Body Story

• Early mammals

• Emotion and reason

• Young and primitive

The history of the brain goes back 450 million years, to the time when the first fish appeared, equipped with small, simple brains. About 150 million years later, reptiles evolved with slightly more sophisticated brains. As well as handling the basic body functions, the reptilian brain had, loosely speaking, three 'compartments' managing other physical capacities: one in the front for smell, one in the middle for vision and one in the rear for balance and co-ordination. In addition, the reptilian brain was 'hard-wired' for reflex actions, such as breathing and the heartbeat, and with the basic instincts necessary for survival and the preservation of genes, such as the sexual drive and the 'fight or flight' response to danger.

• Early mammals
Mammals evolved about 200 million years ago. They inherited the basic functioning and hard-wired drives of the reptilian brain, but the primitive brain grew larger as they developed other instincts, such as the parental instinct to care for the young.

There was another key change during this period. Because dinosaurs ruled the earth and were active during the day, mammals were largely nocturnal. The 'vision' part of the mammal's brain therefore shrunk, while the 'smell' part grew: two swellings developed, which became the cerebral hemispheres or cerebrum.

Later, when the ruling reptiles died out, mammals were safe to walk the earth by day and vision reassumed its former importance. But although the part of the brain responsible for vision consequently expanded, the part responsible for the sense of smell stayed the same size, and the cerebral hemispheres became too large to fit inside the cranium. The brain therefore began to wrinkle and fold.

• Emotion and reason
In the millions of years following the first appearance of mammals, the cerebral cortex — the layer of grey matter on the outside of the cerebrum — completely enveloped the primitive brain. But the primitive brain is still highly active and at times vies with the cerebral cortex for control, pitting the ancient, hard-wired dictates of another era (instinct and emotion) against the more flexible and subtle responses of our 'new' brains (reason). And it remains responsible for our basic survival instinct. The hypothalamus, for example, a walnut-sized part of the primitive brain, is thought to contain the centres for aggression and the 'fight or flight' response. If life is threatened, the primitive brain takes over, and we react instinctively — and sometimes violently.

• Young and primitive
In babies, the primitive brain takes precedence, because we are born with a partially-formed cerebral cortex (see Born stupid). As well as controlling the essential, involuntary body functions, the primitive brain enables babies to demand what they need to stay alive.

When the brain demands food, for example, the primitive brain activates an automatic survival mechanism: the baby cries, demanding attention. And the primitive brain has another vital feature: a built-in training programme to help the cerebral cortex — the part of the brain capable of learning — to develop. For example, babies have a reflex to look at things that will improve their vision, such as strong, bold patterns or things moving in front of them.

Sight and speech | Primitive brain | First year

Born stupid

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Brave new world

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