For much of the 20th century, scientists believed that the first settlers of the Americas could only have arrived one way. As the conventional story went, an ice-free super highway opened up across the Bering Land Bridge toward the end of the last ice age, allowing people from Eurasia to follow big game like bison and mammoths down through the interior of North America.

New archaeological discoveries have challenged that narrative in recent years. And a study published in the journal Nature offers further evidence that this northerly corridor wasn’t the first route to the continent.

University of Copenhagen researchers Eske Willerslev, Mikkel Pedersen, and their colleagues found that this harsh route only became viable for human migration 12,600 years ago—when the first plants and animals showed up in the region. Meanwhile, archaeologists have ample evidence that people were living in the Americas long before then.

“We know conclusively that human groups were in the interior before that date—perhaps as early as 15,000 calibrated radiocarbon years before present—so it is highly unlikely that they came south through the corridor,” said Michael O’Brien, an anthropologist and current academic vice president of Texas A&M University–San Antonio, who wasn’t involved in the study. “A more likely scenario is that they came south along the Pacific coast.”

For the study, Pedersen and colleagues drilled sediment cores from beneath the frozen surface of two lakes in western Canada: Charlie Lake and Spring Lake. These were among the last areas to lose their ice cover when the two huge ice sheets that blanketed the region (the Laurentide and Cordilleran ice sheets) split during the end of the last glacial maximum, around 15,000 years ago. The retreating ice opened up a path some 1500 kilometers long into the interior of North America.

With the sediment cores, the scientists were able to reconstruct a history of environmental conditions along this route based on algae, pollen and other plant matter, fossils, and ancient DNA trapped in the layers of chilly soil. They concluded that before 12,700 years ago, patchy grass was the only life along the ice-free route. Slowly, plants like sage brush and willow started to change the barren corridor into a steppe landscape. By 12,600 years ago, bison arrived. About 2000 years later, the route started to look more lively as it became populated by jackrabbits, voles, and other small mammals, which were followed by mammoths, elk, and predators like bald eagles. The route likely became impassable for humans and big mammals again about 10,000 years ago, when dense coniferous forests started to grow.

The results of the study suggest the route was only usable between 12,600 and 10,000 years ago. This narrow window is too late to match with the once-prevailing “Clovis First” hypothesis. The Clovis people, who are named after their characteristic fluted stone spearheads first found near Clovis, New Mexico, were thought to have been the first inhabitants of the Americas. The earliest Clovis points show up in the archaeological record about 13,500 years ago. It was long believed that they got here by crossing the Bering Land Bridge sometime before then.

Recently, several before-Clovis sites have been discovered in the Americas. Fossilized feces more than 14,000 years old have been found in Oregon’s Paisley Caves. Stone tools alongside mastodon bones in Florida were recently found to be 14,550 years old. And much further away from northwestern Canada, in southern Chile, humans inhabited Monte Verde at least 14,000 years ago (and possibly even earlier).

Alternate migration routes have been put forth in the past, such as the controversial Solutrean hypothesis, which posits that the first Americans actually came from Europe, not Asia, via a North Atlantic route. But many anthropologists now favor a Pacific coastal route to explain how the first people got to the Americas, though more research is needed to fully understand how these intrepid settlers traveled (perhaps by boat).

“Such a study has been needed for quite some time now,” said Vanderbilt College archaeologist Tom Dillehay, who wasn’t involved in the new study. Dillehay, whose excavations at Monte Verde in the 1970s revealed the site's ancient age, challenging the Clovis First theory—and long considered suspect as a result—told mental_floss that this type of study is just the beginning. “I would like to see more studies of this nature done in other areas of the corridor to confirm this hypothesis—especially at the entrance and exits points of the corridor.”