Humans began migrating to the Americas roughly 15,000 to 20,000 years ago. They didn't bring their dogs with them.
"We were interested in seeing if we could see when dogs arrived in the Americas," said study author and graduate student Kelsey Witt. "It's assumed they came with humans, but no one has actually looked at it."
Dogs very likely were domesticated in Asia and then brought over to the Americas, Witt said. "What we know about ancient dogs so far is that by the time Europeans arrived, they were pretty widely spread across the Americas. They were used to haul supplies, used as guards, had religious significance for some, used as a food source."
For this study, researchers analyzed the remains of 84 dogs, including 34 from a single burial site in Illinois that dates back about 1,000 to 1,400 years. They specifically examined mitochondrial DNA, which is inherited maternally and allows a direct look back at a lineage. Previous studies only had access to about 40 such dog remains.
What these researchers found is that the genetic diversity of the dogs was only about 10,000 years old. And the newly studied dogs were closely related, which suggests that humans may have been breeding the animals, Witt said.
Some caveats to these findings: Witt said it's possible their results were skewed because they were working with very old and disintegrated DNA. "You can't always sequence as much as you might like," she said. Her next project involves working with a much longer stretch of DNA to see if she can replicate similar results.
The dogs arriving in the Americas were more genetically similar to European wolves, and a prevalent theory is that some went on to interbreed with American wolves, Witt said.
Given that dogs have been an integral part of human life for so long, learning about their ancient history also means we are learning about our own.
Witt said sequencing the DNA of ancient humans can be extremely challenging, especially when people don't want their ancestors' remains destroyed or disturbed. Dogs can serve as a good proxy. Also, although humans in the American Midwest were burying dogs about 1,000 years ago, they weren't burying people.
The late arrival of dogs suggests that there may have been a second, major wave of migration of humans as well. And once dogs arrived in the Americas, they quickly spread all over the place, which tells us more about human movement.
So, could dogs have arrived in North and South America on their own? "The assumption," Witt said, "is that humans chose to bring them" — for some of the same reasons people have dogs today, she said.
"I would imagine they were brought because they were useful to the people bringing them. For whatever reason, they felt they would be helpful as they were migrating."
Those ancient humans must have missed the memo the first time around.