Even Darwin in his own book points out people who had these ideas before him. But that's not enough, you can't just go "ha here's an idea" and leave it at that. You have to work to prove it, and that's what Darwin did. The idea itself wasn't so revolutionary but everyone else's was just slightly wrong. Some for example believed evolution worked on all beings except humans, because we were work of god, and similar. Darwin came up with a correct one by using theirs as a base and then made his own, tested it and saw it worked. The amount of work Darwin invested in this theory alone was staggering. In fact the book itself "The Origin of Species" was something he was forced to write because people got afraid his work will go undocumented and he'll die soon. So he wrote that book as a summary with more books to come later in expanded and detailed form, which never came.