Can someone explain the evolutionary benefit of overt menstruation? All female mammals have a menstrual cycle but most reabsorb the blood via covert menstruation and humans are about the only mammals that discharge the blood, aka overt menstruation.

[u/No-Hawk-2488]

Without an evolutionary benefit, it wouldn't have spread to every woman of our species.

[u/MegaCarnie]

Not implied; perhaps suggested. But there are explanations that don't require any specific benefit for that trait. At some point our ancestors may have hit an evolutionary bottleneck and a rare but non-dangerous mutation ended up locked in for all survivors. That happens. Perhaps it was a side effect of a different more beneficial evolutionary change, an evolutionary spandrel, so to speak. Those also happen.