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brains. Relative to body size, the human brain is three times larger than might be expected in a primateâand primates already have enlarged brains compared to other mammals. The top part of our brain, known as the cortex, has mushroomed during the evolution of hominids. Rita Carter describes this in her book Mapping the Mind: âOne and a half million years ago the hominid brain underwent an explosive enlargement. So sudden was it that the bones of the skull were pushed outwards, creating the high, flat forehead and domed head that distinguish us from primates. The areas that expanded most are those concerned with thinking, planning, organizing and communicating. The development of language was almost certainly the springboard for the leap from hominid to human. It gave our ancestors lots to think about, and new brain tissue was needed. The frontal lobes of the brain duly expanded by some 40 per cent to create large areas of new gray matter: the neo-cortex. This spurt was most dramatic at the very front, in what are known as the pre-frontal lobes. These jut out from the front of the brain, and their development pushed the forehead and frontal dome of the head forward, reforming it to the shape of a modern skull.â

Our brains are organized into distinct areas. First, at the top of the spinal column, at the base of the skull, there are cells sensitive to smell and light. This corresponds to the fish brain. On top of this lies a clump of cells called the cerebellum, which coordinates movement. Together the two layers form the reptilian brain. Further areas lie on top of this, including the thalamus (involved in the primary sensory processing of vision, sound, and touch), the amygdala (involved in emotion), the hippocampus (involved in learning and memory), and the hypothalamus (involved in motivation and behavioral regulation). This corresponds to the mammalian brain, which also has an additional top layer of cells known as the cortex. Some mammals have more cortex than others. In humans, the cortex balloons out of all proportions.

The architecture of the human brain incarnates our hereditary connection to other vertebrates, in their order of evolutionary appearance: first fish, then reptile, then mammal. But the human brain differs from other animal brains in that it is equipped with specialized neuronal circuitry to deal with language. For decades, scientists believed that two specific parts on the left side of the human cortex, known as Brocaâs area and Wernickeâs area, function as âlanguage centers.â But recent research based on brain imaging shows that language is handled by many different brain regions working in parallel. As Susan Greenfield writes in her book Brain Story: âOne of the most startling discoveries from such research is that saying just a single word causes a unique pattern of activity to ripple through the cortex. The experience of the word âscrewdriver,â for example, causes a part of the brain called the motor cortex to light up. The motor cortex is involved in controlling movement, so perhaps this word triggers memories of handling a screwdriver to become active. Obviously, language cannot be the preserve of just Brocaâs and Wernickeâs areasâit involves an eruption of associations and memories that are different for every word.â

Humans have remarkably big brains compared to the rest of their bodies. Our children come into the world so top-heavy that they take months just to sit up. Their heads are so large that our species has by far the highest maternal death rate during birth. And young humans require long years of nurturance, education, and compassion for their brains to reach full potential. Humans also have by far the longest childhoods and adolescences, and human parents sustain compassion longer than parents from any other species.

Having a large number of neurons relative to body size certainly seems to enhance intelligence, as octopuses and humans demonstrate. But if intelligence is defined as the capacity to gauge the world and make correct decisions, there is some doubt that humans are as