The founding population then split into two groups approximately 20,000 years ago, the genetic evidence suggests. According to the researchers, these groups were the newly identified Ancient Beringians and the ancestors of all other Native Americans. There are two possible explanations for where this split occurred, and how the separation affected subsequent migrations.
The first possible scenario is that Ancient Beringians and the ancestors of other Native Americans diverged somewhere in Asia 20,000 years ago. Both populations then could have moved along different routes, or at different times, through Beringia.
The second possible scenario is that Ancient Beringians and the ancestors of other Native Americans diverged in what is now Alaska 20,000 years ago. The latter group then could have moved south of the ice sheets at a later date.
"Regardless of whether scenario one or two is correct, there was a single population of ancestral Native Americans, and they at least migrated to an area where they were genetically isolated from ancestral East Asians and ancestral North Eurasians," Potter said.
Moreno-Mayar noted, "By 20,000 years ago, most of North America was covered by two vast glaciers, so whenever Native Americans made it to Alaska, they most likely still had to wait until a viable route into mid-latitude America was made available."
As for why people then migrated so much, prior research suggests at least one reason. They appear to have been following the movements of prey animals. These could have included migrating schools of fish and/or terrestrial mammals such as woolly mammoths, steppe bison, and caribou.
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Geological and ecological evidence suggests that the northwest American coastal route was ice-free and potentially navigable by 16,000 years ago, while the American interior was ice-free by 15,000–14,000 years ago.
A debate now exists as to precisely when humans first colonized the Americas and if those first routes were over land or water. Moreno-Mayar and his team admit that the genetic evidence cannot yet provide definitive answers to these questions.
Archaeological findings ranging from everything to an ancient fishhook to fossilized excrement provide clues, though. Evidence for human settlements at Triquet Island, Paisley Caves, and the Channel Islands along the western North American coast all date to around 13,000–14,000 years ago.
The sites Quebrada Jaguay, Quebrada Tacahuay, Quebrada Santa Julia, and Monte Verde along the western South American coast date to about 13,000 years ago.
On the eastern coast of North America, the site Page-Ladson in Florida dates to approximately 14,500 years ago. Stone tools and mastodon remains suggest that humans there were hunting big game. Yet another early site is Huaca Prieta, north of Peru, which dates to 15,000–14,500 years ago.
Taken together, the genetic and archaeological evidence then suggests that people first expanded out of Beringia by about 16,000 years ago.
Archaeologist Todd Braje of San Diego State University, who recently co-authored a paper published in Science on Native Americans, told Seeker that "the first Americans likely arrived along the Pacific Coast — not crossing the open Pacific but migrating along the Pacific Rim in boats in a step-wise fashion."