Average life expectancy of a species on this globe is just a few million years. From an external point of view, it would be nothing special if humankind suddenly disappears. We have been here for sometime. With humans no longer around, evolutionary processes would have an even better chance to fill in all those ecological niches which have been created by human activities. As we change the world, and as thousands of species are lost every year because of human activities, we provide a new and productive environment for the creation of new species. Thus, humankind is very creative with respect to providing a frame for new evolutionary trajectories, and humankind would even be more creative, if it has disappeared altogether. If somebody (unfortunately not our descendents) would visit this globe some time later, they would meet many new species, which owe their existence the presence and the disappearance of humankind.
But this is not going to happen, because we are doing science. With science we apparently get a better understanding of basic principles in nature, we have a chance to improve quality of life, and we can develop means to extend the life expectancy of our species. Unfortunately, some of these scientific activities have a paradoxical effect resulting in a higher risk for a common disappearance. Maybe, science will not be so effective after all to prevent our disappearance.
Only now comes my dangerous idea as my (!) dangerous idea. It is not so difficult to come up with a dangerous scenario on a general level, but if one takes such a question also seriously on a personal level, one has to meditate an individual scenario. I am very grateful for this question formulated by Steven Pinker as it forced me to visit my episodic memory and to think about what has been and still is "my dangerous idea". Although nobody else might be interested in a personal statement, I say it anyway: My dangerous idea is my belief in science.
In all my research (in the field of temporal perception or visual processes) I have a basic trust in the scientific activities, and I actually believe the results I have obtained. And I believe the results of others. But why? I know that there so many unknown and unknowable variables that are part of the experimental setup and which cannot be controlled. How can I trust in spite of so many unknowables (does this word exist in English?)? Furthermore, can I really rely on my thinking, can I trust my eyes and ears? Can I be so sure about my scientific activities that I communicate with pride the results to others? If I look at the complexity of the brain, how is it possible that something reasonable comes out of this network? How is it possible that a face that I see or a thought that I have maintain their identity over time? If I have no access to what goes on in my brain, how can I be so proud, (how can anybody be so proud) about scientific achievements?