Developmental roots of cortical bone distribution and mandibular robusticity in early hominins.

Mandibular morphology in early hominins reflects both developmental processes and functional demands, yet the ontogenetic emergence of robust mandibular traits remains poorly understood. Here, we present cross-sectional analyses of cortical bone development during early mandibular ontogeny across Paranthropus robustus (N = 7), Paranthropus boisei (N = 1), Australopithecus africanus (N = 1), early Pleistocene Homo erectus (N = 1) and Homo habilis (N = 1), Pan troglodytes (N = 29), Pan paniscus (N = 34), and Homo sapiens (N = 24). We examined the cortical thickness and the structural properties at two key regions of the mandible-the symphysis and the corpus between the deciduous molars. We find that cortical bone thickness displays strong positive allometry with mandibular height across all taxa, with relative cortical thickness increasing significantly during early ontogeny. This pattern reflects normal ontogenetic bone growth, potentially reinforced by rising masticatory demands. In P. robustus, disproportionate cortical thickening and elevated resistance to bending-measured by the second moments of area-emerge early and become more pronounced throughout development. In contrast, the early Pleistocene Homo shows cortical thickening and second moments of area comparable to those of H. sapiens, even though H. habilis shares external morphological features with Paranthropus. Across all taxa, the mandibular robusticity index decreases during early ontogeny, reflecting proportionally greater increases in corpus height. However, this measure does not differentiate groups in infants and slightly older juveniles, whereas second moments of area reveal taxon-specific developmental patterns. These results highlight the value of second moments of area and, to a lesser extent, cortical thickening and robusticity index, for understanding ontogenetic and functional divergence among early hominins. They further suggest that robust morphology of Paranthropus stems from a distinct developmental trajectory established early in life.