Cranial Morphometrics and Postcranial Affinities of Sahelanthropus tchadensis: A Multivariate Re-evaluation of Phylogenetic Position, Genetic Drift and Hominin Congruity in Miocene-Pliocene Chronology

The Late Miocene hominid Sahelanthropus tchadensis occupies a pivotal yet contentious position in early hominin evolution. Since its discovery in 2001, debates have persisted regarding its phylogenetic placement. Current research places S. tchadensis ostensibly as a stem hominin, a basal hominid, with some research positioning the taxon as an early member of the Gorillini lineage. Recent craniometric analyses have reinforced hominin affinities, particularly in basicranial and dental metrics, while postcranial studies have generated conflicting interpretations regarding locomotor behavior. In this paper we present a comprehensive multivariate re-evaluation integrating (1) three-dimensional geometric morphometric analysis of cranial landmarks from the virtual reconstruction of TM 266-01-60-1, (2) comparative dental metrics across extant apes and Plio-Pleistocene hominins, and (3) quantitative reassessment of femoral and ulnar morphology incorporating newly available data from TM 266-01-063, TM 266-01-050, and TM 266-01-358. Whereas principal analysis of the cranial features including the vertically oriented upper face, the reduced subnasal prognathism, and an anteriorly positioned basicranium, places S. tchadensis within the hominin morphospace intermediately between African apes and australopiths. It is the detailed dental metrics that align the species unequivocally with Plio-Pleistocene hominins rather than extant apes, and in the supplementary re-analysis of femoral neck-shaft angle, diaphyseal cortical thickness distribution, and cross-sectional geometry a substantial overlap with both early hominins and extant hominids is uncovered. The additional presence of an obturator externus groove and femoral tubercle also further suggests some capacity for extended hip posture comparable to Ardipithecus ramidus, but without the full suite of australopith locomotor adaptations challenging claims of habitual bipedality. We interpret these findings through the lens of population-genetic models incorporating Late Miocene effective population sizes, rates of genetic drift, and incomplete lineage sorting. We propose that S. tchadensis represents a stem hominin exhibiting mosaic evolution, with cranial and dental derived characters emerging under strong directional selection for dietary and social functions, while postcranial morphologies persisted due to weaker selective pressures on locomotion in wooded habitats. This re-evaluation supports a defined hominin status for Sahelanthropus while cautioning against over-interpretation of individual anatomical systems in isolation, emphasizing the need for integrative phylogenetic frameworks that accommodate clearly delineated classification in a case of heterogeneous trait evolution during the chimpanzee–human divergence.