Dissertations, Theses, and Capstone Projects

Date of Degree

6-2023

Document Type

Dissertation

Degree Name

Ph.D.

Program

Anthropology

Advisor

Jeffrey T. Laitman

Committee Members

Eric Delson

William Harcourt-Smith

Timothy Smith

David Friedland

Subject Categories

Anatomy | Biological and Physical Anthropology | Sense Organs

Keywords

Inner Ear, Vestibular system, Anthropology, Human evolution

Abstract

Our sense of balance is among the most central of our sensory systems, particularly in the evolution of human positional behavior. The peripheral vestibular system of the inner ear comprises the organs responsible for this sense; the semicircular canals (detecting angular acceleration) and otolith organs (utricle and saccule; detecting linear acceleration, vibration, and head tilt relative to gravity). The vestibular organs are often considered a single system, with most research focusing on the semicircular canals. The otolith organs, by comparison, remain largely unexplored despite their central role in balance. Consequently, this lack of knowledge limits understanding of vestibular functional morphology and hinders the ability to reconstruct the evolution of positional behavior within the human lineage.

This dissertation creates a morphometric model of the otolith organs and encasing bony morphology (otolith system) in order to provide a better understanding of human vestibular structure. It then applies the model to a sample of extant anthropoid primates and fossil hominins to elucidate the comparative and evolutionary aspects of the human otolith system. The morphometric model is generated using new visualization modalities combining spherical harmonic-based and landmark-based shape analyses. The model establishes the relationship between the external bony and internal membranous tissues of the otolith system which enables the reconstruction of soft tissue from bone. This approach is a powerful tool to assess functional aspects of the otolith system from bony morphology and fossil remains.

Results reveal the otolith system as a distinct third component of the inner ear with its own structural and evolutionary constraints which differ from those of the semicircular canals. Novel visualizations of bony and membranous structure of the otolith system also suggest linkages to several variables. These include: 1) sex-related differences in size; 2) shape-related differences related to provenance in Homo sapiens; 3) a functional signal (related to head and neck orientation) tied to shape; and 4) size-related differences related to phylogeny.

There is also evidence of evolutionary change in the hominin otolith system. Specimens representing Australopithecus africanus and Paranthropus robustus exhibit major morphological differences compared to H. sapiens, while La Chapelle aux saints 1 (representing Homo neanderthalensis) shares most features with H. sapiens. These morphological differences relate to neck orientation and thus enhance our understanding of postural shifts within the hominin lineage.

This dissertation offers a new viewpoint into the evolution of the human inner ear and transforms our current conceptions of the balance organs. Furthermore, the morphometric model enhances our understanding of spatial relationships between the otolith organs and cranial structure providing the foundation to assess the evolution of a prime neurosensory system essential for human function, and more broadly, all vertebrate life.

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