HOW America was originally colonised is a topic of perennial interest at the AAAS. Until recently, the earliest uncontested archaeological evidence of people living in the New World came from Swan Point, in Alaska. This dates back 14,400 years. Linguists, however, maintain that the diversity of native languages in the Americas could not have arisen so quickly. Conventional models of linguistic evolution assume tongues separate in the way populations of organisms do—so that the flow of vowels, words and grammatical structures between groups must cease before new languages can emerge, just as a cessation of gene flow gives rise to new species. This suggests it would take at least 50,000 years for a single population speaking a single language to diversify and spread through the Americas in a way that yielded the pattern heard today. Since Native Americans’ genes do, indeed, indicate they all derive from a single population, this discrepancy in timing is a paradox.

That paradox may be close to resolution. Recent digs have pushed the physical evidence of America’s settlement back in time. Meanwhile, as the meeting heard from Mark Sicoli, a linguist at the University of Virginia, in Charlottesville, a different model of linguistic evolution brings the common ancestor of Native-American tongues forward. Apply a few error bars to the results and the two estimates touch—at about 25,000 years ago.

The problem with explaining linguistic evolution in pure Darwinian terms is that words are not genes. Species, once separate, do not exchange genetic information because they do not interbreed. Languages, though, can exchange grammatical and semantic elements when they meet, which can speed up diversification. Dr Sicoli thus turned to computational phylogenetic analysis, an area of linguistic research that tries to work out whether and how such interaction may have taken place.

From the thousand or so Native-American languages he chose four dozen spoken in Alaska and northern Canada, the part of the Americas closest to humanity’s point of entry from Asia. He and his colleagues created a database that recorded, for each of them, 116 linguistic features such as sounds, parts of words, the functions of these parts and the ways a language combines words into phrases. They then used this to identify the influences of languages on each other. They also added geographical information, plotting the flow of linguistic change along the Pacific coast and through the river valleys. This nearly halved the time needed to give rise to the modern situation if the languages had evolved independently from a single common ancestor. That suggests the process of divergence may have begun as recently as 25,000 years ago.

John Hoffecker, an archaeologist at the University of Colorado at Boulder, drew attention to a study of an archaeological site called Bluefish Caves. 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, Dr Sicoli 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.