A human bone discovered near the Tucson, Arizona, home of missing 84-year-old Nancy Guthrie has been determined to be prehistoric and unrelated to her disappearance investigation, authorities said. The Tucson Police Department said the bone was found at North Craycroft Road and East River Road and was quickly identified as possibly "prehistoric" in nature. "This will be a prehistoric anthropological investigation," said James Horton, a spokesperson for the Tucson Police Department. "This is not a criminal investigation.