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Evidence of ancient humans in San Diego
San Diego Natural History Museum paleontologist Don Swanson points to a rock fragment near a large piece of a mastodon tusk at the excavation site. (San Diego Natural History Museum Photo)

Scientists say the patterns of breakage in mastodon bones found 25 years ago near a San Diego highway suggest that humans battered the beast 130,000 years ago.

That’s a shocker, because before now, the oldest widely accepted evidence of human habitation in North America goes back only about 16,000 years. If the scientists are right, that makes the place they studied, known as the Cerutti Mastodon site, the oldest archaeological site in North America.

“It’s somewhat mind-boggling to have 130,000 years proposed,” said University of Oregon archaeologist Dennis Jenkins. He has found previous evidence for human habitation in 14,000-year-old preserved poop but wasn’t involved in the latest study, published today by the journal Nature.

The researchers behind the newly reported find came upon their evidence way back in 1992, when they made a routine check of a construction site for a noise-reducing berm near State Route 54 in San Diego. The site was promising enough for the scientists to dig 10 feet below the level of the construction crew’s excavation.

What they found was intriguing: Bones, tusks and molars from a mastodon were buried alongside large stones that appeared to have been used as hammers and anvils for breaking up the bones. That posed a puzzle: Apparently, humans were in what is now Southern California long before they were supposed to be.

The site would have been close to sea level back then, with a meandering stream running through it, said study co-author Tom Deméré, curator of paleontology and director of PaleoServices at the San Diego Natural History Museum. “A very nice place to live, I’d think, 130,000 years ago,” he told reporters.

But the researchers knew they had to be sure about what they thought they were seeing. “Of course, extraordinary claims like this require extraordinary evidence,” Deméré said.

Over the years that followed, researchers painstakingly put the puzzle pieces together. They studied how the break marks on bones and the chip marks on stones, and concluded all those marks could only have resulted from tools being used intentionally.

To test their hypothesis, they even beat away at bones from dead elephants, including a leg bone from Tanzania and a carcass that had been buried in Colorado three years earlier.

“The bone was extremely fresh, and smelled very bad. I almost wished that I never continued that experiment when we dug it up. But we did,” said lead author Steve Holen, director of research at the Center for American Paleolithic Research. Holen and his strong-armed colleagues produced similar patterns of breakage and chipping, using stone hammers modeled after the ones that were found at the construction site.

Uranium-thorium dating of bone samples indicated that the mastodon was 130,000 years old, with a conservative margin of error of plus or minus 9,400 years.

“It’s hard to argue with the clear and remarkable evidence,” said co-author Richard Fullagar, an archaeologist at the University of Wollongong in Australia.

Nevertheless, there are arguments. Skeptics wonder whether the site may have been disturbed, or whether the dating estimates could be off. Also, the bones showed no evidence of the cut marks typically associated with butchering.

The researchers insist that there are no signs the site had been disturbed at the depths they excavated, that the dating estimate is as solid as it can be. They surmise that the hunters may have been after the marrow inside the bones.

The University of Oregon’s Jenkins raised a wider concern.

“It’s not so much to do with the dating – that seems to be the standard kind of stuff that’s been applied in Africa,” he told GeekWire. “It’s the fact that they have this huge gap between what’s accepted and what they’re claiming. … Where have these people been?”

The researchers speculate that these people may have been ancient Asians, perhaps Neanderthals or Denisovans, who figured out a way to get to North America. They may have walked from Siberia and Alaska, over a land bridge that disappeared about 135,000 years ago. Or they may have sailed around the coasts of Asia and North America to land in California.

The early humans may have died out, leaving little trace of human existence until a fresh wave of immigrants swept in more than 100,000 years later. Such scenarios are mere speculation, however, because no human remains were found at the Cerutti Mastodon site.

That’s problematic for the researchers’ case, but Holen said such remains are hardly ever found at ancient settlement sites. Instead, researchers typically have to piece together the evidence from stone artifacts and animal bones, just as Holen and his colleagues have done.

“To find human remains that date to 130,000 years ago, we think, would be a truly exceptional find,” he said.

Rocks and bones at mastodon site
Fossil bones and rocks are spread out at the Cerutti Mastodon site. The positions of the femur heads, one up and one down, broken in the same manner next to each other, are unusual. Mastodon molars are located in the lower right-hand corner next to a large rock composed of andesite, which is in contact with a broken vertebra. Upper left is a rib angled upwards, resting on a granitic pegmatite rock fragment. (San Diego Natural History Museum Photo)

Perhaps the findings seem so incredible because scientists have previously missed seeing the evidence at other sites. “A lot of this evidence has fallen in the academic cracks between archaeology and paleontology,” Holen said.

And perhaps the potential evidence has been there, but scientists just haven’t accepted it. As an example, Jenkins pointed to the Calico Hills site in California’s Mojave Desert, where researchers uncovered a “rock ring” that dates back to somewhere between 135,000 and 200,000 years ago.

Holen and his colleagues say they’re willing to have other scholars check their findings, or run new rounds of tests on the objects they excavated. They’re even thinking about finding a way to revisit the Cerutti Mastodon site, which today lies buried once more beneath the highway berm.

“People thought they knew the answer [to the question] of when humans came into North America, so they didn’t go look in these older geological deposits,” Holen told GeekWire. “I know that I never looked at this age of geological deposits in my entire career, until I started working on the Cerutti Mastodon site. I know that I will now be looking at older age deposits, and I would invite all the skeptics to do the same.”

In addition to Steven Holen, Deméré and Fullagar, the authors of the Nature paper, “A 130,000-Year-Old Archaeological Site in Southern California, USA,” include Daniel Fisher, James Paces, George Jefferson, Jared Beeton, Richard Cerutti, Adam Rountrey, Lawrence Vescera and Kathleen Holen.

Specimens recovered from the Cerutti Mastodon site are on display at the San Diego Natural History Museum, and digital 3-D models of a selection of the specimens can be viewed interactively at the University of Michigan Online Repository of Fossils.

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