One thing I've been thinking about recently is how lucky humanity was to have developed nuclear weapons precisely when we did. One could imagine a multitude of technologies that potentially pose an existential threat - rogue AI, grey goo, genetically engineered super-viruses, etc. Even mundane technologies like prolonged fossil fuel consumption or CFCs can have disastrous consequences if a society does not realize it is capable of causing its own extinction and thus never stops. While it's possible to anticipate some of the dangers, the roads that lead to these technological endpoints are paved in good intentions - there are so many good reasons to advance computers or genetics, etc. Preventing proliferation of these threats might not even be possible once the technologies advance far enough. Further, all these existentially threatening technologies become possible once a civilization learns the same few principles - if you have the knowledge of physics and chemistry to develop one, you're within a few decades at most of developing any of the others.
How fortunate then are we that the first really existentially threatening technology we developed was the atomic bomb? It's a technology that is so obviously destructive, and so distantly related from any useful application, so alien in its method of function, that it's perhaps the only technology in history to be near universally seen as too dangerous to use at scale. It requires both secret knowledge and immense industrial capacity to make even a single device, nonetheless enough devices and delivery systems to actually pose that existential threat, and thus we only need a small handful of large nation states to agree not to use them. It's arguably the very easiest existential threat to handle. Since developing them, we have become cognizant of our civilization's threat to itself, and now every advancement we make is met with the question "does this pose any existential risk?" which our forebears never asked, and which we ourselves might not have asked otherwise.
But it's not just that we developed nukes first. The timing down to the month was critical. Our first nuclear weapons were dropped in the last days of the worst war our planet had ever known. We only had a few, and the decision to drop them was made by people who did not witness nor fully comprehend their destructive potential. As it happened, in the peace that came about after the end of the second world war, there was an opportunity to observe the devastation the bombs had wrought, for the hatred that had led to their use to subside, and for the nuclear monopoly to be broken before another major war could start. But imagine if the bomb was developed a bit earlier, say midway through WW2, we might have dropped dozens or even hundreds of bombs, completely normalizing their use and irradiating large sections of continents before anyone really second guessed it. Conversely, if the weapons were developed in peacetime, either before or after the war, then the threat might go completely unrecognized and no one would think twice about opening the next war with an all out nuclear attack, or perhaps without the wartime funding we would just never have developed them at all and never recognize our species' ability to destroy itself. It's very well possible that if we had discovered the neutron a few years earlier, or if the US had not committed so many resources to the Manhattan project, or if Japan had surrendered a little earlier, we might now be extinct.