Date of Degree


Document Type


Degree Name





William Harcourt-Smith

Committee Members

Christopher Gilbert

Jeffrey Laitman

Bernard Wood

Subject Categories

Anatomy | Animal Structures | Anthropology | Biological and Physical Anthropology | Body Regions | Ecology and Evolutionary Biology | Evolution | Medical Anatomy | Musculoskeletal System | Nervous System | Veterinary Anatomy | Zoology


Brachial plexus, primate evolution, primate locomotion, systematics, soft tissue


Primate evolutionary history is inexorably linked to the evolution of a broad array of locomotor adaptations that have facilitated the clade’s invasion of new niches. Researchers studying the evolution of primates and of their individual locomotor adaptations have traditionally relied on bony morphology – a practical choice given the virtual non-existence of any other type of tissue in the fossil record. However, this focus downplays the potential importance of the many other structures involved in locomotion, such as muscle, cartilage, and neural tissue, which may each be influenced by separate selective forces because of their different roles in facilitating movement. This dissertation is an investigation into the evolution of primate anatomy with an emphasis on the peripheral nervous system, particularly that of the brachial plexus, its intraspecific patterning, and its interactions with muscle in relation to changes in locomotion across clades. As the primate nervous system directs voluntary motor movement to the limbs, thereby facilitating locomotion, its morphology may be expected to vary with primate locomotor proclivities and/or limb anatomy. This prediction has not been explicitly tested. The anatomy of the peripheral nervous system was studied using a comparative approach both within 29 genera of primates and among non-primate clades via extensive primary dissection and a broad literature search in order to better understand its evolution. Data on spinal nerve level contributions, axon combination and branching morphology, nerve distribution pattern, and neural relationships with other soft tissues are detailed with photographs and standardized descriptions for 79 specimens and 123 individual plexuses. 99 characters generated from observations made during dissection were then analyzed using a parsimony-based phylogenetics approach to evaluate the evolutionary patterns presented by the brachial plexus in primates. The phylogenies generated with the brachial plexus characters did not perfectly mirror commonly accepted primate phylogenies, suggesting that while there is some evolutionary signal contained in the plexus, its morphology may also be influenced by forelimb function. As robust hypotheses exist regarding extant primate phylogenetic relationships and evolutionary histories, character evolution was mapped onto existing molecular trees to better understand how the individual structures that comprise the brachial plexus may evolve independently or in concert at different taxonomic levels. The rate of brachial plexus evolution in clades and leaf taxa was then assessed, demonstrating a marked heterogeneity in the structure both within and among clades. Taxa that have undergone recent locomotor shifts since divergence from their most recent common ancestor, and particularly those who exhibit some amount of suspensory behaviors, exhibit the highest rates of evolution observed here. Notably, several ape genera exhibit brachial plexus evolutionary rates significantly higher than the primate mean, running counter to the notion that hominoids have undergone an evolutionary slowdown relative to other primates.

As the true unit of homology in the peripheral nervous system is a subject of ongoing debate, several levels of discussion are necessary to understand the variation in primates and their place in the broader spectrum of tetrapod diversity. Macroanatomy, microanatomy, development, and comparative anatomy are explored in a broad context to evaluate the evolutionary trends of the primate peripheral nervous system and are discussed in detail.