Smooth muscle contractions involve the same nerves and muscles in orgasm as in childbirth contractions.

Uterine contraction > Labour and pregnancy, In orgasm https://en.wikipedia.org/wiki/Uterine_contraction

Birthgasm https://en.wikipedia.org/wiki/Birthgasm

In terms of evolutionary origin and adaptation and natural selection (mutation, crossover, selection), all amniotes (reptiles, amphibians, mammals) have a clitoris.

Animal clitoris https://en.wikipedia.org/wiki/Animal_clitoris