The biggest single threat is willful human stupidity. And I'm not talking about a nuclear holocaust, I'm talking about the human race breeding for stupidity and breeding against intelligence, which we've been doing for centuries. It's beginning to look as though human technology and industrialization were just an anomaly, and are going to last for only a very short while longer before the human race devolves into a state of barbarism resembling that of ancient times. The killer for humanity will be when we collectively lose our ability to read and write. I see that coming, and not too far down the pike.