Genetic studies suggest postmenopausal women are so important in terms of ensuring the survival of grandchildren that we evolved genes to keep their brains healthy.
This is also theorized to be a similar reason that gay people persist throughout history; they are an extra care giver or hunter, gatherer etc. who has no offspring of their own to worry about, so they help ensure the family groups genes get successfully protected in a holistic way.
People talk about genetics as if it’s just about individuals, but really it’s just as much about group success. If I’m a gay member of a family and my siblings reproduce, statistically my genes are still getting passed on (since all us siblings are just different combos of our parents). So if I ensure my nieces and nephews survive my genetics succeed as well, just kind of blended up.
Plus, a few percent chance of no reproduction isn’t exactly a huge penalty, depending on what benefits a trait may bring. The relative narrowness of the human birth canal is another example. Enough average width to avoid lethal outcomes to inevitable natural variation would result in pretty big gait issues.
