The Spinal Cord in Hominin Evolution

Abstract

Spinal cord dimensions increased in both relative and absolute terms during the course of hominin evolution. Recent discoveries demonstrate the presence of a fully developed human‐sized cervical spinal cord in Australopithecus afarensis at 3.6 million years before present (Ma) and in the cervical and thoracic regions of Homo erectus at 1.8 Ma. The fossil evidence suggests that these hominins possessed a fully human‐like postcranial neurological substrate for precision coordination of the arm and hand, corroborated by corresponding skeletal anatomy of the hand and shoulder, as well as discoveries of penecontemporaneous lithic use and putative throwing manuports in the archaeological record. These new fossils provide insight into the neurobiological underpinnings of sophisticated manual behaviours, and overturn prior thinking on the evolution of the central nervous system in the human lineage, indicating that the evolution of the hominin spinal cord was completed well in advance of advances in brain size. Key Concepts The brain and spinal cord are both larger in humans than those of the African great apes. New Australopithecus afarensis fossils from Woranso‐Mille, Ethiopia, demonstrate that hominin cervical spinal cord enlargement was completed by at least 3.6 million years ago. Evidence from Homo erectus now shows that hominin thoracic spinal cord enlargement was completed by at least 1.8 million years ago. These new discoveries reveal the neurological capability for fine‐motor coordination of the arm, hand, and thoracic muscles in our smaller brained ancestors. It is now apparent that spinal cord expansion was completed prior to brain size expansion in the human lineage.

Human-specific progenitor sub-domain contributes to extended neurogenesis and increased motor neuron production

“…Spinal motor neurons (MN) innervate muscles and sympathetic ganglia, controlling body posture, voluntary movement and the function of visceral organs 13,14 . The human spinal cord generates >4 times the number of motor neurons compared to mouse (Supplemental figure 1A), concordant with the increase in the size of our spinal cords and complexity of human motor skills [15][16][17][18][19][20] . Accordingly, human motor neuron genesis spans approximately 2 weeks from Carnegie stage 11 (post-conception day 26-30) to Carnegie stage 17 (post conception day [41][42][43][44], compared to ~2 days from embryonic day 9.5 to 11.5 in mouse 15,21 .…”

Semiotics and the Origin of Language in the Lower Palaeolithic

“…The fossil evidence now indicates that modern-like speech and auditory capacities had evolved by at least 430,000 years ago in the ancestor of Neanderthals and Denisovans (Martínez et al 2004(Martínez et al , 2008Gómez-Olivencia et al 2007;Dediu and Levinson 2013;Steele et al 2013; Aboitz 2018). The neurological control of breathing to produce articulate speech may have evolved as early as 1.8 Ma with Homo erectus, but was not present in australopithecines (Meyer 2016;Meyer and Haeusler 2015;cf. MacLarnon and Hewitt 2004).…”