How the First Americans Got There

About eleven and a half thousand years ago, people in what's now central Alaska buried two babies in a hearth. One was less than six weeks old, and the other was a late-term stillbirth. They stayed there until 2013, when archaeologists found their remains. And this week, a group of researchers published a detailed analysis of one of the babies' genomes, its full genetic code in the journal Nature. Turns out that this one baby's DNA can tell us a lot about our history as a species. It's the most direct evidence we have for how people first came to the Americas, and what happened to them afterward. Over the years, researchers have proposed lots of different theories for how the first Americans got here. And these days, there are at least a few things that almost everyone agrees on. At some point, probably more than 14,000 years ago, people crossed over the Bering Land Bridge, between what are now Siberia and Alaska. From there, they spread through North and South America, splitting off into Northern Native American and Southern Native American genetic groups, which are still around today. There are plenty of unresolved questions about this, though, like when exactly the migration happened, and why the ancient Native American and East Asian genomes are so different. Enter the babies from this week's paper. The researchers were only able to isolate enough DNA for a full genome analysis from the 6-week-old baby known as USR1. They sequenced USR1's genome, and compared it to both ancient and modern genomes from more than 2,500 people. They found that USR1 wasn't nearly as closely related to the Northern and Southern Native American groups as you might think. It's clear they had a common ancestor, but according to the team, the baby's genetic code was so different that it's part of a whole separate lineage, a third group that they're calling the Ancient Beringians. That supports what's known as the Beringian standstill model, which suggests that people first came to the areas near the land bridge, aka Beringia, something like 30,000 years ago. But then the Ice Age peaked, and the ice sheets and glaciers made travel so difficult that the people in Beringia were basically cut off from the rest of the world for thousands of years. Hence the standstill part of the name. According to the model, once the climate warmed up a little, people weren't trapped anymore, and they started to move out into the rest of North and South America. If the first Americans were stuck in Beringia for a while, breeding amongst themselves and developing new mutations in their DNA, these changes wouldn't be shared with the East Asian population they came from. So you'd expect there to be a lot of diversity in the ancient Native American genome, and the ancient Beringian lineage tells us there was. The findings also fit what researchers found back in 2015 when they looked at the baby's mitochondrial DNA. That's the separate, smaller genetic code in the cell's mitochondria, as opposed to the nucleus. They were able to get good samples from both USR1 and the other infant, USR2. Even though the mitochondrial genome doesn't have as much information as the nuclear genome, it's still a useful tool. And it was enough to show that the two babies were each part of genetic lines you don't generally find in today's Northern Native American populations. In other words, it showed that the babies must have come from a pretty diverse population. And now that we have USR1's nuclear genome, we know that there's more to it than that. The ancient Beringians were a whole separate group. And by comparing USR1's genome to those of other groups, the researchers were also able to flesh out the Beringian standstill model and come up with a much more solid timeline of when and how the first Americans moved in. To be clear, we don't know for sure that this is exactly how it went. But by combining this new evidence with what archaeologists have found in the past, here's what the team thinks happened. People first settled in the area of Northeast Asia near the land bridge around 36,000 years ago. But for a while after that, there were still people going back and forth to the rest of East Asia and genes being shared between the populations. Then, about 24,000 years ago, the cooling climate isolated them from each other. Around 21,000 years ago, USR1's ancestors, and the ancestors of both the Northern and Southern Native American groups, split into two separate genetic lines. It's not clear whether they crossed over the land bridge before or after the split, but either way, they still couldn't get very far because of all the ice. Then, sometime around 15,700 years ago, the Northern and Southern Native American groups split off from each other. And this was right about when the climate finally warmed up a bit and people started to move farther east, eventually spreading throughout the Americas. About 4,000 years later, two babies were born — and died — in eastern Beringia. And now, 11,500 years after that, their remains have brought us the closest we've ever been to understanding how the indigenous peoples and cultures in the western half of the planet came to be. Thanks for watching this episode of SciShow News! And a special thanks to our President of Space, S.R. Foxley. Thanks, S.R. Also, if you liked this video, you might like our sister channel, PBS Eons, which is all about the history of life on Earth, including some very weird times like when whales walked or when oxygen almost killed everything. If you want to check it out, just go to youtube.com slash eons.