In 1868 Charles Darwin wrote this about sexual reproduction: "We do not even in the least know the final cause of sexuality... The whole subject is as yet hidden in darkness." Sex dominates among Earth's species, but asexual organisms seem to have several advantages; they do not spend energy seeking a mate, or run the risk of never finding one, or expose themselves to sexually transmitted diseases, and can reproduce faster than sexual organisms because they have only daughters, each able to reproduce on her own. Given the benefits of asexual reproduction, why is sex so common? The answer remains "hidden in darkness." That Darwin was unable to solve the problem of sex is understandable, but it is something of a scandal that it is still in the shadows more than150 years later. Why We Have Sex discusses the evidence for and against the most popular theories of sex, and offers a new approach. Sex is shown to be linked to another controversial topic of evolution - the formation of new species. Why We Have Sex explores the impact of these ideas on human evolution. We walk upright and have big heads, but we also have sex exclusively with other primates that share these traits. Why We Have Sex asks: Why was Tarzan sexually attracted to the human Jane, and not to one of the apes that he was raised with? The answer to this question leads to fascinating speculation about our evolutionary past, and our future.