Evolution and Scaling in Mammalian Brains

Citation

Bush, Eliot Christen (2004) Evolution and Scaling in Mammalian Brains. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/P9FG-7K79. https://resolver.caltech.edu/CaltechETD:etd-03292004-144927

Abstract

Here I look at three stages in the evolutionary development of mammalian brains. Chapter one addresses how connectivity in neocortex scales with brain size. This is of evolutionary interest because it helps define the basic mammalian condition. Neocortical white matter increases disproportionately in large brains. This might reflect increases in the number of connections per neuron. It might also reflect scaling in axon diameter. I compare these hypotheses by examining white matter-gray matter scaling in cerebellum. Because the white matter of cerebellum lacks cortico-cortical connections, the connectivity theory predicts that cerebellar white matter should not hyperscale relative to gray matter. I have measured white matter and gray matter volume in a large sample of mammals and I find that cerebellar white matter does not hyperscale. This supports the proposition that neocortical hyperscaling reflects an increase in the number of connections per neuron in large brains.

In chapter two I use independent contrasts analysis to examine the scaling of frontal cortex in a large sample of mammals. I find significant differences in scaling between primates and carnivores. Primate frontal cortex hyperscales relative to the rest of neocortex and the rest of the brain, and the primate slope is significantly greater than that for carnivores. This suggests that there are substantial differences in frontal cortex structure and development between the two groups. Combined with with anatomical differences, it suggests that primates have evolved a number of unique adaptations in frontal cortex.

Chapter three examines the evolution of brain size in anthropoid primates. Living anthropoids have larger brains than strepsirrhines. What about early anthropoid fossils? I measure brain size in the early anthropoid Parapithecus grangeri using computed tomography. I find that relative to the living anthropoids, Parapithecus had a small brain for its body size. Thus large brains did not develop at the same time as a number of other anthropoid adaptations.

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