Think about life expectancies then and now. Well, I don't really know what the cavemen's life expectancies were, but at least compare a couple of hundred years ago with today. Have we improved our longevities despite our diet, or because of it?
I think the reason we live longer is because of better medicine and doctors. No longer are we relying on blood letting.....although there are still a few instances where leeches are medicinally used. These days, we have antibiotics, inoculations, anti-venoms, incredible diagnostic machinery, etc.
I think it’s the science and medicine that has increased our lifespan even though our diets have gotten a lot worse. Just look at how much of science is dedicated to fixing the problems caused by our diets such as obesity, diabetes, high cholesterol, etc.