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Is theory about peopling of the a bridge too far? Some argue that humans flourished for thousands of years on a fertile intercontinental until melting opened the route to the Americas. But major gaps in the evidence remain.

Traci Watson, Science Writer

Twenty thousand years ago, the Earth was hostile thousands of years. An idea originally devised to territory. Hulking glaciers smothered much of North explain genetic variation among Native Americans is America. Deserts claimed swathes of every , now reaping support from fields as disparate as linguis- and winters were 40 degrees colder than today in tics and paleoecology. Recent analyses of massive some spots. But far to the north lay a sprawling, unlikely DNA databases and ancient DNA have only shored haven that stretched from modern-day to . up the genetic underpinnings of the theory, known This was Beringia, a refuge of and grasslands as the Beringian Standstill Hypothesis. And archae- dotted with wildflowers, ponds, and scrubby ological work offers tantalizing—though disputed— trees. and roamed its plains for thou- evidence that people were living in Beringia at the sands of years. And, if a controversial theory is to be hypothesized time. believed, people did too. Skeptics remain, and proponents admit that it will Researchers once envisioned Beringia, an area that be a challenge to find conclusive proof that people included the now-submerged land bridge connecting lingered in a landscape now drowned by more than to Siberia, as flyover country, a mere corridor to 100 meters of seawater. Still, the idea “has legs,” says the New . But growing evidence suggests that evolutionary anthropologist Drew Kitchen of the Uni- bands of hunter-gathers from tarried there for versity of Iowa, who worked on the hypothesis after

About 20,000 years ago, when the Earth was still in the throes of the last major ice age, humans are thought to have inhabited Beringia, which stretched from modern-day Canada to Siberia. The general vicinity includes well-established archeological sites (labeled here) from before and after that ice age. But little evidence supports the presence of humans in Beringia at the right time. Bones at the Bluefish Caves site have been dated to the ice age, but that timing is controversial. Reproduced with permission from ref. 8 and adapted by Lucy Reading-Ikkanda (graphic artist).

5554–5557 | PNAS | May 30, 2017 | vol. 114 | no. 22 www.pnas.org/cgi/doi/10.1073/pnas.1705966114 Downloaded by guest on September 28, 2021 Cut marks from stone tools scar a mandible from the Bluefish Caves archaeological site, suggesting a human presence. The mandible dates to 24,000 years ago, but the site’s age remains a matter of debate. Reproduced with permission from ref. 7.

others proposed it a decade ago. Back then, Kitchen that Native American mitochondrial DNA includes wasn’t sure the idea would endure. “I wouldn’t have four unique variations absent in related Asians. staked my life on the model,” he says. “I am pleasantly When the Tamm group read the Brazilians’ work, gratified that it has been shored up.” the idea of a Beringian layover “fit with the patterns we were seeing,” says team member Ripan Malhi of of an Idea the University of Illinois at Urbana–Champaign. The The Standstill Hypothesis’s pedigree stretches back result was a landmark 2007 paper making the case for almost to Columbus. Less than a century after the a “Beringian standstill” (3). The authors argued that explorer landed in the , a Spanish mis- people leaving Asia had “paused in Beringia,” iso- sionary wrote that the first humans to reach the lating themselves from their ancestors so long that Americas might have come by land from Asia. The their mitochondrial DNA evolved new variations seen term “Beringia” was coined in 1937 by Swedish bot- only in their descendants in the New World. anist Eric Hult ´en.As defined today, Beringia stretches In this scenario, the Asian emigrants probably from Siberia’s Verkhoyansk Mountains in the west to reached Beringia before the full brunt of the Last the in Canada in the east. It includes Glacial Maximum, a period of glaciers and fierce cold – the now-vanished Bering Land Bridge, the corridor that lasted from roughly 27,000 19,000 years ago. Once in Beringia, the emigrants could go no further, between Alaska and exposed by falling seas thanks to the ice sheets blocking the door to the rest during the last major ice age. At the time, an immense of the Americas. Tamm’s group hypothesized that peo- sheathed most of Canada, blocking access ple were isolated in Beringia for up to 15,000 years until from Beringia to the Americas. they could venture into the unknown lands to the east. The notion of Beringia soon became useful across The model was popular from the start, Malhi says, disciplines. In the 1970s, biological anthropologist but “for a really long time, there were no formal sim- Emoke} Szathmary, ´ now an emeritus professor at ulations or actual testing of this hypothesis.” That’sno Canada’s University of Manitoba, and physical anthro- longer the case. pologist Nancy Ossenberg of Canada’s Queen’sUni- versity proposed that genetic and physical similarities Mounting Evidence between two modern-day Native American groups had In the past 2 years, several powerful studies have arisen when different founding populations exchanged buttressed the original genetic findings. One large genes while living in Beringia (1). In the 1990s, Brazilian consortium of scientists from , the United scientists wrote that the people who became the Native States, and elsewhere focused, as Tamm’s group did, Americans might have “settled and diversified” in on mitochondrial DNA, some of it from Native Amer- Beringia, building up genetic diversity before entering ican mummies dating back hundreds of years. The the New World (2). results showed Native Americans were genetically A decade later, Estonian geneticist Erika Tamm isolated from their source population for 2,400–9,000 and colleagues were puzzling over a trove of mito- years (4). Another large consortium examined nuclear chondrial DNA, which is passed down only from DNA and found evidence for an ancient separation mother to child. Tamm and her colleagues noticed between the ancestors of modern-day Siberians and

Watson PNAS | May 30, 2017 | vol. 114 | no. 22 | 5555 Downloaded by guest on September 28, 2021 modern-day Native Americans. This team estimated (8), confirm humans did not abandon Beringia even the separation at 8,000 years, perhaps less (5). The when the climate was far chillier and drier than to- fact that nuclear DNA points to the same conclusions day, Pitulko says. as the mitochondrial DNA, Kitchen says, is “reaffir- But Pitulko allows that conditions at Yana River mation that the mitochondria [effect] we’re seeing might have gotten tiresome. If your local temperature is real.” averages 5 degrees, “you’ll probably think about The new mitochondrial DNA results suggest only getting to some other place 5 degrees warmer, which 2,000 women gave rise to the inhabitants of the New makes a big difference,” he says. World, says Lars Fehren-Schmitz, an author of the Humans seeking more hospitable real estate could mitochondrial DNA paper and a human ecologist at aspire to southeastern Beringia, which Hoffecker calls the University of California, Santa Cruz. That translates the “Riviera” of the glacial maximum. During that cold to a standstill population numbering in the few tens of and arid period, Beringia’s most productive ecosys- thousands. Fehren-Schmitz and his colleagues also tem was the area now just offshore of southern Alaska, modeled the capacity of Beringia during the Last according to modeling of the ancient vegetation Glacial Maximum, given the vegetation and fauna. based on local temperatures, carbon-dioxide levels, They found that 18,000–54,000 people could have and other conditions. Ice-age sediment drilled from lived off the land, which is roughly in line with their the nearby sea floor included pollen from and population estimate. (9). Unless that pollen eroded out of older In early April, Malhi and his colleagues published sediments, it indicates relatively mild conditions, yet more genetic evidence providing subtle evidence says paleoecologist Nancy Bigelow of the University of a standstill (6). His group found that four prehis- of Alaska Fairbanks. The whole of the was toric individuals—including one from more than 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 These were not people who are scared away by a little helped support herds of mammoths, bison and other bit of snow.” large game. —Lars Fehren-Schmitz Linguistic as well as genetic evidence implies that humans were situated to take advantage of this bounty. Researchers examined languages from Siberia, Alaska, 10,000 years ago—who lived on the Alaskan and and northern , looking for structural traits, British Columbian coasts had DNA characteristic of such as the presence of plural pronouns. Using these the so-called northern lineage of Native Americans. traits to build an evolutionary-tree–like diagram, they People of that lineage still live in the , as well as found that the Siberian languages evolved alongside in the northern interior of the continent. Malhi says the Native American languages (11). This finding that one possible explanation for the lineage’s ap- supports “at least a period of occupation and di- pearance is that it existed in Beringia. He confesses to versification within the Beringian area, and probably having harbored serious doubts about the Standstill somewhere within the southwestern Alaskan area,” Hypothesis after hearing of another mechanism that says study coauthor Gary Holton of the University of could explain the genetic patterns. But his new paper Hawai’iatManoa. (6) has made him more of a believer. “I’m more con- vinced than I was a year ago,” he says. Alternate Route Also supporting the hypothesis’s validity is new Despite all of the evidence, the case for a Beringian data from Bluefish Caves, an archaeological site in the standstill remains open. The genetic patterns seen in Canadian . Researchers working there in the Native Americans could have been created by “ge- 1970s and 1980s excavated animal bones with signs netic surfing,” Malhi says. That’s rapid genetic change of human butchering. Now scientists have used ra- seen when a small population expands rapidly in a diocarbon techniques to show that people occupied large area, as happened when humans finally spread the site as early as 24,000 years ago, the heart of the across the New World; Malhi says it could have pro- most recent severe ice age (7). If confirmed, the data duced the DNA relationships researchers see without are strong evidence that “somebody was in Beringia a long genetic isolation. during the ,” says archaeologist The sparse archaeological record is particularly John Hoffecker of the University of Colorado, Boulder. troubling. Bluefish Caves provides some of the only Humans clearly lived on the other side of Beringia archeological evidence for human occupation of too. At several sites on the Yana River in far Beringia during the glacial maximum. But that timing northern Siberia, researchers found -ivory provokes skepticism, and if it’s correct, it places artifacts including a tusk that had been scraped to humans at a spot in Beringia close to the ice sheet, make spear points or the like. The artifacts date to which was much more barren than the land bridge. between 21,000 and 24,000 years ago, says Vladi- “For people to be that at Last Glacial Maxi- mir Pitulko of the Russian Academy of Sciences, who mum times you would need a lot of compelling evi- heads research at the Yana sites. Those dates, part dence to demonstrate that,” says Ben Potter of the of research that has been accepted for publication University of Alaska Fairbanks. “I’m not convinced.”

5556 | www.pnas.org/cgi/doi/10.1073/pnas.1705966114 Watson Downloaded by guest on September 28, 2021 There is a middle course. Perhaps people paused such as mountains, says Fehren-Schmitz. “These were on their way to the New World, but not in Beringia. not people who are scared away by a little bit of Genetic data do not specify where the standstill took snow,” he adds. It also seems likely that the standstill place, only that the people who gave rise to Native site was within striking distance of North America. Americans were genetically isolated. “It’s unlikely to The models show that people entered the Americas actually be in Beringia, because we don’t have evidence from wherever they were waiting as soon as the of people there between 25,000 and 14,000 years ago, glaciers began to pull back. Either they were on the the time period when this supposedly took place,” ar- doorstep of the Americas at that time, or the instant gues Potter, who points to alternative spots in Siberia. the ice sheets retreated they sprinted “very, very fast,” One recently proposed option centers on the Kitchen says. Japanese island of Hokkaido and the neighboring Finding an Asian site that checks all of the boxes is Russian island called Sakhalin. During the glacial not easy, Hoffecker says, although he’s open to maximum, Sakhalin was connected to Hokkaido, the credible alternative locales from critics. “I’m all ears,” mainland, and some of the nearby Kuril Islands. Hoffecker says. “But I haven’t heard it yet.” Starting 26,000 years ago, signs of human habitation Beringia is the most plausible site to many re- on Hokkaido, Sakhalin, and the Kurils rose sharply, searchers, but it’s also problematic. The region is according to the number of archaeological sites and nearly devoid of signs of a human presence during the other data (12). That implies an influx of people, says Last Glacial Maximum; whether that’s because there archaeologist Ian Buvit with the National Park Service wasn’t such a presence or it hasn’t been found is un- in Anchorage. And this happened shortly before the clear. The argument is likely to continue until re- number of human sites in southern Siberia fell sharply, searchers recover ancient DNA from a population that as if people were fleeing from one place to another. shows initial signs of genetic isolation. Or perhaps But every proposed incubation site has its weak- researchers will stumble across a site that holds un- nesses. It would’ve been difficult for people living in deniable evidence of a human presence during the Siberia to be genetically isolated from their neighbors, glaciers’ reign. “I hope someday it is going to be as the genetics patterns demand. Genetic isolation found,” Pitulko says. “I am optimistic. ... It might be requires not just harsh climate but physical barriers, this coming summer. Who knows?”

1 Szathm ´aryEJE, et al. (1978) Are the biological differences between North American Indians and Eskimos truly profound? Curr Anthropol 19:673–701. 2 Bonatto SL, Salzano FM (1997) A single and early migration for the peopling of the Americas supported by mitochondrial DNA sequence data. Proc Natl Acad Sci USA 94:1866–1871. 3 Tamm E, et al. (2007) Beringian standstill and spread of Native American founders. PLoS One 2:e829. 4 B, et al. (2016) Ancient mitochondrial DNA provides high-resolution time scale of the peopling of the Americas. Sci Adv 2:e1501385. 5 Raghavan M, et al. (2015) Genomic evidence for the and recent population history of Native Americans. Science 349:aab3884. 6 Lindo J, et al. (2017) Ancient individuals from the North American Northwest Coast reveal 10,000 years of regional genetic continuity. Proc Natl Acad Sci USA 114:4093–4098. 7 Bourgeon L, Burke A, Higham T (2017) Earliest human presence in North America dated to the Last Glacial Maximum: New radiocarbon dates from Bluefish Caves, Canada. PLoS One 12:e0169486. 8 Pitulko V, Pavlova E, Nikolskiy P (2017) Revising the archaeological record of the Upper Pleistocene Arctic Siberia: Human dispersal and adaptations in MIS 3 and 2. Quat Sci Rev 165:127–148. 9 Hoffecker JF, Elias SA, O’Rourke DH, Scott GR, Bigelow NH (2016) Beringia and the global dispersal of modern humans. Evol Anthropol 25:64–78. 10 Willerslev E, et al. (2014) Fifty thousand years of Arctic vegetation and megafaunal diet. 506:47–51. 11 Sicoli MA, Holton G (2014) Linguistic phylogenies support back-migration from Beringia to Asia. PLoS One 9:e91722. 12 Buvit I, Terry K (2016) Outside Beringia: Why the northeast Asian record does not support a long standstill model. PaleoAmerica 2:281–285.

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