Complex sentence: Over 99% of the history of humanity has occurred within prehistoric cultures, who did not make use of writing, thereby not leaving wrritten sources, the only way to learn about prehistoric societies is to use archaeology.
Split into simpler sentences: Over 99% of the history of humanity has occurred within prehistoric cultures, who did not make use of writing, thereby not leaving written records about themselves that we can study today. Without such written sources, the only way to learn about prehistoric societies is to use archaeology.