In the fall of 1992, a construction crew made an unusual discovery during a freeway expansion in a coastal area of San Diego County. Buried deep within the silty soil were the bones, tusks, and molars of a mastodon, an elephant-like mammal that once lived in North America. When archaeologists took a closer look, they found signs that humans had battered the animal shortly after its death with the large stones buried alongside it.
But the real surprise came when geologists determined the age of the bones. Radiometric dates showed the mastodon had been buried for about 130,000 years. If correct, this means that ancient humans were in coastal California many tens of thousands of years before they were thought to be in the Americas.