Darwin got his idea presumably from mixing his prior education with observations of nature. At this time, many other people collected observations about nature, much more thoroughly than in previous centuries. The more observations on nature are made, the more likely repeated patterns are discovered.

So one way to answer this question is that every intelligent person, without knowing about the theory of evolution, will eventually come up with a theory of common descent of diverse biological life, if they continuously learn about observations from nature.

So what mankind before Darwin lacked was not a single genius with a brilliant idea, but just all the observations about the millions of plant and animal species on the planet, beyond just knowing a handful of farm animals (as many as would fit a boost in pairs), and a few plants.

And with any theory of common descent (no matter what species are involved), a curious person would investigate on the mechanics of common descent, and eventually discover hereditary trait rules, genetics and mutations (though it might take a few generations to develop the technology and run certain experiments).

If in the other hand an intelligent person was kept in the dark about the wealth of knowledge from observing nature, common descent theories would seem unlikely (e.g. a common ancestor of a duck and a cat does not seem possible to a person without biological knowledge), and without common descent, life form emerging from anything else than design would seem difficult to argue for.

It is not the theory of evolution that supports itself and refutes intelligent design, it is the wealth of knowledge about nature and biology that inevitably leads to the theory of evolution as sufficient and most reasonable explanation for life.

Darwin got his idea presumably from mixing his prior education with observations of nature. At this time, many other people collected observations about nature, much more thoroughly than in previous centuries. The more observations on nature are made, the more likely repeated patterns are discovered.

So one way to answer this question is that every intelligent person, without knowing about the theory of evolution, will eventually come up with a theory of common descent of diverse biological life, if they continuously learn about observations from nature.

So what mankind before Darwin lacked was not a single genius with a brilliant idea, but just all the observations about the millions of plant and animal species on the planet, beyond just knowing a handful of farm animals (as many as would fit a boost in pairs), and a few plants.

And with any theory of common descent (no matter what species are involved), a curious person would investigate on the mechanics of common descent, and eventually discover hereditary trait rules, genetics and mutations (though it might take a few generations to develop the technology and run certain experiments).

If in the other hand an intelligent person was kept in the dark about the wealth of knowledge from observing nature, common descent theories would seem unlikely (e.g. a common ancestor of a duck and a cat does not seem possible to a person without biological knowledge), and without common descent, life form emerging from anything else than design would seem difficult to argue for.

It is not the theory of evolution that supports itself and refutes intelligent design, it is the wealth of knowledge about nature and biology that inevitably leads to the theory of evolution as sufficient and most reasonable explanation for life.

The key guide that leads to unguided design is common descent, which was already suggested before Darwin, see https://en.m.wikipedia.org/wiki/Common_descent, https://en.m.wikipedia.org/wiki/Evidence_of_common_descent, https://en.m.wikipedia.org/wiki/History_of_evolutionary_thought