Most mammals (the males, anyway) have a baculum, or penis bone. Human males, however, do not. Inasmuch as men and women have the same number of ribs, some people have suggested that the human baculum had to be sacrificed in order to make Eve (see here for a survey). For my part, I think it is just as well to have an upright posture and no baculum.
Now comes a report in Science to the effect that the penises of most male mammals are covered with spines of keratin, the substance of which horns are composed. Owing to a gene deletion, however, male humans lack these spines and therefore are not horny.
Additionally, we see that the human penis not only has no backbone, but also no spine.