A picture of pre-Colombian U.S. residents is emerging, based on genetic mutations
, Photo: Creative Commons
07 of April 2016 10:53:42
SARAH KAPLANTHE WASHINGTON POSTIt all began with a tiny, brave band of people who strode across the land bridge connecting Asia to Alaska and into the unknown. Within a few millennia they would conquer the Americas, laying the groundwork for civilizations as varied as any in the world — the prehistoric creators of Clovis points; the Pueblo, who cut palaces straight into the sheer faces of cliffs; the mound builders of the Mississippian culture; the vast empires of the Incas and the Maya.Their diversity was enshrined in their DNA: A new study of mummies and other remains from across the Americas found at least 84 genetic lineages from the pre-Columbian period.But then, 500 years ago, those lineages vanished. A glance at a history book will explain why."When Europeans arrived, some of those populations were wiped out completely," lead author Bastien Llamas, a geneticist at the University of Adelaide, told Science.[caption id="attachment_11668" align="alignleft" width="300"] By conducting genetic research on human remains, scientists are learning about relationships between pre-Colombian tribes. Photo: The Washington Post/Courtesy of Mohammad Younas Nawandish[/caption]Although indigenous communities still thrive in Americas, no living human seems to carry any of the ancestral DNA documented in Llamas' study, which was published in the journal Science Advances. While that doesn't necessarily mean that the genetic diversity of the pre-Columbian era has been completely wiped out — other lineages not traced in this study may exist, and it's still possible that the 84 lineages live on in a population that hasn't been sampled yet. But the find demonstrates what the researchers bluntly term the "high extinction rate" for indigenous American people.But that sobering statistic is just the end of the story of humans in the pre-Columbian Americas. From their analysis of ancient genetic material, Llamas and his colleagues reconstructed a complicated narrative spanning 23,000 years and three entire continents.They started by sampling 92 skeletons and mummified bodies whose owners lived somewhere in South America between 8600 and 500 years ago for what's called mitochondrial DNA — compressed loops of genetic material that exist inside the hundreds of tiny structures that power each human cell. Every human inherits this material exclusively from his or her mother, so it's a good way of tracing matrilineal lines of kinship back through time.MtDNA is also useful for figuring out the relationships between populations, and measuring how they evolved. Since scientists know roughly how many mutations genetic material is likely to accumulate over the course of a given number of years, they can tally up the differences in mtDNA between two groups and derive from that the number of years it's been since they were last in contact.Using this technique, Llamas and his colleagues figured out when the initial American settlers left Siberia via the land bridge over what is now the Bering strait. Their answer: roughly 23,000 years ago.From there, it seemed that a group of about 2,000 child-bearing women (suggesting a population of some 10,000 people total) spent the next 6,000 years in genetic isolation — meaning that they weren't intermarrying with other groups or branching off into separate ones. They just hunkered down together, and lived as their ancestors did.That matches most scientists's understanding of the early colonization of the Americas. In 20,000 BC, Alaska and pretty much all of Canada would have been encased in a vast ice sheet. Traversing it was impossible - nothing could survive the 3,000 mile trek across a barren glacier, let alone a motley crew of prehistoric humans.
(The study) is confirming a picture that has been emerging. It challenges archaeologists to catch up with the genomics people, because they're creating models for us that need to be tested."-Jon Erlandson, archaeologist at the University of Oregon in EugeneWhat happened next is still subject of a long-running debate — one that Llamas's genetic reconstruction might help resolve. Historically, scientists have believed that early settlers known as the Clovis people migrated south through an ice-free corridor that formed as the glaciers melted at the end of the Ice Age, roughly 11,500 years ago. That "Clovis first" theory, though dominant for most of the 20th century, is a tricky one: Recent radiocarbon analysis of human remains discovered at an ancient site in Chile found they were about 14,000 years old. Either the analysis was wrong, or the theory was.A second and increasingly popular possibility is that the people of Beringia, as the land bridge is known, built themselves some boats, traveled south and began settling the American coastline long before the inland ice had melted. The Science Advances study backs up that theory: It found that the genetic diversity of early Americans exploded about 16,000 years ago, suggesting that they'd made it to the warm, wide open spaces of the North and South American continents and flourished. By 14,000 years before present, humans inhabited the Americas from Alaska to Chile.Llamas and his colleagues acknowledge that DNA can only illuminate part of the narrative. For one thing, their skeleton samples came primarily from Bolivia, Chile and Peru, Llamas told the Christian Science Monitor. For another, mitochondrial DNA only offers clues about matrilineal lines of kinship — men, and women who didn't have children, are left out of the equation."The conclusions that we have drawn from this dataset may change in the future when we gather more data," he said. "It's an ongoing story."But Jon Erlandson, an archaeologist at the University of Oregon in Eugene who wasn't involved in the research, says the findings are something of kick in the pants for other scientists studying early Americans."(The study) is confirming a picture that has been emerging" about how and when humans first arrived in the Americas, he told Science. "It challenges archaeologists to catch up with the genomics people, because they're creating models for us that need to be tested."