From Fossils To Mind Workshop 2021: Christopher M. Smith

The Importance of the Otolith System in Human Cognition

Christopher M. Smith


1 – Department of Anthropology, The Graduate Center, City University of New York;
2 – Center for Anatomy and Functional Morphology, Icahn School of Medicine at Mount Sinai;
3 – New York Consortium in Evolutionary Primatology 


The evolutionarily ancient vestibular system (VS) is central to many cognitive processes including postural and gaze stabilization, and 3D spatial orientation. Growing evidence suggests that the otolith portion of the VS in the inner ear (utricle and saccule; detecting linear acceleration and gravity), provides an essential gravitational frame of reference for cognitive processes fundamental to our concept of the human mind (e.g., bodily self-consciousness, spatial memory, and social cognition). By providing such a framework, the otolith system is foundational for interpreting sensory signals in 3D space and facilitates signal integration with higher cognitive processes. Investigation into the evolution of human cognition would benefit from a better understanding of concomitant evolution in the otolith system. This presentation reviews the evidence that differences in brain and vestibular structure among extant hominoids may be associated, suggesting the importance of studying this prime sensory system in understanding human cognitive evolution.

We conducted a meta-analysis of published data and our own preliminary morphometric work to determine potential associations of vestibular and brain differences among hominoids. Published data combined with our own results point to an expansion of the inner ear vestibule in hominids, with the greatest expansion found in modern Homo sapiens, potentially reflecting differential otolith function. It has also been noted that relative increases in hippocampal volume occur alongside expansion of the saccule in Hylobates lar. Pairing such data with known metrics of brain evolution suggests correlated trends in the expansion and reorganization of cerebellar structure in great apes compared to non-ape anthropoids. These results illustrate the potential connection between observable morphological differences in the otolith system and brain structure. Accordingly, the otolith system can provide a novel starting point by which to investigate cognitive behaviors that are difficult to interpret in the fossil record, including spatial navigation and social behavior.