McCrone begins with two assumptions: that "self-consciousness must have a biological basis" and that the mind evolved.

Language is one of the defining human characteristics; indeed it is language that has permitted our species to learn how to control the environment around us rather than being forced to adapt to it. Language permitted self-awareness and self-consciousness.

Being intelligent is hard work. The brain uses about one fifth of the oxygen intake even though it's only about one fiftieth of the body's weight. During the climatic changes of the Miocene era some 10 million years ago, the apes which had flourished in the rich forest environment were forced to adopt a land-gait and leave the trees. Most of the ape lines became extinct, a process almost completed today; they had reached an evolutionary dead-end. Only the human line of apes survived. Because two-legged movement is not as efficient, nor as fast as four-legged, these strange upright ancestors of ours developed social organizations for the common defense (also a characteristic of the few remaining apes like baboons and chimps.) Still, this alone was not enough for several early hominid lines became extinct, unsuccessful experiments of God.

The Australopithecines, with strong jaw for chewing up the tough roots and plants of its diet disappeared with the advent of the colder ice age. Our direct ancestors, with smaller jaw, a more varied diet, and the ability to cook, were better suited to adapt to the change in environment. The last 3,000,000 years have been dominated by the ice-age with only brief 10,000 - 20,000-year long interruptions of more temperate climates (we near the end of the most recent one now.) These periods placed terrible stress on the animals that had developed warm coats and had adapted to colder climates. Many species died out. The lightweight homo line with his intelligence and flexible diet was again successful. Another advantage was food-sharing -- almost unique to humans -- and pair bonding. But language, appearing it is thought with home sapiens, was to make a crucial difference. "Language paved the way for all the special abilities that we so value abilities such as self-awareness, higher emotion and personal memories."

McCrone examines how various basic mental abilities work such as thought, memory and learning, in order to appreciate the structures that language expanded. ( )