This article is from the Yolo County FAQ, by David F. Prenatt, Jr. with numerous contributions by others.
As long ago as 40,000 years, the aboriginal inhabitants of California
(known collectively as the "California Indians") first arrived in Yolo
County. The most recent aboriginal inhabitants of Yolo County were
primarily the Patwin ("the people" in their native tongue). Like most
aboriginal Californians, the Patwin relied heavily upon oak tree
acorns as their primary food supply. The Patwin employed a basic
monetary system that was recognized by their neighboring culture
groups and helped prevent most conflicts over territories and
resources. This monetary system also formed the basis for an
extensive trade network that stretched throughout California,
notwithstanding the dearth of long distance travel and communication
between different culture groups in aboriginal California.