It goes back to Einstein, Oppenheimer, and the nuclear scientists in the years after WWII. They were aware of the terrible destructive weapons they had made possible. Way back then they were urging that nuclear weapons be outlawed. They were warning then that either we realized we lived in one world or we would bring about no world — One World or None — which they made into both a book and a movie.
They began the Doomsday Clock more than 70 years ago now. And now that clock is closer to Nuclear Doomsday than ever in history — ticking ever more loudly no longer in minutes but in seconds.