human brain extra-large

Physiologically, the function of the brain is to exert centralized control over the other organs of the body. The brain acts on the rest of the body both by generating patterns of muscle activity and by driving the secretion of chemicals called hormones. This centralized control allows rapid and coordinated responses to changes in the environment. Some basic types of responsiveness such as reflexes can be mediated by the spinal cord or peripheral ganglia, but sophisticated purposeful control of behavior based on complex sensory input requires the information integrating capabilities of a centralized brain.
 We cohabit in cities of tens of millions of people and, despite what the media tell us, violence between individuals is extremely rare. This is because we have an extremely large, flexible and complex "social brain".
 Neurobiologist Steven Rose suggests that even this is not enough – we would still need know how these connections have evolved over a person's lifetime and even the social context in which they had occurred. It may take centuries just to figure out basic neuronal connectivity.


Humans have no bodily artifacts like horns or sharp nails or teeth, but working in large groups and having tools allowed us to become the leader predator, hunting animals as large as mammoths to extinction.
Our social groups are large and complex, but this creates high stress levels for individuals because the rewards in terms of food, safety and reproduction are so great.
Oxford anthropologist Robin Dunbar argues our huge brain is primarily developed to keep track of rapidly changing relationships. It takes a huge amount of cognitive ability to exist in large social groups, and if you fall out of the group you lose access to food and mates and are unlikely to reproduce and pass on your genes.
The anatomy of the chimpanzee brain is strongly controlled by their genes, whereas the modern human brain is extensively shaped by the environment, no matter what the genetics.
Human brain is pre-programmed to be extremely flexible; its cerebral organisation is adjusted by the environment and society in which it is raised. So each new generation's brain structure can adapt to the new environmental and social challenges without the need to physically evolve.