Great question to ponder. Surely people would have seen their own reflections in water, but the mirror itself would have made "Reflection Technology" for "Artificial Introspection" more scalable. I suspect the mirror offered modern people a new viewpoint, allowing one to see how one is perceived by others. I do not think selfhood and identity would have been affected. My main question is about when people came to behave differently than apes. Douglas Hofstadter's "The Mind's I" may have a few hints on perception of self for you.