Researchers studying the skulls of some of the earliest known Americans — including a skull from Baja California sometimes called the Peñon Woman, individuals from South American sites, and the Kennewick Man remains — noted that the cranial morphology of some early specimens appeared to differ from the “generalized Mongoloid” morphology that characterized many later Native American populations.

The interpretation of these differences was contested: some researchers argued they reflected a different founding population, others argued they reflected normal variation within a founding population that was subsequently transformed by genetic drift and natural selection as populations adapted to diverse environments, and still others questioned whether the morphological comparisons were being made with sufficient methodological rigor.

The morphological debate intersected with the Solutrean hypothesis in ways that generated popular attention disproportionate to the hypothesis’s scientific standing.

If ancient American skulls showed affinities to populations outside what would be expected from an Asian migration, and if Clovis technology could be traced to European Solutrean origins, then perhaps the peopling of the Americas was more complex — or more European — than the standard model allowed.