Many invasive species get into new ecosystems only with human help. They can’t do it on their own, but once they’re there… Asian carp and zebra mussels.

I'm not in favor of a full-on precautionary principle for everything.

But if you take on too many 1 in 1000 or 1 in 10000 risks, eventually things go badly.

Russian roulette or reckless driving looks safe-- for awhile.

Note the person who really advanced the view of "igniting the atmosphere" with fission was Teller back in 1942, who was about the most pro-nuclear scientist you can find. They didn't just shrug and push the button: Bethe and others did a lot of math to conclude that it was exceptionally unlikely.

(Was it too much of a risk, given that we didn't have much experimental data about nitrogen-nitrogen cross-sections? Probably not, but we can't conclude that it was a reasonable risk purely on the basis of "we didn't die.")