The skull was relatively long and narrow, with features that some researchers described as resembling certain populations from Southeast Asia, Australia, or even historical European populations, rather than the broader, rounder crania that characterized most Native American groups.

This observation was not unique to Spirit Cave.

A handful of other very ancient American skeletal remains — including the Kennewick Man, found on the banks of the Columbia River in Washington State in 1996 and dated to approximately 8,500 years ago, and remains from Lagoa Santa in Brazil — showed similar apparent morphological differences from later Native American populations.

This observation led a group of researchers to propose what became known as the Paleoamerican hypothesis — the idea that the earliest inhabitants of the Americas were not the direct ancestors of modern Native American populations but rather a distinct group that had arrived in the hemisphere via a different route, possibly from Australia or Southeast Asia, and that had been subsequently replaced or absorbed by a second, later wave of migrants from northeastern Asia who became the ancestors of modern Native Americans.

The implications of this hypothesis were significant.