The bones of giant steppe bison and clues left by their ice age hunters have led scientists to conclude that people likely colonized North America south from Alaska along the Pacific coast, and not through the Rocky Mountains, according to a new study.

The first ancient people in America are believed to have arrived from Siberia, across a land bridge now submerged under the Bering Strait. Exactly when that crossing was made and how the people then spread across the Americas are still mysteries – if not for want of tantalizing hints.

Those hints are spread across two continents and suggest a complicated, even contradictory, story about how people conquered the west. Evidence of human societies has been found as far east as the Florida panhandle, dating 14,500 years ago, and as far south as Chile, dating more than 15,000 years ago.

These discoveries, with a host of other finds, have challenged the traditional story that people swept into America in a single migration 13,500 years ago. In the new study, researchers from the University of California Santa Cruz decided to follow the example of ancient hunters, and track bison through the mountains.

“The interesting thing about the [Rocky Mountain] corridor is that it was open until about 21,000 to 23,000 years ago, when the ice sheets to the west and east came together and completely separated populations,” Pete Heintzman, a researcher and lead author on the paper, told the Guardian.

Through DNA analysis of 78 bison fossils, Heintzman and his team found there were two distinct populations to the north and south, and were able to trace when the animals migrated and when and where they interbred. The analysis showed that the northern and southern bison started mingling in the open pass about 13,000 years ago.

The discovery means that the mountains must have cleared of ice more than a thousand years after humans had colonized the south, suggesting humans first settled the Americas along the Pacific coast.

“It’s really hard to think of any other ideas,” Heintzman said. “Fourteen to 15,000 years ago, there’s still a hell of a lot of ice around everywhere. And if that wasn’t opened up you’d have to go around the ice, and going the coastal route is the simplest explanation.”

The Rocky Mountains corridor is still important, he said, for its role as a route for later migrations north and south. Evidence from human artifacts, he noted, suggests “people probably migrated from the south back up to the north through the ice-free corridor – so they’re circling around the mountains” in pursuit of bison.

Distinctive spearheads of the Clovis people, named after the New Mexico site where artifacts were first found, have been found south of the corridor dating to 13,500 years ago. But a Clovis site in Alaska dates to no more than 12,400 years old.

Because of tidal erosion, Heintzman said, few archaeological sites have been found along the Pacific coastline as evidence that ancient humans used it to migrate south. In the north, “people keep finding sites, and the dating’s getting better”, he added, but “there are only a handful of them in Beringia [the Bering Strait land bridge]”.

A researcher of the Yukon Paleontology Program with the skull of a Pleistocene steppe bison. Photograph: Government of Yukon

Bison were painted and hunted around the world and were among the most common animals in North America. They survived the mass extinction that killed off the mammoths, dire wolves, American lions and 9ft-tall sloths.

More than 6ft tall at the shoulder, steppe bison were much larger than living American bison, and a small population survived past the ice age until a few centuries ago. Modern American bison are descended from the northern steppe giants, Heintzman said, even though they live south of their ancestors’ range.

The researchers published their findings in the journal Proceedings of the National Academy of Sciences.