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Humans may have reached North America 100,000 years earlier than we thought

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New insights from mastodon remains

San Diego Natural History Museum Paleontologist Don Swanson pointing at rock fragment near a large horizontal mastodon tusk fragment.
Photo: San Diego Natural History Museum

Humans may have reached North America 130,000 years ago — over 100,000 years earlier than we thought, say researchers who studied mastodon remains in San Diego, California.

Right now, most evidence suggests that humans arrived in North America around 15,000 years ago. But a team of archaeologists analyzing mastodon remains found that the bones showed signs of being broken by human tools, and were buried in ways that suggested human work as opposed to death from natural causes. Most importantly, uranium-thorium dating found that the bones were about 130,000 years old. The results, published today in the journal Nature, change what we know of the history of human migration. Current evidence suggests that humans arrived in North America by crossing from Siberia into Alaska over the Bering land bridge. The authors of today’s paper suggest that perhaps early humans came by a coastal route, since other evidence shows that they’d reached the Asian islands by sea more than 100,000 years ago.

San Diego is home to the Late Pleistocene Cerutti Mastodon site, which was excavated in the early 1990s. There was a single mastodon discovered there, along with some hammerstones and stone anvils. It’s the oldest archaeological site in the Americas, but until now researchers weren’t successful at dating the remains.

A view of two mastodon femur balls, one faced up and once faced down. Neural spine of a vertebra exposed (lower right) and a broken rib (lower left).
Photo: San Diego Natural History Museum

The Cerutti archaeological site is “pristine” as far as geological sites go, says study co-author Steve Holen, co-director of the Center for American Paleolithic Research. It hasn’t been disturbed by geological processes, “so we can eliminate all of the natural processes that break bones like this,” he says.

There are a few different reasons the scientists think humans interacted with these mastodon bones. First, the bones were found in unusual clusters — for example, one mastodon tuft was vertical and some femur bones were found side by side — that suggests they were buried, as opposed to the result of the mastodon dying naturally, according to co-author Thomas Demere, a paleontologist with the San Diego Natural History Museum. Many of the bones had distinct spiral fractures, implying they were broken while fresh and not because of, for instance, animals chewing on them years later. Plus, the ends were broken off, which suggests that humans did this to get to the marrow inside.

Also, the fragments looked like they had been hit with hard objects, in a way consistent with the hammerstones and anvils that were found nearby. And the tools themselves had use marks that the researchers say were unlikely to be caused just by age and the environment. The team also experimented by breaking elephant bones with large rock hammers and anvils. “We reproduced the same kind of fracture patterns that we see on these mastodon limb bones,” says Holen.

But others are more skeptical. “I was astonished, not because it is so good but because it is so bad,” Donald K. Grayson, an archaeologist at the University of Washington, told the New York Times of the study. Other archaeologists called the results “inherently ambiguous” and suggest that the markings could still have been caused by natural factors, despite Holen’s statements.

When it comes to dating the bones, the usual method of radiocarbon dating didn’t work well because there wasn’t enough collagen, though some tries suggested that the bones were at least 60,000 years old, which is about as far back as radiocarbon is able to date. The team then turned to uranium-thorium dating, which suggested the bones were about 130,000 years old.

“The results are completely astounding, as it’s very unexpected that there would be any evidence of humans in North America that are this old,” says Beth Shapiro, a professor of paleogenomics at the University of California, Santa Cruz who was not involved in the study. Though she believes the data has been “reasonably interpreted,” she notes that uranium dating is not typically used for bones and may not be the most accurate method. But the fact that the bones were too old for radiocarbon dating is notable in itself. “I trust the radiocarbon that they’re older than 60,000 years, and if they’re human-modified bones older than that, even that is astounding because we don’t have a record of human occupation there,” she says.

April 26, 2017 1:27 PM EDT Update: The post has been updated to include opinions from more experts.