Humans are off the scale. Modern human brains are about threefold larger than those of our earliest hominin ancestors and living great ape relatives. Across animals, brain size is tightly correlated with body size. But humans are the extreme outlier when gauged against this typical scaling relation. The average adult human brain is roughly three pounds, which is approximately 2 percent of body size. But it consumes an outsized 20 percent of the body's energy budget because of high levels of electrical activity by neurons and the metabolic fuel it takes to transmit chemical signals from one brain cell to the next.
Detailed comparisons of human brains with those of our close living primate relatives, including chimpanzees, have shown that the parts of the cerebral cortex involved in higher-order cognitive functions, such as creativity and abstract thinking, have become especially enlarged. These cortical areas, known as association regions, mature relatively late in postnatal development. Some of the long-range neural connections that link these association areas to one another and to the cerebellum (the latter plays a role in voluntary movement and learning new skills) are more numerous in human brains as compared with other primates. These human-enhanced networks are loci for language, toolmaking and imitation. Even ancient reward systems in a subcortical area called the striatum, a hub of activity for the brain-signaling molecule dopamine, appear to have been reshaped in human brain evolution. That change most likely increases attention to social signals and facilitates language learning.
Where did our big brains come from? The hominin fossil record points to a general trend toward increased cranial capacity during the past six million years or so. That is when our lineage split from the last common ancestor we shared with chimpanzees and bonobos. Scientists consider a constellation of interrelated features of human biology to be associated with our large brains—slower growth through the stages of childhood, a longer life span, and more involvement in raising offspring by fathers and grandparents to assist mothers. Extended brain growth after birth means that significant events that lay the groundwork for cognition take place in a rich social and ecological context.
Another clue to what makes us different from chimpanzees and other intelligent species comes from compelling research that has uncovered genetic and molecular changes that occurred during the long course of the brain's evolution. A look at some of the distinctive features of the human brain follows.