From Science Daily (press release, no known paper).
People inhabiting southern Utah's Escalante Valley used hand mills some 10,000 years ago to grind seeds to make flour. This kind of economy is generally considered Mesolithic¹ (transitional from Paleolithic hunter-gatherer to Neolithic "production" economy) in other contexts and it is very revealing to find ancient Native Americans doing the same that their contemporaries in West Asia, North Africa or China were doing about that same time.
Hunt continued however but it's very possible that the millers of Utah were already creating the conceptual and economical scenario for the eventual Neolithic revolution (farming) in America.
Update: Julien Riel-Salvatore has a nice article on these findings.
¹ Note: I follow the school that uses the term Mesolithic only for cultures that do display that transitional economy, other contemporary cultures that do not show any sign of transition towards Neolithic are best called Epipaleolithic. However I must mention that others use the term Mesolithic indiscriminately for all post-Glacial pre-Neolithic cultures.