His thesis is based on an examination of life from simple bacteria to monogamous mammals. Bacteria are essentially asocial and asexual, resorting to a form of mating with some gene exchange under certain environmental conditions. Bisexuality occurs at a second, more advanced, level of organism complexity, where there is some gain from the greater variety that gene-mixing confers. Full-fledged male and female genders are common at a third stage of evolution when social competition and a sufficiently stable environment provide opportunities for optimal gene exchanges. At this point Hapgood sees male sexuality evolving to ""serve"" what is essentially the female ""manufacturing"" role. Males compete and females select. Only at a fourth stage do male and female roles evolve to the point where lengthy pair bonds and even monogamous matings occur, with the male investing much time and effort in providing and protecting the female and even sharing in parenting. On the surface the thesis has a more benign cast than the ""sex-for-the-selfish-gene's sake"" theories prevalent. But the theories are really akin. They share the common sociobiological belief in the grand logic of nature, holding that much of human behavior is reducible to a ""reproductive imperative,"" or the equivalent--all of which is obvious.