I don't think it's the act of invention that's the hard part, but without long-distance communication or trade, the same inventions had to be made over, and ove

I don't think it's the act of invention that's the hard part, but without long-distance communication or trade, the same inventions had to be made over, and over, and over again. If a tribe died out due to disease/war/famine, etc, any advancements they had made were lost.