Dissertations, Theses, and Capstone Projects

Date of Degree


Document Type


Degree Name





Michael E. Steiper

Subject Categories



Hominoids, intraspecific diversity, population genetics


Cranial shape diversity within hominoids has been previously studied with the aim of understanding how levels of diversity in extant species compare with extinct hominin specimens. This dissertation addresses the question of why cranial shape diversity differs among extant hominoids. Levels of intraspecific cranial shape diversity are highly varied among hominoids. For example, Sumatran orangutan cranial shape diversity is more than twice that of all living humans. Here, the population history of each species, or sub-species, is considered as a force potentially structuring phenotypic variation. It is already well established that population history has shaped patterns of modern human cranial diversity across the world. Yet, to date, no one has considered that the independent population histories of other apes may have also influenced their cranial diversity through evolutionary time. Genetic data from non-coding loci reveal the population history of each taxon. Nucleotide diversity levels reflect non-selective evolutionary processes--such as mutation, drift, migration or fluctuations in population size. For each taxonomic group in this study, genetic diversity and the effective population size (Ne) are compared with cranial shape diversity to determine the strength of the relationship between these two data-types. 3D cranial landmark data are divided into two separate analytic units, which represent independent developmental modules. Shape variance of the cranial vault, and the face, are evaluated together, and then separately. The following taxa are included in this work: Homo sapiens, Pan paniscus, Pan troglodytes troglodytes, Pan troglodytes verus, Pan troglodytes scweinfurthii, Gorilla gorilla gorilla, Pongo pygmaeus, Pongo abelii, Symphalangus syndactylus, Hylobates moloch, Hylobates pileatus and Hylobates klossii. Results show a strong positive correlation between intraspecific cranial shape diversity and nucleotide diversity across all taxa. Species that are more genetically diverse, and have larger effective population sizes, show more cranial shape diversity. The relationship between cranial vault shape diversity and genetic diversity is stronger than in the face. Variation in the face is likely driven by sexual dimorphism in certain species, which may overwrite any signal of population history. This work provides new evidence of the strength of non-selective pressures--such as random mutation and genetic drift--on skeletal elements such as the cranium. Traditionally, biological anthropologists have looked to adaptation by natural selection as the primary explanation for patterns of skeletal diversity. The advent of large population genetic data-sets--which document random evolutionary changes in the genome--have enabled genetic variability to function as the null hypothesis for explaining phenotypic diversity. If phenotypic diversity mirrors neutral genetic diversity, non-selective evolutionary forces may sufficiently explain patterns of phenotypic diversity without invoking a selective explanation. This project increases our understanding of extant hominoid cranial evolution and therefore elucidates the complex framework within which extinct hominin species diversity should be evaluated.

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