The article mentions people who produce more Klotho, so clearly that variation has occurred: > Some people carry a genetic variation that causes them to produc

The article mentions people who produce more Klotho, so clearly that variation has occurred: > Some people carry a genetic variation that causes them to produce higher levels of Klotho than average in their bodies. Dr. Dubal and her colleagues identified a group of healthy old people with the variant and tested their cognition. > They scored better than people who make an average level of Klotho. “It’s not like they didn’t undergo cognitive decline,” said Dr. Dubal. “It’s just that they started off higher.” I'm a bit torn between your perspective and the parent comment. I accept the logic of your argument. But that doesn't mean the parent comment couldn't be correct. Klotho could have side effects (doesn't everything have side effects?) and that could be why we (and mice) don't produce more of it. I don't think that's a bad hypothesis. Of course it's just a hypothesis and, as you point out, it doesn't logically follow that our production of Klotho is perfectly optimized (Optimized for what? Fitness? Fitness is a moving target). But, on the whole, I'm skeptical of a hormone that makes us smarter and wards off brain degeneration but does nothing else.