John Hoffecker, an archaeologist at the University of Colorado at Boulder, drew attention to a study of an archaeological site called Bluefish Caves.
博尔德科罗拉多大学的考古学家John Hoffecker对 “蓝鱼洞”考古遗址非常感兴趣。
This is in Yukon, a Canadian territory that abuts Alaska.
Some of the remains found in these caves date back 24,000 years.
They include stone tools and the bones of horses, caribou and bison, all with marks which imply those bones have been stripped of their flesh by such tools.
A third line of evidence, a genetic analysis, adds weight to all this.
It compared 31 modern genomes from the Americas, Siberia and various Pacific islands with 23 ancient genomic sequences from archaeological sites in the Americas.
The comparison suggested that Native-American genomes diverged from their Siberian ancestors no earlier than 23,000 years ago.
It also showed that the Native-American line was isolated for at least 8,000 years before big genetic splits within it took place as people spread through their new homeland.
Combining everything, then, it seems that the band of brothers and sisters whose descendants first populated the Americas lived somewhere between 25,000 and 23,000 years ago.
Very neat, if it were not for the fact that archaeological evidence appears to show that areas outside Alaska and Yukon were colonised rapidly, starting soon after 15,000 years ago.
That could be because the ancestral band and its descendants were confined for much of the intervening period to a region known to palaeogeographers as Beringia.
This was composed of what are now eastern Siberia, bits of Alaska and Yukon in the Americas, and the Bering Strait between them (which was then dry land).
Parts of Beringia were habitable wetlands and grassland steppe.
But the North American ice sheets to its east would have blocked any passage beyond.
That could account for the 8,000 years of genetic autarky in the ancestry of Native Americans,
for it was not until the ice sheets retreated (starting about 16,000 years ago), that anyone in Beringia would have been able to pass to the rest of the Americas.
To explain how languages might have continued to diversify in a genetically stable population within Beringia,
DrSicoli suggests its members may have lived in different habitats, separate enough for linguistic diversification,
but mixing often enough to maintain a single gene pool.
The answer to the question, “how was America peopled?” seems tantalisingly close.