maximum, dropping sea levels by as much as 400 feet. As the glaciers melted, sea levels once again rose, reaching their current levels about 6,000 years ago.
Siberia and Alaska were linked by the Bering land bridge during the long glacial period. The name, Bering land bridge, is a bit of a misnomer as it was really a huge swath of land north, in between and south of Siberia and Alaska, at the present sites of the Chukchi Sea, the Bering Strait and the Bering Sea, respectively.
When Beringia was at its largest extent, it measured around 1,000 miles from north to south and as much as 3,000 miles from Siberia's Verkoyansk Range east to the Mackenzie River in Canada.
O'Rourke says that the theory that humans inhabited the Bearing land bridge for 10,000 years "helps explain how a Native American genome (genetic blueprint) became separate from its Asian ancestor."
"At some point, the genetic blueprint that defines Native American populations had to become distinct from that Asian ancestry," he explains. "The only way to do that was for the population to be isolated. Most of us don't believe that isolation took place in Siberia because we don't see a place where a population could be sufficiently isolated. It would always have been in contact with other Asian groups on its periphery."
"But if there were these shrub-tundra refugia in central Beringia, that provided a place where isolation could occur" due to distance from Siberia, O'Rourke says.
The team points to the Tartu study which involved mitochondrial DNA (mDNA), which is the genetic information passed on by mothers, sampled from 600 Native Americans through the Americas. The study findings revealed that the unique genome or genetic blueprint of Native Americans arose sometime before 25,000 years ago. However, it did not spread through the Americas until about 15,000 years ago.
"This result indicated that a substantial population existed somewhere, in isolation from the rest of Asia, while its genome differentiated from the parental Asian genome," O'Rourke says. "The researchers suggested Beringia as the location for this isolated population, and suggested it existed there for several thousand years before members of the population migrated southward into the rest of North and, ultimately, South America as retreating glaciers provided routes for southern migration."
"Several other genetic-genomic analyses of Native American populations have resulted in similar conclusions," he adds.
"For a long time, many of us thought the land bridge was a uniform tundra-steppe environment" – a broad windswept grassland devoid of shrubs and trees, O'Rourke says. This was disproved by sediment cores recently drilled from the Bering Sea and the coast of Alaska that contained pollens of trees and shrubs.
That "suggests Beringia was not a uniform tundra-steppe environment, but a patchwork of environments, including substantial areas of lowland shrub tundra," O'Rourke says. "These shrub-tundra areas were likely refugia for a population that would be invisible archaeologically, since the former Beringian lowlands are now submerged."
"Large herd animals like bison or mammoths likely lived on the highland steppe tundra because they graze. Many smaller animals, birds, elk and moose (which browse shrubs instead of grazing on grass) would have been in the shrub tundra," he adds.
While previous studies suggest that the lowlands of Beringia had summer temperatures somewhat identical to those of today, O'Rourke says that the "local environments likely were not as daunting as many have assumed for years. They probably hunkered down pretty good in the winter though. It would have been cold."
Researchers studying how early Native Americans moved south along the Pacific coast as the glaciers receded and sea levels rose have long cited the notion that rising sea levels covered evidence of human migration to the Americas. O'Rourke says, however, that this theory has not been used before to explain the scarcity of archaeological sites in Alaska and Siberia—areas that were highlands when the land bridge was exposed.
O'Rourke's team note that archaeological sites must be found in Beringia if the long human layover theory is to be confirmed. Most of these sites are underwater; however, some evidence of human habitation in shrub tundra might remain above sea level in low-lying portions of Alaska and eastern Chukotka.
Elias says, "This work fills in a 10,000-year missing link in the story of the peopling of the New World."
After the maximum, two sets of archaeological remains dated to less than 15,000 years ago.
"One represents a late migration from Asia into Alaska at that time," he says. "The other has no obvious source outside Beringia and may represent the people who are thought to have sheltered on the land bridge during the glacial maximum.
"If we are looking for a place to put all of these people during the last glacial maximum, Beringia may be the only realistic option," concludes Hoffecker.