Cosmos - Carl Sagan [147]
But suppose what you had to do was so complicated that even several billion bits was insufficient. Suppose the environment was changing so fast that the precoded genetic encyclopaedia, which served perfectly well before, was no longer entirely adequate. Then even a gene library of 1,000 volumes would not be enough. That is why we have brains.
Like all our organs, the brain has evolved, increasing in complexity and information content, over millions of years. Its structure reflects all the stages through which it has passed. The brain evolved from the inside out. Deep inside is the oldest part, the brainstem, which conducts the basic biological functions, including the rhythms of life—heartbeat and respiration. According to a provocative insight by Paul MacLean, the higher functions of the brain evolved in three successive stages. Capping the brainstem is the R-complex, the seat of aggression, ritual, territoriality and social hierarchy, which evolved hundreds of millions of years ago in our reptilian ancestors. Deep inside the skull of every one of us there is something like the brain of a crocodile. Surrounding the R-complex is the limbic system or mammalian brain, which evolved tens of millions of years ago in ancestors who were mammals but not yet primates. It is a major source of our moods and emotions, of our concern and care for the young.
And finally, on the outside, living in uneasy truce with the more primitive brains beneath, is the cerebral cortex, which evolved millions of years ago in our primate ancestors. The cerebral cortex, where matter is transformed into consciousness, is the point of embarkation for all our cosmic voyages. Comprising more than two-thirds of the brain mass, it is the realm of both intuition and critical analysis. It is here that we have ideas and inspirations, here that we read and write, here that we do mathematics and compose music. The cortex regulates our conscious lives. It is the distinction of our species, the seat of our humanity. Civilization is a product of the cerebral cortex.
The language of the brain is not the DNA language of the genes. Rather, what we know is encoded in cells called neurons—microscopic electrochemical switching elements, typically a few hundredths of a millimeter across. Each of us has perhaps a hundred billion neurons, comparable to the number of stars in the Milky Way Galaxy. Many neurons have thousands of connections with their neighbors. There are something like a hundred trillion, 1014, such connections in the human cerebral cortex.
Charles Sherrington imagined the activities in the cerebral cortex upon awakening:
[The cortex] becomes now a sparkling field of rhythmic flashing points with trains of traveling sparks hurrying hither and thither. The brain is waking and with it the mind is returning. It is as if the Milky Way entered upon some cosmic dance. Swiftly the [cortex] becomes an enchanted loom where millions of flashing shuttles weave a dissolving pattern, always a meaningful pattern though never an abiding one; a shifting harmony of sub-patterns. Now as the waking body rouses, sub-patterns of this great harmony of activity stretch down into the unlit tracks of the [lower brain]. Strings of flashing and traveling sparks engage the links of it. This means that the body is up and rises to meet its waking day.
Even in sleep, the brain is pulsing, throbbing and flashing with the complex business of human life—dreaming, remembering, figuring things out. Our thoughts, visions and fantasies have a physical reality. A thought is made of hundreds of electrochemical impulses. If we were shrunk to the level of the neurons, we might witness elaborate, intricate, evanescent patterns. One might be the spark of a memory of the smell of lilacs on a country road in childhood. Another might be part of an anxious all-points bulletin: “Where did I leave the keys?”
There are many valleys in the mountains of the mind, convolutions that greatly increase the surface area available in the cerebral cortex for information storage in a skull of limited size.