A Sunken Bridge the Size of a Continent
A remote Arctic land may hold a vital missing chapter from human history. The only problem? It disappeared at the end of the last ice age.
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The planet was cold. Vast sheets of ice extended over the northern half of North America and parts of northeast Asia, forming treacherous frozen barrens. In between, death lurked in many forms. Massive predators—giant short-faced bears, Beringian cave lions, scimitar cats, gray wolves—prowled the steppes and green valleys, hunting for prey. And bands of Homo sapiens traversed the landscape too, searching for food and skirting the white barrens.
It was the height of the last ice age, a time known as the Last Glacial Maximum, which lasted from about 26,500 to 19,000 years ago. The world looked entirely different from today—not only was the flora and fauna rather strange, but its very geography was unrecognizable. So much water was locked up in ice that sea levels were around 120 meters lower than now, laying bare vast stretches of ocean floor along the coasts. Islands became peninsulas. Underwater plains became inland valleys and meadows.
Few places were more transformed than the Bering Strait. As the waters subsided between eastern Siberia and western Alaska, a swath of dry land emerged where wooly mammoths and other animals eventually roamed. For thousands of years, Asia and North America were one continuous expanse—until the world warmed again, and rivers and streams carried meltwater from the ice sheets back to the ocean.
Today, if we think about this forgotten land, we imagine it as a narrow, frozen corridor that small bands of ancient Asian migrants hurried over as they made their way east to the New World, where they gave rise to the first people of the Americas. But that picture is proving increasingly misleading. The “Bering land bridge” wasn’t a bridge at all, for instance—at its greatest extent, it was a landmass roughly as large as Australia, stretching 1,600 kilometers north to south and 4,800 kilometers east to west, from Canada’s Mackenzie River to Russia’s Verkhoyansk Mountains. Scientists call it Beringia.
And it wasn’t a frozen barren, according to fossil and pollen evidence. Wildflowers and shrubs bloomed there in summer, and animals as diverse as steppe bison, western camels, Pleistocene horses, antelope, and wooly rhinoceros roamed its steppe lands and tundra. At a time when much of the northern hemisphere groaned under thick ice, Beringia was largely ice-free, with a range of climates that may not have been too different from those experienced in Alaska today.
This new picture raises a key question: did the first migrants to the Americas really race over Beringia just as the great ice sheets were melting? Or did they arrive much earlier, taking refuge there during the coldest days of the Ice Age? In eastern Siberia and northwestern North America, archaeologists are now searching for campsites and artifacts of ancient migrants. At one site, the quest has turned up tantalizing hints that humans may have called Beringia home around 21,000 years ago. And in labs across North America and Europe, researchers are combing through human DNA and dental patterns for other evidence.
Like the ancient Beringians themselves, these scientists are striking into the unknown, struggling to arrange the clues into a story that makes sense. Yet as we hold up a torch to illuminate fragile molecules of DNA here and fragments of mammoth-ivory vessels there, a picture is slowly coming to light.
One of the scientists searching for Beringia’s lost past is John Hoffecker, an archaeologist and human paleoecologist at the University of Colorado’s Institute of Arctic and Alpine Research. With a sturdy build and graying beard, Hoffecker looks like he could be an Arctic explorer himself. But though he’s spent years working in the North, he has done so as a scientist, conducting field research and studying the paths that our human ancestors took as they dispersed eastward across Eurasia and to Alaska and beyond.
Hoffecker was less than inspired by his first introduction to Beringia. For decades, scientists had considered the region a “glacial hell” of frozen soil and howling winds. And because the earliest confirmed human evidence in the Americas at the time dated to roughly 12,000 years ago, most researchers assumed that humans were unable to survive in Beringia during the coldest part of the last ice age. Most believed that people only ventured across as the world warmed, and the land bridge began thawing out.
But this hypothesis began to fall apart in the 1990s, as major archaeological discoveries pushed back the date of human migration to the New World. Along the southern tip of the Americas, at a Chilean site known as Monte Verde, archaeologists excavated a campsite of hunter-gatherers who sheltered in tents and feasted on seaweed at least 14,500 years ago, and perhaps as many as 18,500 years ago. Other archaeological finds—including the recent discovery of 14,500-year-old stone tools at a site now on the bottom of Florida’s Aucilla River—also cast doubt on the story of humans rushing over the land bridge and settling the Americas as the ice melted.
Further complicating the story, archaeologists working in northeastern Siberia in 2002 discovered that humans had adapted to Arctic conditions much earlier than once believed. Nearly 32,000 years ago, at a group of sites located at latitude 71° north (along the Yana River and well above the Arctic Circle), hunters butchered mammoths, carved ivory vessels, and sewed warm, tailored clothing with eyed needles. On the eve of the Last Glacial Maximum, they had already mastered Arctic living—a prerequisite for surviving in Beringia.
As the deep freeze set in, though, humans in many parts of the world abandoned their old hunting grounds, moving to areas where food was still abundant enough for survival. Those living along the Yana River may have fled south, but recent research suggests they had another option: heading east to Beringia.
Changes in the jet stream had created a relatively welcoming climate there—according to one model, parts of southeastern Beringia may have been as much as 8 °C warmer than today. Winter temperatures plummeted to -40 °C in some areas—cold, but for people adapted to life on the northern Siberian steppe, survivable. Spring brought the return of light, and a thawing of the soil above the permafrost. The forbs began to unfurl their leaves. The first flowers—from little blue asters to white clover—injected color into the brown tundra. By summer, bison, mammoths, and horses fattened on the land. Flora as diverse as shrub willows, flowering plants, grasses, and sphagnum moss all flourished, and fossil studies point to a wealth of fauna, from beetles, lemmings, and ptarmigan to moose and grizzly bears. If Beringia was a refuge for plants and animals, why not people?
For years, these questions troubled Hoffecker, like a song he couldn’t get out of his head. But because he and other archaeologists hadn’t been able to find any clear traces of humans in Beringia that dated to the Last Glacial Maximum, he brushed the questions aside. There was simply nothing more compelling to replace the old model.
In 2007, that changed. Geneticist Erika Tamm from the University of Tartu in Estonia and 20 colleagues analyzed mitochondrial DNA data from 601 indigenous Americans and 3,764 indigenous Asians for clues to the peopling of the New World. Their evidence, published in the journal PLOS One, suggested that somewhere along the journey to the New World, ancient Asian migrants had lingered long enough to develop the new genetic lineages that came to characterize indigenous populations in the Americas. A rapid migration some 12,000 to 13,000 years ago couldn’t account for these new lineages: the migrants must have been isolated somewhere for thousands of years. The most likely spot, the team suggested, was Beringia.
Enter the Beringian standstill model.
Imagine you lived in northeastern Siberia some 30,000 years ago, along the banks of the Yana. Even in the best of times, surviving in this lake-studded tundra requires special skills and relatively complex technology. Your clothing is carefully tailored, much like the parkas and pants of the historical Inuit, whose winter suits were made up of nearly 100 different pieces, including hoods, mittens, boots, and mechanical parts such as drawstrings to tighten things. You construct windproof shelters to keep yourself alive in below-zero temperatures.
Lately, though, it’s been colder. There are no climate models telling you that the Earth is changing; no maps telling you where to go. Perhaps you edge farther and farther from the ice sheets. Or maybe you follow the herds of caribou, bison, and other big game as they migrate toward food. Perhaps you head eastward, to places no one has ever been. You follow the rising Sun, your breath hanging in puffs of golden light in the morning air.
As you make your way across Beringia, you carry things: weapons, animal hides, babies on hips. It’s a slow journey; along the way, people die and are born. Maybe you reach a pleasant valley where game is abundant, and several families decide to stay. Others move on, seeking something better, until they can’t move on anymore. The way is blocked by ice. Some go back, until they, too, reach impassable ice.
It’s impossible to know the intricacies of the routes the migrants took to the New World. It wasn’t a single march or a mass migration, but a meandering dispersal across a vast and complex landscape. In all likelihood, these ancient Beringians honed their survival skills along the way, devising new technologies and tools. And if Tamm and her colleagues are right, they stayed so long in Beringia that it changed their genetic makeup. It became part of them.
John Hoffecker was deeply immersed in his own research when the Beringian standstill paper came out; he didn’t give it much thought to begin with. But a few years later, while working on a textbook about the evolution and dispersal of modern humans, he checked the paper again. It dawned on him: the Beringian standstill model accounted for the inconsistencies that had been nagging him for years. Researchers working in other disciplines were coming to the same conclusion—Beringia would have been habitable during the deep freeze, and ancient humans may have hung out there for thousands of years.
But proof in the form of artifacts or campsites remained elusive, and with good reason, says Hoffecker. “The obvious explanation for why we might not have archaeological sites dating to the Last Glacial Maximum is because, climatically, the mildest area [of Beringia] is now underwater,” he says.
The more Hoffecker thought about it, the more excited he got. Eventually, he had to talk to someone. He had long known Dennis O’Rourke, a prominent geneticist at the University of Kansas who studied genetic variation in the indigenous people of the Americas. And he knew O’Rourke was critical of the Beringian standstill model. Given his own giddiness, it was exactly what Hoffecker needed—someone who would bring him back to reality by pointing out all the problems with the hypothesis.
At the time, O’Rourke was working in the Arctic village of Kotzebue, Alaska, where cellphone service is spotty. He was walking from the airport when his phone buzzed. Hunching against the wind, he strained to hear the person on the other end. “And I’m rambling incoherently,” Hoffecker recalls, laughing. “I knew Dennis would be skeptical, and I was bracing myself. I remember him saying, ‘Well, it’s awfully convenient that these sites are all underwater.’”
O’Rourke saw other problems, too. If a small number of Asian families migrated into Beringia and were subsequently isolated there by ice sheets to the east and west, for example, their descendants would likely have experienced what is known as a population bottleneck—a drastic reduction of genetic variation detectable among their descendants. But there was little trace of such a bottleneck. In fact, a 2007 study led by Sijia Wang, then a PhD candidate at University College London, showed only a minor loss of genetic variation among indigenous Americans compared to the worldwide average—just six to seven percent.
But today, O’Rourke’s skepticism of the standstill model has pretty much evaporated. A recent analysis of genetic data shows that as many as 8,000 to 10,000 people may have migrated into Beringia and become isolated—far more than O’Rourke thought possible only a few years before. And new estimates of biological productivity suggest Beringia could have indeed supported that many people. “I’m a convert, in a sense,” O’Rourke says. “Most, if not all, of my early questions regarding the Beringian standstill model have been resolved with new, additional data and modeling.”
It’s not just O’Rourke and Hoffecker who are warming to a new version of human history, one that diverges from the story we’ve been telling for decades. In February 2016, with funding from the US National Science Foundation, Hoffecker brought the world’s top Beringia scientists to Colorado for a two-day meeting of the minds. Everyone from entomologists to dental experts shared research and discussed the plausibility of the Beringian standstill model.
Some scientists walked away from the workshop unconvinced. Ben Potter, a University of Alaska archaeologist, has been scouting for early sites in Alaska for the past 15 years, using remote sensing as well as ground and helicopter surveys to identify likely looking spots before digging. In all, he and his colleagues have now found 400 new sites in Alaska. But none dates to the Last Glacial Maximum. “With respect to the Beringian standstill, I feel comfortable in saying we’re probably not missing 9,000 years of human occupation in this region,” Potter says. He and several other archaeologists think that humans waited out the depths of the Ice Age somewhere in northeast Asia and didn’t enter Alaska until the ice retreated.
Ripan Malhi, a University of Illinois geneticist and coauthor of the original Beringian standstill paper also harbors doubts. Back in 2007, when he and his colleagues wrote the paper, they estimated that the standstill could have lasted as many as 15,000 years—ample time for new mutations to occur. Today, he says, two major genetic studies point to an isolation period of no more than 8,000 or 9,000 years, and perhaps as few as 2,400 years: too short an interval, in all likelihood, to account for the genetic differences between East Asians and indigenous Americans. “I think the less the time of the isolation period, the less important [the standstill] is from an evolutionary perspective,” Malhi later explained by email. His view of the Beringian standstill model is now “nuanced,” he says, and he’s looking at additional factors that may help account for the unique genetic patterns seen in indigenous people in the Americas.
But other scientists are increasingly convinced that Beringia harbored a thriving human population during the coldest days of the Ice Age. G. Richard Scott, a physical anthropologist from the University of Nevada, Reno, presented a paper comparing the distinctive shapes of tooth crowns and roots among indigenous people in Asia, the Americas, and elsewhere. His dental findings strongly suggested that indigenous Americans descended from a human population long isolated from Asia, most likely in Beringia. And Scott Elias, a paleoenvironmental researcher from Royal Holloway, University of London, talked about beetle remains he retrieved from sediment cores taken from different parts of Beringia. By identifying the species of beetles and the habitats they frequent today, Elias helped confirm climate models showing that central Beringia would have been largely tundra, capable of supporting a host of plant and animal life.
The biggest surprise, though, came from archaeologist Vladimir Pitulko of the Russian Academy of Sciences. While digging in Russia’s now-famous Yana sites, situated in what was once western Beringia, Pitulko and his team excavated an intriguing mammoth ivory artifact. He dispatched a sample from it to the United States for radiocarbon dating. The results took him and his team aback: the sample dated between 21,126 and 21,738 years ago. This artifact and other related finds at the site constitute the first evidence that humans may have occupied Beringia during the Last Glacial Maximum. Hoffecker cautions that more dates will be needed to confirm the finding before it can be published, but he calls it “a dramatic revelation.”
Hoffecker is convinced that other evidence of humans living in Beringia during the Last Glacial Maximum exists, perhaps somewhere in the vast, unexplored reaches of western Alaska, buried under thousands of years of frozen dirt. But he’s also open to other models, stories that we haven’t yet considered. Each discovery opens up new possibilities, and the Beringian standstill, ultimately, is just one of them.
If there’s one thing the scientists agree on, it’s that someday soon, many of these puzzles will be solved. The pace of discovery is accelerating. Every year, new fragments of DNA and new stone and ivory artifacts are brought to light; new pieces to fit into the puzzle. The picture that’s forming may be more complex than anything we’ve yet considered, a story that can’t be told in a paragraph or two in a high school textbook. Looking at it from a distance, as though from space, it won’t be a tale of one group of people moving in a direct line, north to east to south. Instead, it could look far more beautiful, more human, like the pulsing of a universe.
Looking back, we’ll see constellations of people expanding and contracting as they move across the landscape, driven by changes in their environment, by warmth and cold, abundance and scarcity, drought and rain. Long before the pyramids were built in Egypt, long before Stonehenge, these ancient people moved across an ice-choked planet, developing and sharpening the skills that allowed them to survive even the darkest moments of an ice age. Their story is our story, the story of who we are and where we came from. We are their progeny, and like them, we’re simply trying to find a way forward.