Insight into human brain evolution through phylogenetic analysis and comparative genomics.

摘要

As a species, humans are often considered to be unique among mammals, with respect to their large brain size and enhanced cognitive abilities. Humans are the most encephalized mammals, with a brain that is six times larger than expected relative to body mass. Presumably, it is this high degree of encephalization that underlies our advanced cognitive abilities, including the skills needed for complex language and culture. Understanding how large brains evolved can shed light on what makes the human brain unique and introduce possible mechanism for human specific neurodegenerative diseases. This study takes a both a phenotypic and molecular approach to study human brain evolution. First, we traced the evolutionary history of encephalization across mammals through a phylogenetic analysis in order to infer at which point significant changes in brain size occurred. We demonstrate that variation in brain size began in anthropoid primates. Furthermore, we show multiple lineages have evidence of brain expansion, providing support for parallelism in encephalization. To provide molecular evidence for parallelism in brain expansion among primates, we implement a comparative genomics approach and sequenced the brain transcriptome of the second most encephalized primate, the capuchin monkey (Cebus). We then test for similar patterns of adaptive evolution on the capuchin monkey and human lineages and demonstrate that genes with accelerated rates of change on these large-brained lineages share similar biological processes, such as microtubule organization, mitosis, and metabolic processes.