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Skull diversity within the Homo lineage and the relative position of Homo naledi

The discovery of Homo naledi has expanded the range of morphological variation observed within our genus, and has led to new questions surrounding the mosaic nature of morphological evolution. Though the geological dates of this species are currently unknown, its unique morphological pattern (small brain, derived skull features) and possible phylogenetic connections with other hominin taxa suggest a potentially complex evolutionary scenario. Here, we perform a series of multivariate and 3D geometric morphometric analyses on cranial and mandibular remains of Homo naledi to investigate the morphological patterning/relationships between Homo naledi and several species of Homo and Australopithecus. We also explore the potential evolutionary processes acting to differentiate this species, applying statistical tests developed from quantitative genetics theory to evaluate whether genetic drift versus selection is responsible for the observed pattern of variation. Morphometric results indicate that, for the cranium, Homo naledi is most similar to other members of the genus Homo, with closest affiliations to Homo erectus specimens. In contrast, results for the mandible are less clear ; Homo naledi closely associates with a number of taxa, including some australopiths, depending on the analysis. The quantitative genetic tests reveal that for all cases the cranial/mandibular phenotypic diversity seen among Homo naledi and other hominin groups is consistent with drift. Taken together, these results support the notion that it is the combination of features (erectus-like cranium ; less derived mandible) that makes Homo naledi unique, and suggests that drift, and possibly small population sizes, were important factors influencing the evolution of this species.

Homo naledi ; cranial variation

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