The movie uses an ape to convey that Robbie was always perceived as "unevolved" from the start. Everything that happens in his life, from his point of view, is shaped by this perception because he never conformed to what others wanted him to be. Instead, he lived life his own way, much like the lyrics of Frank Sinatra's song, "I did it my way." While he may seem unevolved to others, he lived his life on his own terms, evolving to be free in his mind. That's why there's no explicit mention of him being a monkey in the movie. I love it. Watching it agay btw :)