You're assuming that ethics is the search for universal moral principles. That's not ethics, it's moral philosophy. Ethics is the search for 'right human action

You're assuming that ethics is the search for universal moral principles. That's not ethics, it's moral philosophy. Ethics is the search for 'right human action'. That is, the search for principles on how to live. You're right that for moral principles to be meaningful they require some kind of universal law-giver (nature is one kind of law-giver, but not one we would expect.) If you assume a physicalist universe the best you can hope for in moral philosophy is a law of the stronger, or a more egalitarian notion that what is good for my neighbor is ultimately good for me (something that Plato believed for example,) but this has nothing to with the validity of ethics. If you read Aristotle's ethics, you'll find that he doesn't assume a universal moral code in the same way as our post-Christian culture is inclined.