When science meets history: Sorting out the path of the first Americans

You may have learned a story of the peopling of the Americas in grade school. The tale begins near the end of the last ice age with a mass migration across a land bridge from Asia to Alaska From there, the first Americans spread down into North, then South America along an interior corridor that opened up between thick ice sheets that blanketed what is now Canada. Or perhaps the tale included a Pacific coastal route.

But coming up with a single narrative isn’t actually as straightforward as it may seem. Without written records, we need scientific data to teach us our prehistory. And that data doesn’t always fall into one nice, neat, settled narrative.

When it comes to the peopling of the Americas, scientists are increasingly entertaining the possibility that the story might not be so simple – or so simple to sort out.

“This is an extremely complex issue. There’s not going to be any simple model or simple narrative that covers all of the variants that we see in the early peoples of the Americas,” says Michael Collins, an archaeologist at Texas State University in San Marcos.

Decades ago, scientists discovered stone tools in the United States dated to shortly after an ice-free corridor was thought to have opened up. The tools appeared to be about 13,500 years old, and the ice sheet is thought to have opened up about 14,000 to 15,000 years ago. So it was easy to imagine that the people using those tools (dubbed members of the Clovis culture) followed bison or other rich resources down that corridor and then spread across the region below the ice sheets.

“We didn’t know anything earlier,” says Professor Collins. “It was an elegant model, and it seemed to explain everything.”

So when some archaeologists found signs of even earlier human presence south of the ice sheets – even as far south as Chile – many other scientists were skeptical, and some evidence was even dismissed as impossible. But the evidence piled up. And then paleoecologists found evidence that suggested the corridor might not have been enticing, resource-wise, for humans until about 12,600 years ago.

Some scientists began to proclaim that the peopling of the Americas needed a rewrite. Around the same time, the idea emerged that perhaps the first Americans exploited resources like kelp as they migrated along the Pacific coastline, which would explain some pre-Clovis archaeological sites in the region.

But that rewrite might have come too quickly, says Ben Potter, an archaeologist at the University of Alaska Fairbanks. He and a group of other archaeologists, anthropologists, geneticists, and paleoecologists are working on questions around the peopling of the Americas. Their paper published Wednesday in the journal Science Advances reviews much of the evidence and asserts that there are still many questions to be answered about the Pacific coastal model – and that we may have been too quick to dismiss the ice-free corridor pathway. Neither can be ruled out, they say.

In the paper, they point out that new scientific data has emerged that suggests the corridor might not have been so barren. There were trees there as early as 13,500 years ago, so perhaps the environment was enticing early enough for those early Americans to wander down it after all.

“We’re letting our hypotheses drag us around by the nose,” says Collins, who was not involved in the study. If we get too attached to one of these narratives, he says, it’s difficult to keep an open mind to new data. 

Writing a story or painting a picture?

Part of the problem, Collins says, is that this isn’t like laboratory science. Researchers can’t reproduce each other’s work to refute or corroborate it. Instead, it depends how you assemble the data in context, and consider its quality.

Furthermore, human migration isn’t exactly linear. A group of people doesn’t say, “hey, let’s march ourselves halfway across the globe without knowing what’s there.” Instead, explains Professor Potter, “we’re looking at expansions and contractions of people as habitat gets better or worse.”

That idea leaves open the possibility that humans migrated down both proposed pathways. Or perhaps people set out in waves.

It’s more like painting a picture of a moment in time than piecing together a linear narrative, says Amy Gusick, associate curator of anthropology at the Natural History Museum of Los Angeles County. All the archaeological and genetic clues must be put into the broader environmental context to understand whether or not a given place could even support people.

“We’re trying to understand broadly this world that these individuals may have gotten into at this particular time,” she says. “We’re just building it piece by piece to understand a much broader issue and a much broader question.”

Part of looking at the broader topic of the peopling of the Americas requires letting go of the attachment to figuring out who was first, Potter says. “That one probably could never be answered.”

Although holding too tightly to narratives can be limiting, narratives do serve a purpose, says Collins. Defining the patterns in the data into a narrative allows for more productive dialogues, he says.

“You build a model and expect it to evolve,” he says. “If you are in the business of making models of human behavior in the past, you cannot expect these to be universally accepted. People are going to challenge aspects of it and change will follow. Sometimes the whole idea will just get dumped.”

“There’s so much more to learn,” says Dr. Gusick. Scientists across disciplines – ecologists, archaeologists, and geneticists – are increasingly looking at the topic to add more data. “All these different kinds of input from so many different kinds of researchers that do so many different kinds of specialties all have their part to play in piecing it together.”

With so much curious data, any story about a prehistoric human expansion is far from written in stone. “We don’t begin to have all the answers,” says Collins. “We don’t even have all the questions.” 

The story of the peopling of the Americas is no different. It may never be settled. And that’s what makes it interesting, Gusick says. “One of the great things about science is the continual discovery of things.”