As the grandfather of a pair of five-year-old identical twins, I see every day the power of the genes and the limits to that power. George and Donald are physically so alike that in the bathtub I cannot tell them apart. They not only have the same genes but have shared the same environment since the day they were born. And yet, they have different brains and are different people. Life has escaped the tyranny of the genes by evolving brains with neural connections that are not genetically determined. The detailed structure of the brain is partly shaped by genes and environment and is partly random. Earlier, when the twins were two years old, I asked their older brother how he tells them apart. He said, ‘Oh, that’s easy. The one that bites is George.’ Now that they are five years old, George is the one who runs to give me a hug, and Donald is the one who keeps his distance. The randomness of the synapses in their brains is the creative principle that makes George George and Donald Donald ... George and Donald are different people because they started life with different random samples of neurological junk in their heads. The weeding out of the junk is never complete. Adult humans are only a little more rational than five-year-olds. Too much weeding destroys the soul.
— Freeman Dyson, On evolution and free will, from Origins of Life, 1999.