One theory regarding the rise of man is that we are mutant chimpanzees who have benefitted from continuing juvenile traits into adulthood. An example is that a human's ratio of brain size to body size is closer to that of a baby chimp than an adult. Perhaps the boneless boner is a simple, undeniable advertisement of cardiovascular fitness.

As well, consider the bonobo. It is a chimp-like primate whose social structure is radically different. Their hierarchy is less rigid, they do not wage war on competing tribes like the chimps, and social bonds are reinforced through sex, amongst both the males and females.

For many years it was thought that chimpanzees were our blood brothers, or the closest thing to in the animal kingdom, and the bonobos were outlyer freaks, an evolutionary dead end, a stub of a thing best left to die in the jungle. However in modern times, with benefit of a genetic analysis which is purely mathematical and not clouded by expectations of 'proper' human behaviour, it is clear that bonobos and chimps are equally related to us, and we to them, and that the split between them happened after the split between them and us.