did-you-kno:

Human men used to have a penis
bone. Many animals, like the
chimpanzee, still have a ‘baculum’
to help them mate for longer periods
of time, but when humans became
largely monogamous and men no
longer fiercely competed to impregnate
women, there was less pressure to
have non-stop sex, the need for the
baculum disappeared, and it was
phased out through evolution. Source