What is the difference between Abemennesker and menneskeaber ?
Abemennesker: ape-men, a somewhat outdated term for the prehistoric ancestors of moden humanity (pitanthropicus and associates; Home erectus, Homo habilis and what have you).
Menneskeaber: all primates or at least all non-human primates, depending on whether you are a biologist or a creationist. In any case: Gorillas, chimpanzes, bonobos, gibbons and orangutans.
The rule is that compounds are determined by the last bit and informed by everything before that, so menneskeaber are essentially aber, unusual in their similarity to mennesker, whereas abemennesker are essentially mennesker, unusual in their similarity to aber.