My question is: who taught the teachers?
Sooner or later someone had to make it up themselves. If you look at what philosophy and theatre are (people reasoning about questions and people pretending to be other people for fun), I don't think it's that much of a stretch to say humans just made that stuff up on their own.
I mean if you think philosophy had to be taught to humans, you basically have to say that humans are not naturally able to ask questions or apply logic. Given that non-human animals can seek new knowledge, use basic maths and understand chains of events, I really don't think humans needed help to create philosophy.