Why do you say that? After humans spread across the globe in hunter-gatherer societies, we were pretty unlikely to destroy ourselves.

Today we could easily destroy ourselves with nuclear war. It may not be likely but it's more likely than in prehistory.

Life in those days was very precarious. One bad drought and everyone would have been dead.

A nuclear war is not likely to kill everyone. Plus a nuclear war is not likely these days anyway.

The chance of a nuclear war today is far far lower than some disease in the middle ages, or a drought in pre-history.

Everyone focuses on nuclear weapons, but reality is they are very very unlikely to ever be used, plus illness is a much more severe danger. And the good thing about today is that we are much better prepared to handle a disease than at any other time in history.

And that's why we are less likely to destroy ourself now than at any other time.