The two large frontal lobes at the top of the brain that make up about 85% of its weight in modern humans. Our high level mental functions, such as speech and processing complex information, occur in the cerebrum. The cerebral cortex, or outer surface, is deeply convoluted. About 2/3 of the human brain surface lies within these folds. The amount of cerebral cortex folding, and subsequently the surface area of the brain, increased as humans evolved from Homo habilis to Homo erectus and ultimately Homo sapiens.