Complex and diverse patterns of neurocranial development in Australopithecus, Paranthropus and Homo.

Even though poorly understood, early ontogeny may have influenced the distinct morphologies and behaviors of Homo sapiens, fossil hominins and extant African apes. Leveraging an extensive high-resolution computed tomography dataset, we investigated previously unexplored relationships between facial, basicranial and endocranial development during early ontogeny in African apes, H. sapiens, Australopithecus afarensis and Paranthropus robustus. We found that a delay in endocranial growth in Pan paniscus infants does not necessarily indicate a uniform trend toward delayed development, suggesting more complexity in developmental processes than previously thought. A. afarensis exhibited protracted endocranial growth compared to P. robustus, a contrast even greater than that between P. paniscus and Pan troglodytes, suggesting that endocranial development in A. afarensis more closely resembles that of H. sapiens than P. robustus. Unlike the shared delayed development of H. sapiens and Homo erectus, A. afarensis and P. robustus show contrasting developmental patterns. We therefore propose that early ontogeny in Australopithecus and Paranthropus was variable, showing no consistent pattern of developmental delay or acceleration across species. Our finding also enhances our ability to identify neonatal cranial remains in the fossil record, providing new opportunities for expanding much needed sample size and advance paleobiological studies.