A test of the Archaic <i>Homo</i> Introgression Hypothesis for the Chiari malformation type I

The Chiari malformation type I (CM-I) is a herniation of the cerebellum through the foramen magnum. Its proximate cause is accepted to be an unusually small occipital bone. However, its ultimate cause remains unclear. In 2013, Fernandes and colleagues hypothesized that individuals develop CM-I because some of their cranial development-coding genes derive from three extinct <i>Homo</i> species that have smaller basicrania than is typical for modern humans-<i>Homo erectus</i>, <i>Homo heidelbergensis</i>, and <i>Homo neanderthalensis</i>. Here, we report a study in which we used 3D data and Geometric Morphometrics to evaluate this hypothesis. We began by investigating whether CM-I is associated with significant differences in cranial shape in a sample of living humans. Subsequently, we compared the crania of living humans with and without CM-I to fossil crania assigned to <i>H. erectus</i>, <i>H. heidelbergenesis</i>, <i>H. neanderthalensis</i>, and <i>H. sapiens</i>. The study's results were mixed. The first set of analyses identified significant shape differences between the crania of people with CM-I and the crania of unaffected people, which is in line with the hypothesis. In contrast, the second set of analyses did not support the hypothesis. They indicated that the crania of living humans with CM-I are only closer in shape to one of the extinct species, <i>H. neanderthalensis</i>. The other two extinct species were found to be closer in shape to living humans without CM-I. This is contrary to the main prediction of the hypothesis. Together, our results suggest the hypothesis should be narrowed to focus on introgressed genes from Neanderthals.