The Americas were the last continents conquered by humanity. Now we know that those that settled there were a hardy group that first had to survive the Arctic
SOME 15,000 years ago, a small band of pioneers stood on the threshold of a new world. To the south were the Americas, 40 million square kilometres of virgin territory including wide-open prairies, dense rainforests and high mountain chains. An epic journey was about to begin – but only because a remarkable adventure had just ended.
Before these original American frontiersfolk ventured south, their forebears had spent millennia scratching a living in the desolate regions just south of the Arctic circle. Once they had arrived in the north, global temperatures plunged and the climate became bleaker still.
Faced with worsening conditions, these original pioneers stayed put, spending thousands of years isolated from the rest of humanity. Their fate is now coming to light, and it is clear that something remarkable happened during those missing years. The people who would eventually conquer the Americas evolved some unusual adaptations to survive, and it turns out that this genetic legacy can help trace their descendants today.
We don’t know exactly when humans first reached the New World. The consensus is that the first Americans arrived fairly recently, about 15,000 years ago. It is also widely believed that they did so via Beringia – an area centred on the Bering Strait between Siberia and Alaska, which was dry land at that time.
This implies that the story of the first Americans began with a subarctic odyssey. Whereas Europe and Asia has been home to hominins for almost 2 million years, it seems none of the earliest inhabitants – including Homo erectus, Neanderthals and Denisovans – strayed much above 55° north, roughly in line with the top of what are now Ireland and Kazakhstan. “There’s every reason not to do it,” says Ben Potter at the University of Alaska Fairbanks. It isn’t just cold. There aren’t as many animals at higher latitudes, making hunting difficult.
On current evidence, modern humans, rather than our extinct relatives, were the first to enter northern Eurasia. John Hoffecker at the University of Colorado, Boulder, suspects that key inventions opened up this subarctic region. Tailored clothing was probably necessary to cope with cooler conditions. And snares offered a solution to sparsely distributed prey. Used by modern Arctic inhabitants, they are essentially automated hunting devices that operate 24 hours a day, helping people exploit territories covering thousands of square kilometres.
“These people could have been cut off from the rest of humanity for 15,000 years”
There are disputed signs of human activity in northern Eurasia 45,000 years ago. But it is at a 32,000-year-old site in the Arctic circle that the archaeological record really begins. On the banks of the Yana river in Siberia, at the western end of Beringia, archaeologists have found hundreds of stone and bone tools, including sewing needles. The people near the Yana hunted reindeer, woolly rhinos and birds. Hares were also caught, perhaps for their pelts, if modern Arctic peoples are any guide.
Life in Beringia wasn’t unrelentingly grim. Those inhabiting the Yana site had time for artistic pursuits: it has yielded dozens of beads carved from ivory and bone. Archaeologists also found strips of decorated ivory that may have been used as hairbands, and shallow ivory dishes that perhaps served ritual purposes.
There were human remains at the site as well – two tooth fragments – and we learned this year that both contain preserved ancient DNA. The teeth belonged to children who were part of a population related to, although not directly ancestral to, modern Native Americans. But the game-changing discovery was about the size of the community. These children were so genetically distinct from each other that the Yana population may have numbered more than 500.
It is a startling finding, says Hoffecker. The Yana site was remote from the southern centres of human activity in the late Stone Age, so we might expect the population to have been tiny, isolated and inbred. “But it’s a fairly healthy and robust group,” he says. This suggests there was a degree of migration into and out of Beringia, keeping populations there genetically diverse. But even though this group may have been flourishing, there is no good evidence that these people pushed further east, despite Siberia and the Americas then being connected by dry land.
There they stayed for generations. Then, about 30,000 years ago, Earth began to lurch into a severe cold snap called the Last Glacial Maximum. Ice sheets grew across North America, and the route from Beringia to the New World froze shut. It was some 15,000 years before conditions began to improve and the route reopened. The Yana site was abandoned at the beginning of this period.
However, it seems that other groups stayed in Beringia. A decade ago, geneticists including Ripan Malhi at the University of Illinois discovered subtle genetic differences between living Native Americans and East Asians that originated thousands of years before people entered the Americas. The geneticists suggested a solution: perhaps the ancestors of the Native Americans had lived in Beringia, and become isolated as temperatures dropped 30,000 years ago. Their route south was blocked by polar deserts in Eurasia and ice sheets in North America. In theory, these people could have been cut off from the rest of humanity for 15,000 years, plenty of time for random genetic drift to give them distinctive DNA before they entered the New World. The idea was labelled the Beringian incubation or standstill.
Today, most researchers accept that there was an incubation period, although they now think it didn’t begin until about 24,000 years ago, when conditions got really bad, and that it lasted 9000 years at most. However, there is disagreement about where exactly this period of isolation happened.
Potter is convinced that the ancestors of today’s Native Americans did, in fact, retreat south as conditions worsened, and spent the Last Glacial Maximum near Lake Baikal at approximately 53° north. We even have DNA from a boy who died there 24,000 years ago. Like the Yana children, he was related, although not directly ancestral, to the Native American founding population. But other researchers, including Hoffecker, are convinced that the incubation occurred in Beringia itself. The region’s geographical remoteness makes it a more likely place for a population to be isolated for millennia, they argue. Indeed, there is genetic evidence that this happened to other mammals – including elk and brown bears – during the Last Glacial Maximum.
Another clue comes from linguistics. There are similarities between some of the languages spoken in the Americas and Ket – a Yeniseian language spoken in Siberia to the west of Lake Baikal. A language family tree indicates that the oldest representatives of this group were spoken near the Beringian region, suggesting this is where it arose. Ket speakers may be descendants of Beringians who migrated back into Siberia after the Last Glacial Maximum.
Hoffecker points to more reasons why an extended stay in Beringia makes sense. As Earth’s climate cooled and ice sheets began to form, global sea levels dropped dramatically. By 28,000 years ago, the seas may have been 100 metres lower than they are today. As a result, thousands of square kilometres of new land appeared in south-central Beringia. And not just any old land. Although most of the subarctic became cold and arid after 30,000 years ago, climate models and pollen records indicate that this region was humid and mild, perhaps because Pacific currents pushed warm air over it. “There’s no other place on Earth where you got this significant expansion of viable habitat during the peak of the Last Glacial Maximum,” says Hoffecker.
Considering Beringia’s size and its benign climate, one estimate suggests that tens of thousands of people could have lived there during the peak of the Last Glacial Maximum.
There would have been big challenges. During winter, anyone living today above 46° north will struggle to get the sunlight they need to trigger vitamin D synthesis in the skin. “Even if you’re naked outside, you’re not going to get enough ultraviolet light exposure,” says Leslea Hlusko at the University of California, Berkeley. Adults can largely overcome this problem by eating vitamin D-rich foods such as oily fish, but nursing infants would be at risk of vitamin D deficiency, which can weaken the immune system and cause skeletal problems, among other things. She thinks evolution found a solution.
“The people who would eventually conquer the Americas evolved some unusual adaptations to survive”
One variant of a gene called EDAR changes the density of milk ducts within the female breast. Hlusko suspects that this boosts the transfer of nutrients, including vitamin D, from mothers to their infants. The same EDAR variant also leaves people with strikingly thick enamel on the tongue side of their incisors. So she, Hoffecker and their colleagues looked at the global distribution of these curious teeth. They are all-but absent in most regions, present to some degree in east Asia – and almost ubiquitous among current and past Native American populations. To Hlusko, this fits with the idea that the ancestors of Native Americans, isolated in Beringia, evolved the EDAR variant, which helped them overcome their vitamin D problem and gave them thicker tooth enamel as a side effect.
“It’s a great hypothesis,” says Tábita Hünemeier at the University of São Paulo, Brazil. She also thinks the Native American founder population was isolated in Beringia – and that it evolved in response. In 2017, Hünemeier and her colleagues discovered that people in many present-day Native American populations carry unusual versions of a family of genes called FADS genes. People with these variants are better able to process the protein and fatty acid-rich diet that subarctic communities typically consume. The variants may have been naturally selected in the Native American founder population while it was isolated in Beringia, says Hünemeier.
All this research is building a picture of life on the American frontier. But until recently, the Beringian incubation was generally viewed as little more than a curious prologue to the story of the first Americans. This held that the real epic began about 15,000 years ago when the ice sheets retreated and a small, genetically homogeneous population moved from Beringia into the New World.
American adventure
By 14,600 years ago, the founding population had split into two distinct Native American subpopulations. One, the Ancestral B group, apparently stuck largely to the very north of North America, where many of its descendants still live today. The second, the Ancestral A group, gave rise to a famous North American prehistoric culture, the Clovis, and also spread south into Central and South America over the next couple of millennia. Or that’s what we thought. But within the past year, we have learned that the Beringian people fractured into subpopulations during the incubation period. This means that several genetically distinct groups moved into the New World.
The clearest evidence for this comes from DNA locked in the bones of two children who were buried in Alaska, 11,500 and 9000 years ago. Both belong to a genetic group – the Ancient Beringians – that split from the rest of the Beringians about 22,000 years ago. The idea that this happened in Beringia and the two subpopulations then avoided mixing for thousands of years isn’t as implausible as it might seem. Malhi points out that tree species in Beringia went through a similar population fragmentation during the Last Glacial Maximum. Perhaps populations became isolated because each adapted to a unique microclimate within the area, he suggests.
At present, there is no evidence that the Ancient Beringians made much of a contribution to the peopling of the New World. Even after the ice sheets retreated, they seem to have lingered in the Beringia region, eventually vanishing when populations of Native Americans from further south pushed back north into Alaska. However, this may not have been the only subpopulation to branch off during the Beringian incubation. And others might have had a greater wanderlust.
Last year, researchers reported that the Mixe people of Mexico carry a genetic signature unlike that of their neighbours. One interpretation is that, about 9000 years ago, their ancestors mingled with a mysterious group, provisionally dubbed UPopA, which seems to be another subpopulation originating in Beringia some 25,000 years ago.
Then there is the enigmatic Population Y. In 2015, geneticists announced that some members of the Suruí and Karitiana groups living in the Amazon share a curious genetic connection with some Indigenous Australasians. The simplest explanation is that this originated in a prehistoric east Asian group, Population Y, which was ancestral to the first Australasians and also contributed genetic material to the New World via Beringia.
However, geneticists have found no signs of Population Y in ancient DNA from Beringians or North Americans, and some researchers began to doubt its existence. Then, last year, came the discovery that a person who died in eastern Brazil 10,400 years ago carried the Australasian-like DNA. The individual belonged to the Ancestral A line of Native Americans that we know came to occupy South America. Given that Population Y DNA hasn’t been found in Ancestral A remains in North America, the geneticists suggest the two groups met and interbred after they arrived in South America. They say this might indicate that Population Y Beringians were the very first people to push into South America. If so, the inhabitants of early sites there – such as the 14,200-year-old Monte Verde in Chile – left a genetic legacy we now see only in some Amazonian people. It is an idea that chimes with a growing realisation that the peopling of South America was more complicated than previously thought.
Some 15,000 years ago, a small band of Beringians stood on the threshold of a new world. In time, their descendants would create giant artworks in the Nazca desert of Peru, begin humanity’s love affair with chocolate in the forests of Ecuador, and build great civilisations in Mexico. Today’s Native American populations still carry the genetic legacy of those remarkable frontiersfolk.
Read more: https://www.newscientist.com/article/mg24332490-500-the-first-americans-the-untold-story-of-the-pioneers-of-the-new-world/#ixzz60djzePAP