The hypothalamus

What has MCPH got to do with the evolution of human intelligence? It has been suggested that small, cumulative changes in MCPHassociated genes have collectively led to the increase in human brain size. There is a clear relationship between measurable intelligence and brain size within humans and between other mammals. Broadly speaking, herbivores have smaller brains than predators and scavengers. Also, nearly all conditions in which the brain is substantially reduced in size lead to intellectual disability (despite physical abilities often being normal). Humans are regarded (by humans!) as having the greatest cognitive abilities amongst animals, and have proven themselves remarkably adaptable to different conditions on Earth and hence have spread far beyond their ancestral geographical origins. But none of this proves that a larger brain is a cleverer brain, although inherently this might make sense! For most MCPH-associated genes there is evidence for evolutionary selection and change during the monkey/ape/human lineage, as seen for genes involved in fertility and immunity. Taken together it has been speculated that multiple alterations in MCPH genes (and presumably in other genes with which they interact) have led to the threefold increase in brain size between chimpanzees (our closest relatives) and us. The other evolutionary change that is thought to be related to human brain size is the loss of the masseter muscle gene MYH16 following the evolutionary fixation of a premature nonsense mutation, allowing the young human skull to be able to grow more easily, but this relationship still remains to be proven. There is no such clear major change in an MCPH-associated gene that would make it ‘the’ candidate to explain a stepwise increase in human versus ape brain size. It is worth noting, however, that there are only 4 amino acid differences between man and mouse in the 714-aminoacid FOXP2 protein and these are thought to have led to the acquisition of human speech. Whilst there is no doubt that each MCPH gene can affect brain size significantly, they are maybe better viewed as modulators of brain size. And it is for this reason that the study of these genes may eventually lead us to discover the chance change(s) that have led to the emergence of the massive parallel processor that is the human brain.