This similarity was not what would be expected if the Americas had been populated slowly, with populations differentiating gradually from north to south as people spread incrementally across the hemisphere over many millennia.

A slow, gradual southward spread would produce increasing genetic differentiation with distance — the further south you went, the more different the populations would look from those at the northern end.

The pattern the researchers found was different.

People at opposite ends of the Americas — Nevada and Brazil — shared genetic similarities consistent with a common origin much more recent than a slow southward spread would predict.

The simplest explanation was a rapid initial expansion: people had moved from Beringia to the southern tip of South America in a time span short enough that they had not yet had time to differentiate substantially from their common ancestral population.