Baculum (Penis Bone)
Great apes have a baculum (penis) bone (a small bone situated near the tip of the penis between the urethra and corpus cavernosum) and humans do not. The evolutionary reasons and the genetic basis for this difference are unknown. One possible function of the baculum is protection of the urethra.
Postcopulatory sexual selection influences baculum evolution in primates and carnivores, , Proceedings of the Royal Society B: Biological Sciences, 2016/12/14, Volume 283, Issue 1844, (2016)
The evolution of human reproduction: a primatological perspective., , Am J Phys Anthropol, 2007, Volume Suppl 45, p.59-84, (2007)