This might be a tough question, as it is about a very distant and undocumented era. I am not looking for a definite answer, only your ideas.
Our distant hominid ancestors were certainly ''ape-like'', and must have been aware of their relative similarity to apes, right? I know genealogy and knowledge of distant ancestors were neither documented nor passed down at that time, but still, our ''ape-like'' pre-human ancestors must have had a clue about their physical and behavioral similarity to the apes they coexisted with, in the same way modern animals recognize members of their own species.
Now, at what point in evolutionary history did our ancestors start considering themselves unique enough to collectively ''forget'' about their kinship with other primates?
Also, why do you think they did? Linguistic factors? Lifestyle? Lack of education? Insufficient transmission of knowledge from parent to child? Religious reasons? I want your ideas.
Now, at what point in evolutionary history did our ancestors start considering themselves unique enough to collectively ''forget'' about their kinship with other primates?
This may sound strange, but before our ancestors were able to consider themselves unique, the question was moot and, after our ancestors were able to consider themselves unique... the answer was obvious.
By the time humans were intelligent and scientific enough to look around and start noticing the differences and similarities between themselves and other animals, and able to teach people about what they saw in the world around them, it had been millions of years since their evolutionary separation from the other primates. Their ancestors at the time weren't other primates, they were humans.
It's not like the Homo Erectus people used to sit around the campfire at night, telling stories about their Homo Habilis ancestors. For one thing, they didn't even have language! So, the knowledge didn't exist. And, even if it had existed, it couldn't have been passed down.
You can't "forget" something that you never knew.