So we are left with the final puzzle of why man lost his penis bone but retained a structure that serves a similar purpose. For the answer to this, we may need to look to women. Sexual selection works on the premise of ‘honest signals’. Put simply, females need to select high quality males to ensure the best genes for their offspring, and a penis that works predominantly through haemodynamics takes a lot of energy to produce. Therefore this is an honest signal of the most healthy males in the same way that the healthiest stags grow the largest antlers.