> why hasn't any mammal evolved to start destroying them?
I think that evolution selects mostly for reproduction and not longevity. You don't have to live very long to have many kids (in like caveman / hunter-gatherer type of context as this is where evolution is basically), you just need to have great genetics. I guess sticking around for a few years after to protect the kids and provide for them is a bonus, but that part may be handled by the tribe too. But there's not much to be gained (from evolutionary point of view) from an individual living up to a 100 years or so.
I think that evolution selects mostly for reproduction and not longevity. You don't have to live very long to have many kids (in like caveman / hunter-gatherer type of context as this is where evolution is basically), you just need to have great genetics. I guess sticking around for a few years after to protect the kids and provide for them is a bonus, but that part may be handled by the tribe too. But there's not much to be gained (from evolutionary point of view) from an individual living up to a 100 years or so.