To me 'humans' is more appropriate in modern scientific contexts like archaeology or anthropology, or ecology, where it's about comparing our species with other hominids or animals. 'Human beings' is more old-fashioned and perhaps more typical in contrast with fictional or fantasy creatures, like Martians or elves, or supernatural ones, like angels, spirits, gods, that kind of thing.
Maybe. The more I think about it, though, 'humans' is also possible there, and 'human beings' just sounds more old-fashioned.
This may be a pet peeve that goes back to Fowler or someone. (Sorry, I don't have my copy here or I'd check for you, but it's online.) It doesn't make any sense to me, but if it makes them feel more British and that makes them feel like better people, who are we to object.