The massive glaciers of the last ice age retreated from an island off the Alaska Peninsula much earlier than once thought, suggesting that people could have been migrating along Alaska's ice-free southern coast for millennia before any corridor opened over land.
The findings — based partly on analysis of ancient pollen in lake sediments on Sanak Island — offers dramatic indirect support for a controversial theory that North America was settled by sea and not by land.
Instead of some wave of spear-chucking, bison-hunters trudging grimly across the steppes of Beringia through a gap in continental ice, the First Americans could very well have been sophisticated Alaskans paddling boats — savvy beachcombers who fished, hunted marine mammals and scavenged tide pools as they hopped island to bight and made their way from Asia into the New World.
Alaska's rich coastal bounty — after all, the ocean sets a fine table twice each day — may have pulled these prehistoric immigrants farther east and south each year.
"It is important to note that we did not find any archaeological evidence documenting earlier entrance into the continent," explained lead author Nicole Misarti, in this story posted by Oregon State University (OSU). "But we did collect cores from widespread places on the island and determined the lake's age of origin based on 22 radiocarbon dates that clearly document that the retreat of the Alaska Peninsula Glacier Complex was earlier than previously thought."
This new insight into Alaska's prehistory came out of the Sanak Island Biocomplexity Project — a long-running study of the natural history, climate and archeology of the Sanak Island region, a marine enclave about 700 miles southwest of Anchorage and 40 miles off the Alaska Peninsula. In this latest wrinkle, researchers used radiocarbon analysis to date pollen in three different lake cores. Although they were seeking evidence of ancient salmon runs, they kept finding earlier and earlier dates that led to a startling conclusion. The organic material had been deposited into open water at a time when continental ice sheets shrouded much of North America with mile-deep glaciers.
That meant the ice must have already retreated from the Sanak area — exposing a habitat that could have been used by people.
"Pollen data suggest an arid, terrestrial ecosystem by (16,300 years before present)," wrote Misarti, a marine ecologist, and seven co-authors in the study published this week in the journal Quaternary Science Reviews. "Therefore glaciers would not have hindered the movement of humans along the southern edge of the Bering Land Bridge for two millennia before the first well-recognized 'New World' archaeological sites were inhabited."
How did people reach the New World and when?
Sorting out when people migrated from Asia into the New World has become one of the most interesting debates in archaeology. For most of the 20th century, scientists believed that humans walked across the thousand-mile-wide Bering Land Bridge exposed by lower sea levels and spread out into North America through openings in the continental ice sometime about 12,000 to 13,000 years ago. These first people were thought to be big game hunters using distinctive Clovis spear points. But more and more sites have been found that date from times when this terrestrial ice-free route could not have existed, with other evidence also undercutting the theory.
"From Alaska to Brazil and southern Chile, artifacts and skeletons are forcing archaeologists to abandon the older Clovis orthodoxy and come to terms with a more complex picture of earliest American settlement," wrote John Noble Wilford in an older New York Times story with excellent background about the issue. "People may have arrived thousands to tens of thousands of years sooner, in many waves of migration and by a number of routes."
For instance, the oldest well-established archaeology sites at Monte Verde, Chile, and Huaca Prieta, Peru, date back 14,000 to 14,200 years ago, according to the OSU story. Overland routes into the New World would not have opened before 13,000 to 15,000 years ago at the very least, presenting a problem for explaining how humans would have had time to walk to South America.
The Sanak Island dates offer an alternative.
"While not proving that first Americans migrated along this corridor, these latest data from Sanak Island show that human migration across this portion of the coastal landscape was unimpeded," by glaciers after 17,000 years before present, the authors wrote in the paper. "A viable terrestrial landscape (was) in place ... well before the earliest accepted sites in the Americas were inhabited," the authors wrote.
Sanak Island research continues to be trove
Misarti — a post-doctorate researcher at Oregon State University who has accepted a position at the University of Alaska Fairbanks — conducted the latest study as part of a multi-disciplinary project that began as an investigation into the ancient Aleut culture that thrived for 5,000 years along the Alaska Peninsula.
With archaeologist Herbert Maschner at Idaho State University, over the past dozen years the Sanak Island biodiversity team has excavated hundreds of sites, including 33 ancient Sanak villages. (One recent paper describes how they examined 115,000 bits of remains and 163 pounds of ancient shellfish garbage.) Earlier this spring, the team published a detailed analysis of Sanak's prehistoric food web, showing Aleut inhabitants firmly entrenched in a network that involved 500 species with 6,000 links. Other research from the team has discovered that profound shifts in the marine climate can alter the abundance of Alaska fish and marine mammals in cycles that occur over hundreds or thousands of years.
This latest angle uncovered several kinds of evidence for when the glaciers retreated and exposed Sanak Island. First, the lake cores showed that ice was only about 70 meters thick at its height, about half as deep as previously thought. That meant deglaciation could have occurred much faster. They also found lakes ranging in age from 16,500 to 17,000 years. Most intriguing was the pollen, which suggested an environment where prowling humans would have been able to find food to eat.
"We found a full contingent of pollen that indicated dry tundra vegetation by 16,300 years ago," Misarti said in the OSU story. "That would have been a viable landscape for people to survive on, or move through. It wasn't just bare ice and rock.
"The region wasn't one big glacial complex," she added. "The ice was thinner and the glaciers retreated earlier."
Contact Doug O'Harra at doug(at)alaskadispatch.com.