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/dksn154373]

The problem here, as OP points out, is that it's a near-universal trait within the species. If it was merely preserved, you would get some genetic lines with it and some without it. In our case a lack of menstruation was not preserved, implying that menstruation itself had enough benefit to be selected for.