The brain changes continually. Not only during development or ageing, but over generations, too. While the common popular notion is that the human brain of today is identical to that of the stone age man, recent studies of the genetics underlying brain development has shown that the human brain has changed significantly over a far shorter time -- only a few thousand years.
A recent study by Nusbaum et al. in Nature (see full PDF) analysing the human chromosome 8 briefly mentions two regions called the major defensin gene cluster and MCPH1. They speculate that these regions have played a significant role in the expanded brain size that can be observed through hominid evolution.
At the end of the article, Nusbaum et al. open up a whole new field of study:
"(...) the majority of the genes in the region of high divergence in distal 8p play important roles in development or signalling in the nervous system. Notably, the extremely large CSMD1 gene, which lies at the peak of divergence and diversity, is widely expressed in brain tissues. High regional mutation rates and positive selection are generally assumed to be distinct, but it is possible that the former may facilitate the latter by increasing the rate of appearance of potentially advantageous single, or interacting, alleles. It is intriguing to speculate whether the accelerated divergence rate of this region has contributed to the rapid expansion and evolution of the primate brain."
In other words, the study of chromosome 8 might open a whole new field of enquiry about what makes the human brain special. Knowing the genomic makeup of our closes evolutionary peers will also make it possible to study the relative contribution of this region to brain size and, not to forget, the underlying role this change has had for cognitive processes. Yes, brain size is interesting, but at our current standard, we need to know more than that. Increasing brain size is normally NOT thought to work like an inflating balloon where all areas increase equally. Rather, the evolution of areas occur within systems of modules or areas, often underlying one or a few cognitive functions.