Reconstruction of the internal cranial base of an adolescent Homo erectus (Sinanthropus III). An enlightening on the hominids neural straightening, its embryonic modalities and its cognitive implications

Abstract

The internal cranial base is at the crossroad of the central nervous system (CNS), the occluso-postural balance and the psychomotor control learning with the development of cognitive abilities and mental images. Weidenreich in 1943 compared the Sinanthropus (Chinese Homo erectus) with Homo sapiens and assumed that the lesser verticalization of its posterior skull base (basi-sphenoid, basi-occipital and petrous pyramids), and, thus, the higher position of the cerebellar fossa, were the results of the more limited anteroposterior growth of the cerebral neocortex (telencephalon). The lesser internal verticality is characteristic of extinct species of the genus Homo which lived before H. sapiens and is clearly visible in comparative anatomy confirmed by the growing number of fossils. Since then, the telencephalic hypothesis became the paradigm. Nevertheless, Weidenreich's hypothesis has been invalidated in 1987 by one of the authors with the discovery of the embryonic origins of the straightening in Homo sapiens. This verticalization is caused by the succession of complex dynamics of the rhombencephalon (future brainstem and cerebellum with the IV ventricle) from the horizontal becoming gradually vertical. Its dynamics cause the plication of the underlying cranial base initially flat, with the dorso-ventral rotation of the basi-sphenoid. We recall this very important discovery given the difficulty of integrating embryogenesis in the sciences of hominization, and present a protocol of angular measurements in 3D to identify hominid species according to their embryonic straightening which increases from great apes to Homo sapiens. The Sinanthropus III disappeared in 1941, but the original casts of each disarticulated bone stored at the Institut de Paléontolologie Humaine, Paris, allowed a reconstruction of the inner surface by their computed tomography scan (CT) and then their virtual assembly for a future digital reconstruction of missing parts. The 3D printing has allowed the reconstruction with modeling clay constrained by the preserved conformation. Finally, the prototype has been scanned and compared to the CT scan of 55 living Homo sapiens including 30 adolescents and 25 adults with 50% males and 50% females, 41 chimpanzees of increasing age, 3 Javanese Homo erectus of Middle Pleistocene, 5 European Homo neanderthalensis and 2 European Homo sapiens of Upper Pleistocene. The protocol confirms the interest of its application to fossil hominid species (more than 70 specimens) by distinguishing at least three increasing embryonic straightenings: 1) the chimpanzee (Pan), 2) extinct species of Homo with H. erectus and H. neanderthalensis and 3) Homo sapiens. The phylogenetic verticalization corresponds to the prolongation of the dynamics of the rhombencephalon which necessarily had postnatal consequences due to the sudden instability of the cerebellum forced to integrate into the control of psychomotor balance. The first threshold of verticality was a jump in the complexity of the neural connections between cerebral and cerebellar neocortex, the last threshold with Sapiens was a cerebellar “explosion”. The modeling of the different degrees of verticality is therefore necessary to better understand the emergence of cognitive faculties such as the “explosion” of symbolic thought in relation to the straightening of the brainstem and the neocorticalization of the cerebellum.