If humans are always experimenting with nature and using technology, by what standards should we judge that a scientist has crossed a moral boundary?

This is certainly an endless debate. Take, for example, using animals for research. At what point have scientists overstepped their ethical boundaries? Are there even any ethical boundaries when it comes to animal experimentation to potentially cure a myriad of ailments? I'm really not sure. Certainly there have been many Science-Fiction stories alluding to the folly of messing with DNA. Unfortunately your question just asks more questions. Dr. Moreau had some admirable ideas but when man plays God, the result is usually disastrous. I think we must consider if potential progress is worth the risks that we take. A great book that deals, in part, with this is Margaret Atwood's "Oryx and Crake". It's a great read.