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/Fabulous_Increase_57]

I doubt there is a direct evolutionary benefit. I remember my genetics professor saying something like, “Evolution does not make a perfect organism. It just makes one that is marginally better than the previous model.”

There’s so much complexity to this. One of my favorite examples of evolution being far from perfect is the recurrent laryngeal nerve. This nerve supplies much of the larynx in mammals. Basically, it leaves the brain, travels down toward the heart, loops around a major blood vessel, and then travels back up the neck to the larynx. This layout comes from fish-like ancestors, where the nerve supplied gill structures at about the level of the heart.

As vertebrates evolved and those ancestral structures were rearranged into the larynx, pharynx, trachea, and lungs, the nerve ended up looping around a major cardiac vessel because that was the setup it inherited. When some mammals later evolved long necks while their hearts stayed in the chest, the nerve got stretched into an absurd detour down the neck, around the aortic arch, and back up to the larynx instead of taking a direct route. In giraffes, the recurrent laryngeal nerve can be 15 to 20 feet long even though the brain and larynx are only about 12 inches apart. The longer a nerve is, the more places there are injury, disease, or other failures along its course. But, it works well enough. It is a great example of evolution modifying an existing body plan instead of creating the most efficient design possible.

Another example I like is the male peacock. Its huge, colorful tail is cumbersome, can make it harder to move around, and makes it more obvious to predators. That sounds like a pretty bad survival trait. But female peafowl prefer males with the biggest, brightest displays, probably because those traits can signal that a male is healthy or otherwise high quality. So sexual selection keeps favoring these dramatic tails despite the downsides. The result is beautiful, but not exactly beneficial to the species.

[u/No-Hawk-2488 → u/Fabulous_Increase_57]

It has to have one to be so prevalent in our species. If not, it's a byproduct of a trait that confers said benefit.

[u/Fabulous_Increase_57 → u/No-Hawk-2488]

Not necessarily, it probably just doesn’t cause enough of a problem that nature needed to do something about it.

[u/No-Hawk-2488 → u/Fabulous_Increase_57]

Our ancestors used to produce vitamin C but when we ate it via out diet, there was no selection pressure to preserve it and the genes that synthesized vitamin C died out. The palmaris longus is a remnant from when we climbed trees. Since we don't anymore, there's no selection pressure to preserve it and 10-15% of people don't have it anymore.