Map of life expectancy at birth from Global Education Project.

Friday, April 01, 2005

Billions and billions

For reasons that may be obvious to some, I was inspired to pull down my old copy of Broca's Brain. Sagan looks back from the perspective of the late 20th Century on the greatest of human voyages, outward to discover the universe and inward to discover ourselves.

Many modern people, including many scientists, would agree with Siddhartha Gautama that this is really one journey -- that our regarding of ourselves as special, as somehow apart from, and as consequential as the rest of the universe, is illusory. But it is necessary, if we are to make sense of the world, that we pay particular attention to the being that is doing the apprehending, in other words us. The circularity of the project is disconcerting, but not fatal. There is nothing contradictory about turning tools on themselves. It is perfectly sensible to look at a lens through a microscope. And it is simply our nature to be particularly interested in ourselves.

It's obvious to everyone how far the outward journey has taken us. Since Galileo looked through the telescope, our place in the universe has changed so radically that many people simply can't accept it. The universe of billions of galaxies of billions of stars and billions of years, the harsh news that all of humanity is just a bit of slime on a grain of sand in a vast desert, can't possibly be true, so they retreat to the comforting cave of ancient superstition. (Sorry Carl, although you always denied it, you really did say "billions and billions.")

There may be less general awareness of the distance we have gone on the inward journey and the implications of what we have discovered. Broca found that specific parts of the brain are responsible for specific functions. Since his time we have learned an immense amount about where specific cognitive, perceptual, motor and somatic functions are located in the gray and white mass inside our skulls. Our consciousness, our selfhood, our humanity, is not one unitary entity but the sum of innumerable parts, which can be removed or altered one by one. Sometimes the structure of our cognitition turns out to be astonishing to us, as in the stories of Oliver Sacks -- the Man who Mistook His Wife for a Hat, people who can see perfectly but who cannot distinuish among human faces, people who speak with perfect fluency except that they cannot name any fruits.

Our brains evolved. The human claim to distinctiveness among the beings of the earth rests on one feature, the cerebral cortex -- the outer rind of brain cells, which, some 2 million years ago, began an enormous expansion. In evolutionary terms, this expansion was so valuable that it continued even though the enormous heads of fully developed human fetuses barely fit through the birth canal and women's risk of dying in childbirth sharply increased. The cortex could not finish developing in the womb, or even for many years after birth, so human infants remained entirely helpless for at least two years and highly dependent for many more.

But there it is. Every thought, every memory, every sight and sound, every caress, every sting, every emotion that comes to our conscious awareness consists of patterns of chemical signals passing among neurons in our cerebral cortexes. (Pedants: the Latin plural cortices is no longer standard in English, so take it somewhere else.) These phenomena are generated and shaped elsewhere in the brain, but their conscious apprehension is a function of the cortex. If our cortexes are damaged, consciousness can be lost piece by piece -- sight of the left half of the universe, the ability to think about animals, the memory of last Wednesday, the knowledge that this person is your uncle, the color pink, verbs, desire. At what point are you no longer there?

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