Is theory about peopling of the Americas a bridge too far?

The Standstill Hypothesis’s pedigree stretches back almost to Columbus. Less than a century after the explorer landed in the New World, a Spanish missionary wrote that the first humans to reach the Americas might have come by land from Asia. The term “Beringia” was coined in 1937 by Swedish botanist Eric Hultén. As defined today, Beringia stretches from Siberia’s Verkhoyansk Mountains in the west to the Mackenzie River in Canada in the east. It includes the now-vanished Bering Land Bridge, the corridor between Alaska and Russia exposed by falling seas during the last major ice age. At the time, an immense ice sheet sheathed most of Canada, blocking access from Beringia to the Americas.

The notion of Beringia soon became useful across disciplines. In the 1970s, biological anthropologist Emőke Szathmáry, now an emeritus professor at Canada’s University of Manitoba, and physical anthropologist Nancy Ossenberg of Canada’s Queen’s University proposed that genetic and physical similarities between two modern-day Native American groups had arisen when different founding populations exchanged genes while living in Beringia (1). In the 1990s, Brazilian scientists wrote that the people who became the Native Americans might have “settled and diversified” in Beringia, building up genetic diversity before entering the New World (2).

A decade later, Estonian geneticist Erika Tamm and colleagues were puzzling over a trove of mitochondrial DNA, which is passed down only from mother to child. Tamm and her colleagues noticed that Native American mitochondrial DNA includes four unique variations absent in related Asians.

When the Tamm group read the Brazilians’ work, the idea of a Beringian layover “fit with the patterns we were seeing,” says team member Ripan Malhi of the University of Illinois at Urbana–Champaign. The result was a landmark 2007 paper making the case for a “Beringian standstill” (3). The authors argued that people leaving Asia had “paused in Beringia,” isolating themselves from their ancestors so long that their mitochondrial DNA evolved new variations seen only in their descendants in the New World.

In this scenario, the Asian emigrants probably reached Beringia before the full brunt of the Last Glacial Maximum, a period of glaciers and fierce cold that lasted from roughly 27,000–19,000 years ago. Once in Beringia, the emigrants could go no further, thanks to the ice sheets blocking the door to the rest of the Americas. Tamm’s group hypothesized that people were isolated in Beringia for up to 15,000 years until they could venture into the unknown lands to the east.

The model was popular from the start, Malhi says, but “for a really long time, there were no formal simulations or actual testing of this hypothesis.” That’s no longer the case.

In the past 2 years, several powerful studies have buttressed the original genetic findings. One large consortium of scientists from Australia, the United States, and elsewhere focused, as Tamm’s group did, on mitochondrial DNA, some of it from Native American mummies dating back hundreds of years. The results showed Native Americans were genetically isolated from their source population for 2,400–9,000 years (4). Another large consortium examined nuclear DNA and found evidence for an ancient separation between the ancestors of modern-day Siberians and modern-day Native Americans. This team estimated the separation at 8,000 years, perhaps less (5). The fact that nuclear DNA points to the same conclusions as the mitochondrial DNA, Kitchen says, is “reaffirmation that the mitochondria [effect] we’re seeing is real.”

The new mitochondrial DNA results suggest only 2,000 women gave rise to the inhabitants of the New World, says Lars Fehren-Schmitz, an author of the mitochondrial DNA paper and a human ecologist at the University of California, Santa Cruz. That translates to a standstill population numbering in the few tens of thousands. Fehren-Schmitz and his colleagues also modeled the capacity of Beringia during the Last Glacial Maximum, given the vegetation and fauna. They found that 18,000–54,000 people could have lived off the land, which is roughly in line with their population estimate.

In early April, Malhi and his colleagues published yet more genetic evidence providing subtle evidence of a standstill (6). His group found that four prehistoric individuals—including one from more than 10,000 years ago—who lived on the Alaskan and British Columbian coasts had DNA characteristic of the so-called northern lineage of Native Americans. People of that lineage still live in the region, as well as in the northern interior of the continent. Malhi says that one possible explanation for the lineage’s appearance is that it existed in Beringia. He confesses to having harbored serious doubts about the Standstill Hypothesis after hearing of another mechanism that could explain the genetic patterns. But his new paper (6) has made him more of a believer. “I’m more convinced than I was a year ago,” he says.

Also supporting the hypothesis’s validity is new data from Bluefish Caves, an archaeological site in the Canadian Yukon. Researchers working there in the 1970s and 1980s excavated animal bones with signs of human butchering. Now scientists have used radiocarbon techniques to show that people occupied the site as early as 24,000 years ago, the heart of the most recent severe ice age (7). If confirmed, the data are strong evidence that “somebody was in Beringia during the Last Glacial Maximum,” says archaeologist John Hoffecker of the University of Colorado, Boulder.

Humans clearly lived on the other side of Beringia too. At several Paleolithic sites on the Yana River in far northern Siberia, researchers found mammoth-ivory artifacts including a tusk that had been scraped to make spear points or the like. The artifacts date to between 21,000 and 24,000 years ago, says Vladimir Pitulko of the Russian Academy of Sciences, who heads research at the Yana sites. Those dates, part of research that has been accepted for publication (8), confirm humans did not abandon Beringia even when the climate was far chillier and drier than today, Pitulko says.

But Pitulko allows that conditions at Yana River might have gotten tiresome. If your local temperature averages 5 degrees, “you’ll probably think about getting to some other place 5 degrees warmer, which makes a big difference,” he says.

Humans seeking more hospitable real estate could aspire to southeastern Beringia, which Hoffecker calls the “Riviera” of the glacial maximum. During that cold and arid period, Beringia’s most productive ecosystem was the area now just offshore of southern Alaska, according to modeling of the ancient vegetation based on local temperatures, carbon-dioxide levels, and other conditions. Ice-age sediment drilled from the nearby sea floor included pollen from birch and alder (9). Unless that pollen eroded out of older sediments, it indicates relatively mild conditions, says paleoecologist Nancy Bigelow of the University of Alaska Fairbanks. The whole of the Arctic was dominated not by grass but by protein-rich broad-leaf plants, according to a recent study of plant DNA embedded in permafrost (10). Those plants helped support herds of mammoths, bison and other large game.

Linguistic as well as genetic evidence implies that humans were situated to take advantage of this bounty. Researchers examined languages from Siberia, Alaska, and northern North America, looking for

“These were not people who are scared away by a little bit of snow.”

—Lars Fehren-Schmitz

structural traits, such as the presence of plural pronouns. Using these traits to build an evolutionary-tree–like diagram, they found that the Siberian languages evolved alongside the Native American languages (11). This finding supports “at least a period of occupation and diversification within the Beringian area, and probably somewhere within the southwestern Alaskan area,” says study coauthor Gary Holton of the University of Hawai’i at Manoa.

Despite all of the evidence, the case for a Beringian standstill remains open. The genetic patterns seen in Native Americans could have been created by “genetic surfing,” Malhi says. That’s rapid genetic change seen when a small population expands rapidly in a large area, as happened when humans finally spread across the New World; Malhi says it could have produced the DNA relationships researchers see without a long genetic isolation.

The sparse archaeological record is particularly troubling. Bluefish Caves provides some of the only archeological evidence for human occupation of Beringia during the glacial maximum. But that timing provokes skepticism, and if it’s correct, it places humans at a spot in Beringia close to the ice sheet, which was much more barren than the land bridge. “For people to be that far north at Last Glacial Maximum times you would need a lot of compelling evidence to demonstrate that,” says Ben Potter of the University of Alaska Fairbanks. “I’m not convinced.”

There is a middle course. Perhaps people paused on their way to the New World, but not in Beringia. Genetic data do not specify where the standstill took place, only that the people who gave rise to Native Americans were genetically isolated. “It’s unlikely to actually be in Beringia, because we don’t have evidence of people there between 25,000 and 14,000 years ago, the time period when this supposedly took place,” argues Potter, who points to alternative spots in Siberia.

One recently proposed option centers on the Japanese island of Hokkaido and the neighboring Russian island called Sakhalin. During the glacial maximum, Sakhalin was connected to Hokkaido, the mainland, and some of the nearby Kuril Islands. Starting 26,000 years ago, signs of human habitation on Hokkaido, Sakhalin, and the Kurils rose sharply, according to the number of archaeological sites and other data (12). That implies an influx of people, says archaeologist Ian Buvit with the National Park Service in Anchorage. And this happened shortly before the number of human sites in southern Siberia fell sharply, as if people were fleeing from one place to another.

But every proposed incubation site has its weaknesses. It would’ve been difficult for people living in Siberia to be genetically isolated from their neighbors, as the genetics patterns demand. Genetic isolation requires not just harsh climate but physical barriers, such as mountains, says Fehren-Schmitz. “These were not people who are scared away by a little bit of snow,” he adds. It also seems likely that the standstill site was within striking distance of North America. The models show that people entered the Americas from wherever they were waiting as soon as the glaciers began to pull back. Either they were on the doorstep of the Americas at that time, or the instant the ice sheets retreated they sprinted “very, very fast,” Kitchen says.

Finding an Asian site that checks all of the boxes is not easy, Hoffecker says, although he’s open to credible alternative locales from critics. “I’m all ears,” Hoffecker says. “But I haven’t heard it yet.”

Beringia is the most plausible site to many researchers, but it’s also problematic. The region is nearly devoid of signs of a human presence during the Last Glacial Maximum; whether that’s because there wasn’t such a presence or it hasn’t been found is unclear. The argument is likely to continue until researchers recover ancient DNA from a population that shows initial signs of genetic isolation. Or perhaps researchers will stumble across a site that holds undeniable evidence of a human presence during the glaciers’ reign. “I hope someday it is going to be found,” Pitulko says. “I am optimistic. … It might be this coming summer. Who knows?”