APPROXIMATELY TWO BILLION YEARS AGO a pair of single-celled organisms made a terrible mistake—they had sex. We're still living with the consequences. Sexual reproduction is the preferred method for an overwhelming portion of the planet's species, and yet from the standpoint of evolution it leaves much to be desired. Finding and wooing a prospective mate takes time and energy that could be better spent directly on one's offspring. And having sex is not necessarily the best way for a species to attain Darwinian fitness. If the evolutionary goal of each individual is to get as many genes into the next generation as possible, it would be simpler and easier to just make a clone.
The truth is, nobody really knows why people—and other animals, plants and fungi—prefer sex to, say, budding. Stephen C. Stearns, an evolutionary biologist at Yale University, says scientists now actively discuss more than 40 different theories on why sex is so popular. Each has its shortcomings, but the current front-runner seems to be the Red Queen hypothesis. It gets its name from a race in Lewis Carroll's Through the Looking Glass. Just as Alice has to keep running to stay in the same place, organisms have to keep changing their genetic makeup to stay one step ahead of parasites. Sexual reproduction allows them to shuffle their genetic deck with each generation.