"During human evolution, our forebrain became larger as our cerebral cortex increased in size. This means it had to become more folded to fit inside the skull. This gives the outside of the human brain its 'walnut' appearance. Humans have a larger cerebral cortex relative to the rest of the brain than any other animal. The cerebral cortex handles many of our unique skills, like language and problem solving." (source http://www.sciencemuseum.org.uk/exhibitions/brain/136.asp) "When we compare our brain to those of other animals, the first thing that strikes us is its size. Human brains weigh on average 1,300 grams; a squirrel brain weighs six grams. Some of this difference is because, as larger animals, we need more brain to run our bodies. However, the brains of our nearest relatives, the great apes, weigh only 300–500 grams, even though their body size is similar to ours. 'Humans sit on the top of the pile when it comes to relative brain size.' notes geneticist Bruce Lahn (University of Chicago, Illinois, United States). Throughout mammalian and primate evolution, there has been a gradual increase in brain size, superimposed with spikes of fast growth such as the tripling in human brain size that occurred about 1.5 million years ago, 4 million years after the human lineage diverged from that of the great apes. 'Even in the ape lineage, the brain has been expanding but along the human lineage it has really taken off,' says Lahn. In addition, over time, different parts of our brain have increased in size at different rates. The cerebral cortex has expanded more than other areas, and within the cortex, some areas have expanded differentially while others have lagged behind." (source http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0030050)