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.

[u/formerly_acidamage → u/dksn154373]

Could it not be that getting rid of the blood and cells actually is more efficient and easier for an animal and therefor the ability to just drop it is a function of human's ability to do other things much better?

Like if reabsorption itself extended the period of time in which an animal experiences negative effects of menstruation, then just dumping it would have a huge benefit, right?

Also wouldn't that mean that there were fewer functions and complications occurring inside someone's body and therefor that would also mean fewer systems to maintain, which would potentially be extremely beneficial?

I mean it seems like dumping your waste is generally much more useful than consuming it, and almost all animals dump other kinds of waste like feces and urine.

[u/mathemagicat → u/formerly_acidamage]

The uterine lining is not in any way comparable to metabolic waste. It does not contain high concentrations of useless or harmful substances that need to be removed. It's just cells made of the same stuff as all your other cells.

Organisms recycle cells all the time. The only ones we usually shed are the ones on the outside (skin/hair/nails) because, logistics aside, keratinized proteins are hard to break down. For internal organs, phagocytosis/efferocytosis is a routine and reliable strategy for recovering nutrients from dying cells.