In his book The Dragon's of Eden, Carl Sagan looks at the evolution of human intelligence. He points out that there is a statistical correlation between brain mass or size and intelligence in human beings (p. 36). Looking at the brain masses and body masses of different species, for a given body mass, mammals have consistently higher brain mass than fish and reptiles, and primate brains are more massive by a factor of two to 20 times than those of non-primate mammals of the same body mass. Sagan points out that mammals are also much more intelligent than reptiles, so there does seem to be a relationship between brain mass and intelligence.
The species with the largest brain mass for its body weight is Homo sapiens. The next in ranking is the dolphin, which is regarded as extremely intelligent. Although there are some exceptions to the rule, such as the pigmy shrew, which has a mass ratio in the human range, brain to body mass seems to be a predictor of intelligence. Sagan believes that the evidence to date shows that the evolution of mammals from reptiles over two hundred million years ago was accompanied by a major increase in relative brain size and intelligence, and that the evolution of humans since that time has been accompanied by even more brain development (p. 40).
The human brain, says Sagan, contains about ten billion neurons which act as switching elements. Neurons communicate with one another through synapses. It is believed that each synapse, or junction between two nerve cells, is responsible for one particular brain state. The existence of these brain microcircuits suggests that intelligence may not only be related to brain mass, but also to the number of such microcircuits, since they make different brain states possible. Indeed, studies in rats have shown that learning is accompanied by an increase in mass of the cerebral cortex of the brain (p. 45). This suggests that the bigger the cerebral cortex,...