Nobody has ever seriously argued that Charles Darwin was the first person to put forward the idea of biological evolution. Even Darwin's own books spend a copious amount of time discussing the theories of other evolutionists, such as the famous Jean Baptiste Lamarck (the most important "serious" evolutionary theorist prior to Darwin), the famous-but-not-very-scientific ideas of Robert Chambers (author of the popular evolutionary text Vestiges of the Natural History of Selection), or the vague-but-evocative evolutionary ideas that came earlier (including from Charles Darwin's own grandfather, Erasmus Darwin).

What Darwin offered was a new mechanism (evolution by natural selection, as opposed to Lamarck's acquired characteristics, for example), a new framework (gradualism), and — most importantly — enough evidence from multiple lines of investigation to make the case plausible and worth taking seriously. That is what made Darwin important, and what changed the discussions about evolution. (He also contributed many other related ideas, like sexual selection, to these discussions.)

For a long history of evolutionary thought, both before and after Darwin, Peter Bowler's Evolution: The History of an Idea goes over it in great depth.