> I get the birds and electricity reference, but nuclear fission?

That was fusion, but you can look into natural nuclear reactors (I think first discovered in Gabon) for the fission.

https://en.m.wikipedia.org/wiki/Natural_nuclear_fission_reac...

> The conditions under which a natural nuclear reactor could exist had been predicted in 1956 by Paul Kazuo Kuroda.[1] The phenomenon was discovered in 1972 in Oklo, Gabon by French physicist Francis Perrin under conditions very similar to what was predicted.

So not only did we not learn by observing the natural phenomena in this case, we actually predicted it (accurately, it seems).

I think the thesis being debated in this thread is the possibility of learning based on observation. No one was claiming that all discoveries were actually made this way.

You should look into the context of other things that was pretty much discovered by luck. Becquerel found something weird with Uranium and Curie found few other materials with radioactive properties without having any sort of theories or prediction at that time.

The question is that if radioactive elements were not naturally present in Earth, would we have known they exist?

That's true, and you can make an argument that we could theoretically find a new kind of substance which then sidesteps all physical limitations known to man. But that's in my opinion as convincing as saying we'll discover magic and go explore the metaverse with it.