Darwin was not the only person interested in evolution; others who came before him had similar theories and ideas. Darwin was criticized - by Baden Powell in particular - for not including his influences in The Origin of Species. (Read his response to Baden Powell here; Powell's original letter is missing.) Darwin did add a Historical Sketch to The Origin of Species after it was first published - read more about the sketch's history in "The Preface to Darwin's Origin of Species: The Curious History of the 'Historical Sketch'" by Curtis Johnson.
Rebecca Stott's book Darwin's Ghosts details some of Darwin's precursors; read an interview with Stott here.
If you're interested in researching some of these people, start here:
