This model avoids the need for an open-ocean crossing but requires that the Australasian-related population made it to Beringia and then to the Americas without leaving the genetic traces that would be expected if it had mixed substantially with North American populations along the way.

The coastal route model is logistically possible in a way that a direct trans-Pacific crossing is not, but it requires a specific set of population dynamics — essentially, a population that moved quickly along the coast without significant admixture with interior populations — that would need to be verified rather than assumed.

The geographic distribution of the signal is also complicated by the fact that it is not simply “present in South America and absent in North America.” It shows up in some North American populations as well, and its intensity varies in ways that do not perfectly match any simple geographic pattern.

This variability is consistent with the deep ancestral structure model, in which the signal reflects ancient diversity within the founding population that was distributed differently across different descendant groups as a result of the specific population dynamics of the American dispersal and subsequent differentiation.