Turns out that isn't exactly true. You can poly concepts from genetic drift to language and infer what they must have been like. It's revolutionized the field of linguistics.

That may work prehistoric cultures that have fragments in our current languages. This is an incomplete picture because we don’t know about the dead ends. Oral histories disappear when the cultures die out.

Very currently. Just 1,000 years ago people memorized Beowulf to keep the text.

But it is hard for us to imagine times before us, it is true.