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University of Wyoming-led research questions the timing of early human arrival in the Americas

An aerial shot of evergreen trees in a valley.
Todd Surovell
An archaeological site the team studied in Eastern Wyoming called ‘Hell Gap.’ Researchers studied 12 sites total across the continental U.S.

How long ago humans arrived in North America continues to be a hotly debated issue within the science community - on the extreme end, researchers estimate more than 100,000 years ago - but new University of Wyoming-led research indicates it might only have been about 13,000 years ago.

University of Wyoming researchers Todd Surrovell and Sarah Allaun are the two lead authors on the study. They spoke with Wyoming Public Radio's Caitlin Tan.

Caitlin Tan: So, I'm wondering if you guys can give us an overview of your research for those who are unfamiliar?

Todd Surrovell: Well, this research concerns one of the long-standing debates and questions in American anthropology, and that is the timing of the first human arrival to the Americas. This is a question that anthropologists and archaeologists have been debating for easily 100 years, and much longer than that. It's a debate that is lively, that is contentious, and this is just one small part of a long, long history of research.

So, that's in a general sense what we're doing. And specifically, you know, we're looking at a series of sites and analyzing them to try to determine when we first see evidence of humans on the continent, in specific kinds of evidence.

CT: I think what a lot of us learned growing up in school is that the first people that came to North America came from Asia via a land bridge, maybe roughly 14,000 years ago. And so that's kind of where the debate is centered, right?

Sarah Allaun: It's not just where people came from. I think looking at the genetic evidence it's pretty clear that people came from Asia and into North America. But looking at the timing of that event is kind of the question that we still have today.

CT: Okay, and so what exactly did you guys find in your research?

TS: Well, several dates of human arrival have been proposed. I think everybody agrees that humans were on this continent by 13,000 years ago. And everybody would agree that humans were in Alaska before that, probably by 14,000 years ago.

To what extent people were here before that time remains very, very open to debate. And there are a number of models that have been proposed. Some people suggest humans have been on this continent 15,000 years ago. Others suggest 20,000 years ago. Others suggest 40,000 years ago. Others have even suggested that more than 100,000 years ago there were humans or perhaps human ancestors on this continent.

And through our research, we actually came up with a very conservative estimate, [supporting] the traditional explanation for when humans arrived, which is that they arrived in Alaska sometime, around 14,200 years ago. When they arrived in Alaska, the way south was blocked by continental ice sheets, and they managed to breach those ice sheets and make it into the continental United States sometime around 13,000 years ago, or just before that.

CT: And so tell me a little bit about the process of your research and the different sites that you looked at. One of them was in eastern Wyoming, if I understand correctly.

TS: So the way we looked at this question was specifically looking at Stratigraphic Integrity. So, if you find artifacts, for example, in sediments that are 15,000 years old, you might infer that those artifacts are also 15,000 years old, but there's actually two explanations.

One is that people were there 15,000 years ago, and they deposit those artifacts. Another possible explanation is that humans arrived at that place sometime later, let's say 11,000 years ago, and then the artifacts later moved into older deposits.

And the way that artifacts move are things like what we have in Wyoming, like pocket gophers, prairie dogs, ground squirrels. They are constantly turning the ground and moving artifacts around.

So, we're really interested in being able to distinguish between those possibilities. And to come up with a measure, a rough statistical measure for how stratigraphically intact sites are by looking at the distribution of artifacts within them, say vertically, or stratigraphically.

We looked at a series of sites - 12 in all. Three of them were in Wyoming, a handful of sites in Alaska, and then other sites around the continental United States. We asked, "When do we first see clear evidence of stratigraphically intact archaeological occupations?"

CT: So with your research, does this potentially fundamentally change how we have perceived this early human migration?

SA: The answer we came to is that people likely appeared around 13,000 years ago. It is not so much a fundamental change, but rather support for a traditional model of the peopling of the Western Hemisphere. Newer models argue for occupation well before 13,000 years ago, but we weren't able to support that kind of interpretation with the evidence that we looked at.

TS: I think it's important to note that we found support for the very traditional model of New World colonization and yet, I think our conclusion will be found to be pretty controversial.

A lot of people in our discipline have long since abandoned that traditional model, and have argued for the presence of humans on this continent long, long before 13,000 years ago.

So, this is sort of an interesting tension in the field. This traditional and generally well supported model is somewhat controversial.

CT: Why is there such varied research out there? How is it possible that different researchers have come to such different conclusions?

SA: Archaeology can be a tough field. We're dealing with a record that is oftentimes incomplete, especially when we're looking at the peopling of North America. We're talking about 13,000 plus years ago.

So we're looking at the distant past. The evidence that we're looking at can be quite old and sometimes of varying quality because of that, and so a lot of our interpretations are impacted heavily by the quality of the data that are available to us.

TS: And for any given find, there's often multiple interpretations possible.

Caitlin Tan is the Energy and Natural Resources reporter based in Sublette County, Wyoming. Since graduating from the University of Wyoming in 2017, she’s reported on salmon in Alaska, folkways in Appalachia and helped produce 'All Things Considered' in Washington D.C. She formerly co-hosted the podcast ‘Inside Appalachia.' You can typically find her outside in the mountains with her two dogs.
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