In Archaeology terms, prehistoric is normally defined as being before the written word or written record (not to be confused with petroglyphs or "cave art") was kept. Technically, if a culture did not develop a written record keeping system until very late in history, then they could still be considered "prehistoric" even if the rest of the world had moved beyond that phase. There are still tribes of people that are technically prehistoric today, although their numbers are diminishing.
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