An anonymous reader writes: In 1992, archaeologists working a highway construction site in San Diego County found the partial skeleton of a mastodon, an elephant-like animal now extinct. Mastodon skeletons aren't so unusual, but there was other strange stuff with it. "The remains were in association with a number of sharply broken rocks and broken bones," says Tom Demere, a paleontologist at the San Diego Natural History Museum. He says the rocks showed clear marks of having been used as hammers and an anvil. And some of the mastodon bones as well as a tooth showed fractures characteristic of being whacked, apparently with those stones. It looked like the work of humans. Yet there were no cut marks on the bones showing that the animal was butchered for meat. Demere thinks these people were after something else. "The suggestion is that this site is strictly for breaking bone," Demere says, "to produce blank material, raw material to make bone tools or to extract marrow." Marrow is a rich source of fatty calories. The scientists knew they'd uncovered something rare. But they didn't realize just how rare for years, until they got a reliable date on how old the bones were by using a uranium-thorium dating technology that didn't exist in the 1990s. The bones were 130,000 years old. That's a jaw-dropping date, as other evidence shows that the earliest humans got to the Americas about 15,000 to 20,000 years ago. The study has been published in the journal Nature.