Introduction Some time this past year, the earth’s human population rose past 6.5 billion living persons. Roughly 130 million babies were born in 2006, which translates into about 350,000 per day. Impressive as these gures are, they represent but a fraction of an invisible industry. For every human birth, as many as four conceptions never reached fruition-nearly 1.4 million each and every day, and more than half a billion lost human conceptions each year. Having breathed a sigh of relief on behalf of the planet that such potential was not realised, one is left to wonder at the extraordinary inefciency of this biological economy. Why hasn’t natural selection during 6 million years of human evolution honed a sleeker assembly line?.