This is fascinating stuff-I believe if the human race can stop from utterly destroying itself we're on the brink of coming up with some truly spectacular advances in the next 50 years.
Today we could easily destroy ourselves with nuclear war. It may not be likely but it's more likely than in prehistory.
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.