Darwin was brilliant.

There were a number of people who were preaching evolution before Darwin, some of them decades prior to OOS. But they didn't have the right theory to explain it and their arguments were generally weak.

The idea that is going round some intellectuals circles now is that Darwin plagiarized all of his ideas from his grandfather. Such is the power of urban legend.

There were, otoh, people who actually did antedate Darwin in the publication of Natural Selection. He actually lists the ones he knew about in later editions of OOS. Of course he was unaware of their theories when he wrote OOS. However, in each case, the idea of NS is peripheral to the subject matter, and none of them explains the theory in anything close to the detail that Darwin musters.