currentsinbiology:

 Why humans lost their penis bone

Despite slang terms that imply otherwise, the human penis contains no
bones. The same cannot be said for many of our closest evolutionary
relatives: Chimpanzees and bonobos both have penis bones (a macaque one
is pictured), also known as bacula. To find out why some primates have
the feature whereas others don’t, researchers traced the bone’s
evolutionary history through time. The baculum first evolved between 145
million and 95 million years ago, the team reports online today in the Proceedings of the Royal Society B.
That means it was present in the most recent common ancestor of all
primates and carnivores. Why some descendants, like humans, lost their
bacula appears to be due to differences in mating practices: In primates, the presence of a penis bone was most tightly correlated to increased intromission duration,