The remarkable increase in brain size and complexity during human evolution was accompanied by a marked acceleration in the evolution of genes that contribute to the development of the brain, according to a new study.
Primates have large and complex brains compared with other mammals, and this increased size and complexity reached its peak in humans. To investigate whether this rapid expansion was associated with accelerated evolution of specific genes, Dorus and colleagues compared the rates of evolution of 214 genes that have important roles in the nervous system in a primate lineage (between macaques and humans) and in another mammalian lineage (between rats and mice).
The average rate of evolution for these genes was much higher in the primate lineage than in the non-primate lineage. To check whether this was specific to genes that are involved in nervous system function, the authors carried out a similar comparison for 95 crucial 'housekeeping' genes, and found that primates and rodents had statistically identical rates of evolution for these genes. This supported the idea that the rapid evolution of nervous system genes in primates was due to positive selection.
To test this idea further, the authors looked specifically at a subset of nervous system genes — those that are important for the development of the nervous system. These genes showed an even greater difference in the rate of evolution between rodents and primates than did the larger set of nervous system genes. By contrast, genes that are important mainly for physiological functions in neurons or glia showed less acceleration of evolution. As developmental processes should be involved in evolutionary changes in brain size or complexity, the accelerated evolution of genes that are important for brain development supports the idea of positive selection associated with brain evolution.
Finally, the authors compared the lineage that led to humans with that leading to macaques. Human evolution involved much greater increases in brain size and complexity than did macaque evolution, and this was reflected in a much higher average rate of evolution for nervous system genes in the human lineage than the macaque one. So, not only have these genes evolved faster in primates than in other mammals, but in humans they have evolved fastest of all.
ORIGINAL RESEARCH PAPER
Dorus, S. et al. Accelerated evolution of nervous system genes in the origin of Homo sapiens. Cell 119, 1027–1040 (2004)