Possibly the 1970 film [_Skullduggery_](https://www.imdb.com/title/tt0066384/?ref_=nm_flmg_wr_11), 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?

> <sub>[Wikipedia, Skullduggery](https://en.wikipedia.org/wiki/Skullduggery_(1970_film))</sub>