For perfectly good reasons, most people cannot conceive the immense amount of time involved in the grand narrative of evolution. It is hard enough to imagine the passage of a thousand years, but it is nearly impossible to grasp the 7 million or so years that have passed since one species of apelike creature gave rise to the lineages both of chimpanzees and gorillas and of us. And then, after all that time and effort, we are told that there is barely a dime’s worth of difference genetically between us and chimpanzees. Estimates vary from 2 percent to 1.5 percent difference. It all seems confusing. And the more one looks into it, the more confusing it can be.