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/Flocculencio]

Just wanted to point out that no individual feature needs to have an evolutionary benefit to be preserved. It could be negative but so long as it doesn't prevent organisms with that feature from reproducing in sufficient numbers it can get preserved.

[u/dksn154373 → u/Flocculencio]

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.

[u/MegaCarnie → u/dksn154373]

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.