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During winter they would rub their hands together almost intuitively, eventually one of them would make the connection about heat and rubbing.
This is why it was possible for them to do it without communication or written information, it was intuitive, at least intuitive enough for their limited cognitive ability.
Not all hominids produced fire, and the explanation would be that bush fires or as others have said "lightning" was common enough for them to rely on that, and perhaps the weather was always warm for them to not bother rubbing their hands together for heat.
