Penis bones.
Well before neanderthals and modern humans parted ways genetically, penises had spines. The spines allegedly quickened the pace of an erection, and is generally correlated with the more promiscuous species, like cats (who still have penis spines). My theory on this is that neanderthals were ugly, so the spines caused more erections, which caused more population. Then, when we became nice and hairless the spines were no longer needed. Now, we have China. Great plan, Mother Nature.