Evolutionary drivers of encephalization and facial reduction in the genus Homo.

The evolution of cranial morphology in the genus Homo is marked by increasing encephalization and reduced facial robusticity through time. These changes are often assumed to be the product of directional natural selection for larger brain size, presumed to be associated with greater cognitive abilities, and for decreasing facial size to lower energetic costs, as masticatory demands were reduced through food processing and tool use. Here, we test this hypothesis by exploring the goodness-of-fit of the predictions of six evolutionary processes to the observed cranial morphological variation in Homo. We analyzed comprehensive neurocranial and facial 3D landmark coordinate datasets, grouped into eight Operational Taxonomic Units starting with early Homo and representing two lineages, Homo sapiens and Homo neanderthalensis. Our neurocranial analysis found strongest support for models of neutral evolution (Unbiased Random Walk) and stasis across both lineages. Similarly, the face shows strongest support for models of stasis, strict stasis, and neutral evolution (Unbiased Random Walk). Our findings suggest a limited role for gradual directional selection and underscore the importance of stabilizing selection and constraints in our lineage's evolution, highlighting the importance of environmental constraints and possibly cultural behaviors as core drivers of human evolution.