I am worried about the self evident truths but i think there's an excellent way around it to explain the inherent need for universal ethics. For me, a diversity and beautiful complexity are the most important human interests. I think that truly moral decisions promote beautiful complexity and diversity, which natural ecosystems as well as humans, especially when working together, are the absolute best at producing. Likewise, having sex with ones own sibling does not promote diversity in a general, not does it promote adventure-spirit to ad diversity to ones life. Ultimately: morality falls under the metaphysics of aesthetics. I don't read Nietzsche directly but I know he talks about how the aesthetic sense is peculiar to humans and not an "evolutionary trait", seemingly paralleling Singer's words on the non-evolutionary origins about modern moral truths. I hope someone can expand on this one. Thanks