Possibly the 1970 film Skullduggery, based on a novel by "Vercors" variously titled but in English generally called You Shall Know Them. I haven't seen the movie nor read the novel, but from my memory of having read reviews of both in the past it sounds like a possibility. Someone who has seen the movie can perhaps elaborate.
On an expedition in Papua New Guinea, Tropis, a tribe of apelike creatures, are being used as slaves by humans. When one of the Tropis is allegedly murdered, the following murder trial centers round the question: are the Tropis a form of human or animal?