Friday, January 20, 2012

The Genetics and Evolution of Brain Development

by Richard Crews
The human brain has evolved long and arduously over several million years through billions and billions of trial-and-error "experiments." These occur because of chance variations in the genetic blueprints for building some particular child's brain, variations due to mutations or incorporation of bits of DNA from viruses and other organisms. Such variations occur very infrequently in the development of children; if, overall, they were not quite rare--if the development of a new human brain in a growing fetus did not usually proceed flawlessly--the race would die out. This is because most of the experiments fail, in other words, they are lethal to the individual in whom they occur.

Even on the rare occasion when a genetic variation occurs and it is not lethal, it is usually pointless--it has no effective function in the organism. In that case it may fade away into the waste-bin of genetic history as "junk DNA." In fact, most of the DNA carried along on our chromosomes from generation to generation is "junk" which has no effective function in forming or running the organism; working genes lie amid long chains of worthless, meaningless DNA (although one must be cautious about dismissing this "junk" out of hand--there are often, tucked away amid the junk, secretive, subtle, or intermittent effectors).

If, on the rare occasion when one of these variations is not lethal and is not pointless, it brings about a change in the structure or function of the organism; then some potential bit of evolution has occurred. If, further, on the even rarer occasion when some particular variation turns out to be useful to or favorable for the individual, then--depending on the individual's success in reproducing and sending forward that variation to future generations--effective, positive, adaptive evolution may occur.

The growth and development of an individual person--and of that person's individual brain--from the genetic blueprint passed to the individual as genes in the sperm and egg of the parents is a very complicated and therefore rather chancy process. There are a lot of ways it can go wrong. In fact, most fertile eggs that arise from the almost miraculously unlikely joining of an egg and sperm do not produce a baby. Most abort as embryos or else later on some time during fetal development. A woman, in her lifetime, may produce several hundred eggs; a man, several million sperm. Of these, at most a couple of dozen succeed in getting together to form a fertilized ovum which can develop into an embryo, then into a fetus, and so on. And most of the fetuses that are formed abort spontaneously, that is, they fail to develop successfully into a baby. Out of the millions and millions of genetic "starts" a normal couple has, the number of children that a married couple raise in their lifetimes is, on average, between two and three.

Most of these kids grow into normal--regular--average people. Less than one in a million of the adult human beings (who have survived the genetic, fetal, infancy, and childhood lotteries) carries some useful variation that might advance the development of the human species.