the easier it may be to learn, and that cerebral cortex size can therefore correlate with intelligence. Further experiments in rats have verified that within several weeks of learning new tasks, rats develop new neural branches in the cortices that form synapses.
Sagan sees the evolution through time of the amount of information contained in the genetic material and the amount of information contained in the brain. The two curves of these figures, when plotted, intersect at a few hundred million years ago, a time when organisms, for the first time in history, had more information in their brains than in their genes, and this made the brain a symbolic turning point in history. The emergence of mammals, and the later emergence of man-like primates were important advances in the evolution of intelligence (p. 47).
The theory of the tripartite human brain that Sagan discusses comes from the work of Paul MacLean, chief of the Laboratory of Brain Evolution and Behavior at the National Institute of Mental Health. It postulates that we see ourselves through three different mentalities, two of which lack the power of speech. These three mentalities amount to three separate computers in the brain, linked to each other, but each having its own intelligence, subjectivity, sense of time and space, memory, motor and other functions (p. 55). Each brain corresponds to a separate evolutionary step. The three brains can be distinguished neurochemically and functionally, and contain different distributions of the neurotransmitters dopamine and cholinesterase.
The most ancient part of the brain is the spinal cord, the medulla and the pons, which compose the hind brain; and the midbrain. This area contains the machinery for reproduction and self-preservation. The brains of fish and the amphibians are almost completely composed of hind brain and midbrain, i.e. bipolar brains. Reptiles and higher animals also possess a forebrain: they ha...