What do you take to be humanness?

Technically, I wouldn't say we'd lost it if the price of sperm donation rose (from its current negative level) until it stopped being an efficient means of reproduction. But I think you underestimate the threat of regular evolution making a lot of similar changes, if you somehow froze some environment for a long time.

Not only does going back to our main ancestral environment seem unworkable - at least without a superhuman AI to manage it! - we should also consider the possibility that our moral urges are a mixed bag derived from many environments, not optimized for any.