Just based on the capacity of the skull, our brain is 3 – 4 times larger than that of our closest primate relative, the chimp. Most of the increase in size occurs in the cerebral cortex (the gray matter) just under the skull. Our cortex is thrown into folds because there is so much of it. Compare the picture of the mouse brain (smooth) and ours, wrinkled like a walnut http://www.nsf.gov/discoveries/disc_summ.jsp?cntn_id=130442.
We now may have part of the explanation. A fascinating paper http://www.sciencemag.org/content/347/6229/1465.full.pdf studied genetic differences between the progenitor cells from which the cortex arises (radial glia) in man and mouse. They found 56 protein coding genes expressed in our radial glia not present in the mouse (out of 20,000 or so).
One in particular called by the awful name ARHGAP11B is particularly fascinating. Why? Because it’s the product of a gene duplication of ARHGAP11A. When did this happen — after the human line split off from the chimp 6 million years ago. Chimps have no such duplication, just the original
Put ARHGAP11B into a developing mouse and its cortex expands so much it forms folds.
There has been all sorts of work on the genetic difference between man and chimp. There almost too many — [ Nature vol. 486 pp. 481 – 482 ’12 ] — some 20,000,000. Finding the relevant ones is the problem. ARHGAP11A is by far the best we’ve found to date.
Another fascinating story is the ‘language gene’ discovered in a family suffering from a speech and language disorder. It’s called FOXP2. Since the last common ancestor of humans and mice (70 megaYears ago) there have been only 3 changes in the 715 amino acids comprising the protein. 2 of them have occurred in the human lineage since it split with the chips 6 megaYears ago. So far no one has put the human FOXP2 gene into a chimp and got it to talk. For more details see http://en.wikipedia.org/wiki/FOXP2
There is all sorts of fascinating molecular biology about what these two genes actually do in the cell, but that would make this post too long,. This is, in part, a chemistry blog and just what FOXP2 and ARHGAP11A actually do involves some beautiful and elegant chemistry — look up RhoGAP and Winged Helix transcription factors. Ferrari’s are beautiful cars, and become even more beautiful when you understand what’s going on under the hood. Chemistry gives you that for molecular, cellular and organismal biology.