( the growth of brain size and complexity...)Human brain is still evolving

For both Microcephalin and ASPM, Lahn and his colleagues are trying to find out the precise traits that are under natural selection. They are also performing more detailed studies of the two genes in human populations to better understand their evolutionary history. And they are searching for other brain-related genes that have changed under the pressure of natural selection. "We want to know how broad a trend these two genes represent," said Lahn. "Did we get really lucky and hit on two rare examples of such genes? Or, are they representative of many other such genes throughout the genome. I would bet, though, that we will find evidence of selection in a lot more genes."

Lahn and his colleagues are now working to understand how subtle changes in the sequences of these two genes can alter their function in such a way that would result in favorable selection. While there is some evidence from earlier studies that Microcephalin and ASPM code for proteins that regulate the proliferation of brain cells from immature neural stem cells, their function has not yet been determined, said Lahn.