A test of the Archaic Homo Introgression Hypothesis for the Chiari malformation type I

Abstract

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 Homo species that have smaller basicrania than is typical for modern humans-Homo erectus, Homo heidelbergensis, and Homo neanderthalensis. 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 H. erectus, H. heidelbergenesis, H. neanderthalensis, and H. sapiens. 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, H. neanderthalensis. 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.

Keywords: 3D shape analysis; cerebellar herniation; cerebellum; evolutionary medicine; fossil hominin; geometric morphometrics; human evolution; hybridization.

Figure 1.

3D models of Homo sapiens (top two images) and Homo neanderthalensis (bottom two images) crania for visual comparison. The human model was created from DICOM files of an anonymised volunteer patient from the Manchester Centre for Clinical Neurosciences. The Neanderthal model is based on La Ferrassie 1 and was created by LB and TR.

Figure 2.

Landmarks used in the present study, shown on a CT-based 3D model of the cranium of living human without CM-I.

Figure 3.

Histogram depicting the distribution of the scores of the two living human OTUs on the single CV yielded by the CVA. Pink bars = individuals with CM-I. Blue bars = individuals without CM-I. The wireframes illustrate the shapes at the ends of the CV. From top left to bottom right, wireframes show neurocranium in posterior, left lateral, inferior, and right lateral orientations.

Figure 4.

PCA illustrating the shape variation among the living human subsample when PC2 is plotted against PC1. The pink circles are individuals with CM-I; blue circles are unaffected individuals. The wireframes show the shapes at the end of each PC.

Figure 5.

CVA plot depicting the between-group shape variation when CV2 is plotted against CV1. The wireframes illustrate the shape differences between individuals at the positive and negative ends of CV2.

Figure 6.

PCA depicting the shape variance within the entire sample when PC2 is plotted against PC1. The wireframes illustrate the shapes at the extreme end of each PC.