More is Different and Less is More

Perhaps the most consequential difference between humans and our fellow great apes is the radical increase in brain size that occurred in the human lineage.  It seems plausible that that is closely associated with the development of speech and technology that have made us so numerous and ubiquitous.  Oddly enough, though, when genes expressed in the brain are compared, they show less divergence between chimp and human than any others (the most divergent are genes expressed in the testis).

What does seem to have changed in those brain genes is that their expression has be up-regulated - in some ways a human brain is just an overclocked or increased displacement chimp brain.  This is not terribly surprising if one considers recent progress in artificial neural networks - the main difference between the neural networks of twenty years ago and the modern, super smart ones is scale - the number of neurons (nodes) and interconnections.

One prominent difference between chimps and humans at the genetic level is in the genes expressed in jaw musculature.  We have lost the gene for powerful jaw muscles.  Why?  Maybe to make room for cranial expansion to accommodate those increased displacement brains.  We have lost a number of chimp genes.

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