How the First Americans Got There

♩♩Intro♩♩
About 11 and a half thousand years ago, people in what’s now central Alaska buried two
babies in a hearth.
One was was less than 6 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 2500 people.
And 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 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 babies’
mitochondrial DNA.
That’s the separate, smaller genetic code in a 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.
But now that we have USR1’s nuclear genome, we know 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: SR Foxley!
Thanks SR.
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/eons.
♩♩Outro♩♩
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How the First Americans Got There

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