"At the heart of evolutionary anthropology lay the assumption that the human mind was guided by universal, not culturally specific, impulses...This assumption had two important methodological implications. First, it allowed ethnologists to reason by analogy, and to do so with the same certainty with which they reasoned deductively from observation.
Because they believed that all societies evolved through similar stages, developing similar or at least comparable technologies and social institutions along the way, they were perfectly comfortable studying ancient Native American cultures by proxy, deducing their histories from the present lives of people who occupied the same rung on the evolutionary ladder.
The conviction that ancient and contemporary aboriginal peoples might be considered virtually identical allowed the scientists to call their work an empirical science despite the absence of the actual subject matter they claimed to be analyzing."
---from Zuni and the American Imagination by Eliza McFeely