Humans in the Americas Earlier than Previously Thought

DNA Studies Reveal Asians Arrived in North America 20,000-30,000 Years Ago

by Dr. Barry Starr, Stanford University

April 11, 2008

Previous studies had suggested that humans first crossed over from Siberia to Alaska around 13,000 years ago. Two new DNA studies reveal that humans probably came to North America more like 20,000-30,000 years ago.

Some time after these Asian settlers arrived, glaciers or some other barrier kept them from returning back to Asia. And from moving south into North America.

Then, around 19,000 years ago, the Earth started to warm up. This warm up created the Bering Strait which kept these people from going back to Asia. But it allowed them to spread quickly through North and South America. In fact, it only took them a few thousand years to reach the southern part of South America.

Scientists in two separate studies figured this out by reading the entire mitochondrial DNA (mtDNA) sequence of hundreds of Native Americans and Asians. A different group confirmed that humans were in Oregon 14,000 years ago by looking at the mtDNA of some really old poop.

The Three Studies

Old Poop and Mitochondrial DNA

Humans may have crossed over from
Asia earlier than previously thought.

Scientists can learn a lot from poop. They can tell how many pandas are in the wild. They can tell which group of chimps the virus that causes AIDS came from. And they can tell when people were in North America.

Scientists do all of these things by looking at the DNA in this fecal matter. A group of scientists discovered some coprolites (a fancy word for old poop) in a cave in Oregon that they thought might be human.

They thought it was human because the coprolites looked like human poop. In the words of the authors, the 14 coprolites were "…morphologically human based on size, shape, constituents, and color." What piqued these researchers' interest was that they found the poo in a very old part of the cave. From a region of the cave that should have predated humans in America.

When they did carbon dating on these coprolites, they determined that they were around 14,000 years old. This contradicted older theories about the settling of the New World and so they set out to prove that the poo was human.

They did this by looking at the mitochondrial DNA (mtDNA) in the coprolites. There are thousands of copies of this DNA per cell so it is ideal for studies of older samples where lots of the DNA might be degraded.

The researchers did find human DNA in the coprolites. But this didn't prove that the poop was human.

A big problem with studying old specimens like this comes from contamination with modern human DNA. What if they were really looking at some of the researchers' own DNA that had somehow ended up on the coprolites? Or what if human DNA from a more recent sample had leached into this sample?

These can be tricky questions to answer. Luckily, it is relatively easy to tell different people's mtDNA apart.

Proving the Poo is Old Poo

DNA recombines in sperm and
eggs. This makes most DNA
hard to trace.

MtDNA is very different from the rest of our DNA. First off, it is found in the mitochondria and not the nucleus. Second, it passes directly from mother to child. Dad's mtDNA is destroyed as soon as the egg is fertilized.

These properties make mtDNA ideal for ancestry and, in cases like this, identification purposes. Why? Because this DNA does not recombine.

Remember, that with the exception of the Y chromosome, we have two copies of each of our chromosomes. One copy comes from mom and the other from dad.

Just before a sperm or egg is made, each pair of chromosomes swaps some DNA. This is a wonderful process for creating ever more unique individuals. But it means that after a few generations, DNA gets pretty jumbled up and becomes hard to trace.

Because mtDNA has no partner and dad's mtDNA is destroyed, it doesn't undergo recombination. This DNA passes virtually unchanged directly from mother to child. Which means scientists can trace back this DNA for thousands of years along a line from mom to grandma to great grandma, etc.

When the researchers looked at the DNA in the poop, they found that it was Native American mtDNA. They checked the mtDNA of anyone involved in the research or who might have come into contact with the samples and showed that none of them had Native American mtDNA. So the DNA did not come from the researchers.

The next step was to show that the DNA didn't come from other samples in the cave. One of the ways they decided to rule this out was to look for wood rat DNA in the samples.

Wood rat remains litter the cave where they found these coprolites. So if any DNA were going to leach into their samples and contaminate them, it would be wood rat DNA. They found no wood rat DNA in the samples.

They did many other experiments to show that the DNA didn't leach from somewhere else too and all the results were negative. So most likely the poop was human and around 14,000 years old. This means humans were almost certainly in North America longer than had been previously thought.

Divining American History through DNA

Scientists can learn a lot
about history by studying
people's DNA.

Now that researchers had confirmed that humans were as far south as Oregon by 14,000 years ago, the next question is how did they get there? Two research groups came up with a theory based on studying the mtDNA of hundreds of Asians and Native Americans.

Of course, people had done these kinds of studies before. These earlier studies had shown that there were five main groups of Native American mtDNA—A, B, C, D, and X.

Scientists believed that A-D came from Asia as it matched up very well with mtDNA from Siberia. The X group was a mystery though. The latest theory was that these people's ancestors might have come from Europe based on the fact that there were a few X groups
there too.

What is different about these two groups' new research is that they looked at more of each person's mtDNA. The older work looked at just a snippet or two of the mtDNA. The new research looked at all 16,569 DNA bases.

When they did this they were able to see patterns that were hidden before. For example, the X group looked much more similar to the A-D groups than had been previously thought. Most likely the X group was part of the original group that came over from Asia.

The researchers also studied the differences between all of these people. Because mtDNA doesn't recombine, these differences came from the occasional DNA change that crops up in any DNA over time*.

By looking at these changes, scientists could figure out that the founding group was probably larger than scientists originally thought. Older data suggested that as few as 80 people came over from Asia. The new data suggested that the number was probably closer to 1000.

The data also suggested that this group stayed small and did not mix with other groups for thousands of years. Then, around 18,000 or 19,000 years ago, the group suddenly expanded greatly in size.

It is from this kind of data that the researchers came up with their theory about how the Americas were populated. Their theory is that Asians settled in what is now North America 20,000-30,000 years ago.

This group continued to interact with the original Asian populations for many years but then stopped for some reason. The numbers line up with a new ice age which is where the idea that they became trapped by glaciers came from.

The group was then stuck there for thousands of years and never grew much beyond 1000 people. Then when the thaw opened up access to North and South America along the Pacific coastline, the population quickly spread.

It is important to remember that this is just a theory to explain the DNA data the researchers saw. And that more extensive studies may change the storyline. But one thing is for sure; humans were in North America much earlier than was previously thought.

* These mutations can be caused by something in the environment that damages the DNA. Or the cells can do it to themselves when they make a mistake copying their DNA.

Content provided by the Department of Genetics, Stanford University.