In The Ethical Project, Kitcher offers a plausible, defensible theory for a naturalistic origin of ethical systems and values.
Be warned: this text is aimed at professional philosophers and those well-versed in the theoretical debates. Many passages address what appear to be expected arguments from different perspectives, much of which will fly over the heads of the casual reader (among whom I class myself).
For an accessible summary of these ideas, I recommend Kitcher's Life After Faith: The Case for Secular Humanism, chapter 2.
The Ethical Project is an important work, one I found strikingly thought-provoking, despite my difficulties accessing some of the arguments. Four stars.