I thought it was pretty well accepted that it’s basically “Genetic defects happen, sometimes they don’t prevent reproduction and pass down to the next generation, then it’s a lottery, maybe it will last, maybe it won’t, maybe it’s beneficial, maybe it isn’t 🤷”

Maybe at some point a human was born with knees that would never wear out but they died before having the chance to reproduce from being eaten by a tiger, we’ll never know!