The progress of biology involves, inter alia, the discovery of unknown kinds of organisms. The progress of ethics involves, inter alia, the discovery of unknown duties. There is a vague disquiet one feels, however, at the suggestion that one has discovered a new duty, especially one hitherto unrecognized and generally violated by people. It may seem, especially to those with conservative inclinations, unduly revisionary.
But in fact it is no more revisionary than the discovery of unknown species. Of course, once discovered, a revision to some theories may be called for, but the simple fact that there is an unknown species is entirely unsurprising. Likewise, the simple fact that there is an unknown duty should be unsurprising. Actions are presumed to be permissible unless there is a specific argument why they are not. In general, we have very little in the way of positive evidence for the permissibility of an action. Except in cases where the permissibility follows from the obligatoriness, or when we have some divine revelation (e.g., we know that all the actions done by Jesus were permissible), we typically assume the action to be innocent until proven guilty. But the flip side of such a presumption of innocence is that the presumption is highly defeasible, and so it should be no surprise if we should discover a new duty.
Indeed, I have argued elsewhere that moral progress consists largely in the discovery of unknown duties or, equivalently, unknown prohibitions. Some discoveries are simply the result of new situations raising new questions—the discovery by Anscombe, Wojtyla, Paul VI and others that the moral prohibition against unnatural sexual activity generalizes to a prohibition against all contraception (except through abstinence) was triggered by the advent of effective non-barrier methods of contraception. But some discoveries concern activities people have blithely engaged in for centuries, such as our discovery that there is a prohibition against buying and selling people or that it is wrong to execute criminals when lesser penalties are sufficient to prevent crime. Of course, some dispute the correctness of some of the claimed discoveries. And, of course, the discovered prohibitions tend to follow from principles that were previously available.
My last post, on fantasies, is meant to be that kind of contribution to moral progress, albeit more modest.