Hazelnuts are literally a prehistoric food and have been known about throughout known human history. Dozens of archaeological sites have evidence of harvesting and processing of hazelnuts including tools and pits of discarded shells. In ~2800 BCE China, Hazelnuts were known as one of the five essential nourishments given by god. Greek Physician Dioscorides of Anazarbus wrote about the medicinal uses of hazelnuts in his "Peri ulhV iatrikhV" (written in 50-70 CE).

From The Wild Bunch: Exploitation of the Hazel in Prehistoric Ireland (JSTOR Link) by Anne M. G. McComb and Derek Simpson

"The indigenous people of North America had a wide variety of uses for the American Hazelnut (Corylus americana) and the Beaked Hazelnut (C. cornuta); as well as being eaten raw and fresh the kernels were boiled in soups. The Iroquois cooked the meats of hazelnuts with hominy and corn soup, ground them and mixed them in puddings and breads. They also boiled them to obtain their oil, which was skimmed off and used with bread, potatoes, pumpkin, squash and corn. The leftover nut meats were mixed with mashed potatoes and other foods (Kuhnlein & Tuner 1993, 138). The Okanagan Colville people mixed the nut kernels with bear oil or pieces of bear meat or grease, or sometimes berries or cooked roots, then formed them into cakes and dried them."

Fast forward a bit to the period of Western (America) Settlement they were still used in much the same way. Hazelnuts were mixed into savory dishes to add variety to stale diets, especially during the winter since they stored well. They were frequently fed to live stock such as cattle, pigs and fowl (and still are,) starting a few weeks before slaughter to fatten them up and add flavor to meat. Hazelnut flour was a frequent substitute and filler for wheat flour and could be used as a coffee substitute in the deep winter.

These last two alternative uses were especially common here in Oregon, where the first European Hazelnut tree was planted by a retired fur trapper in 1858. They quickly spread across the Willamette Valley due to the rich soil and perfect growing climate that allowed them to be easily grown with minimal work. By the 1880's hazelnuts were among the common food exports from the Willamette Valley to the Oregon and California Gold fields along with beef, eggs, and cheese.

Out of curiosity, I just looked through an 1910 cookbook I own. I found a cake recipe that uses hazelnuts as the flour!