Natural selection is the combination of two ideas: 1. Population characteristics change over time if members of the population are systematically disallowed reproduction. 2. Nature systematically disallows reproduction.

I'm willing to accept that I'm suffering from hindsight bias. But will you at least give me that his theory is much easier to understand than any of the others? And maybe a few guesses on the topic of why it was so hard to think of?

Also, even if an insight is rare, that doesn't mean its bearer deserves credit. Many inventors made important accidental discoveries, and I imagine luck must have factored into Darwin's discovery somehow as well. If 1% of biologists who had gone on the voyage Darwin went on also would have developed the theory, does he still deserve to be on the list of the top ten intellectuals?

Addendum: Here is an argument that ancient scientists and mathematicians don't deserve as much credit as we give them: they were prolific. We have no modern equivalent of Euler or Gauss; John Von Neumann was called "the last of the great mathematicians". There are two possibilities here: either the ancient thinkers were smarter than we were, or their accomplishments were more important and less difficult than those of modern thinkers. The Flynn effect suggests that IQs are rising over time, so I'm inclined to believe that their accomplishments were genuinely less difficult.

And even if making new contributions to these fields isn't getting more difficult, surely you must grant that it must become more difficult at some point, assuming that to make a new contribution to a field you must understand all the concepts your contribution relies on, and all the concepts those concepts rely on, etc.