Assorted mammalian bacula |
Despite the assumptions behind certain slang terms, human males don't really have a penis bone, or baculum, unlike some of our closest mammalian relatives such as chimpanzees and bonobos. The reason why was not really clear until now although evolutionary explanations have been suggested.
The penis bone first put in appearance in mammalian evolution between 145 and 95 million years ago, so was present in the last common ancestor of apps and carnivores, so at some point, and in the case of humans after we diverged from the chimpanzee/bonobo line, the penis bone was lost.