Were humans in the Americas 100,000 years earlier than scientists thought?

What broke the 130,000-year-old mastodon bones in California? Most archaeologists would tell you it couldn’t have been humans, who didn’t leave conclusive evidence of their presence in the Americas until about 14,000 years ago. But a small group of experts now says that the fracture patterns on the bones, found during highway construction near San Diego, California, must have been left by humans pounding them with stones found nearby. If correct, the paper, published this week in Nature , would push back the presence of people in the Americas by more than 100,000 years—to a time when modern humans supposedly had not even expanded out of Africa to Europe or Asia.

“The claims made are extraordinary and the potential implications staggering,” says Jon Erlandson, an archaeologist at the University of Oregon in Eugene who studies the peopling of the Americas. “But broken bones and stones alone do not make a credible archaeological site in my view.” He and many other archaeologists say it will take much stronger evidence to convince them that the bones were fractured by ancient people.

Archaeologists first excavated the Cerutti Mastodon site in 1992, after the construction exposed bones. Over time they found more splintered bones and a smattering of large round rocks embedded in otherwise fine-grained sediment. More recently, Daniel Fisher, a respected paleontologist at the University of Michigan in Ann Arbor, took a close look at the fractures and found patterns he says are consistent with blows from a rounded stone, which leave a characteristic notch at the point of impact. Other chips of bone show what he calls unmistakable signs of being popped off by the impact. “Nobody has ever explained those [characteristic bone flakes] satisfactorily in any way not involving human activity,” Fisher says. He says humans were probably breaking the bones to reach the marrow, or to turn the bone itself into a sharper tool. The nearby stones, hefty and round, show wear patterns consistent with being smashed against bone, the authors say. In experiments, they used that method to break elephant bones and produced identical fracture patterns.

The bones’ collagen, a protein that can be used for radiocarbon dating, was long gone. Instead, the team relied on a dating technique based on the radioactive decay of uranium in minerals within the bone. Several dating experts said the methods were sound and found the 130,000-year date trustworthy. “At face value, these results are about as good as you can get,” says Alistair Pike, a uranium dating expert at the University of Southampton in the United Kingdom.

But that still leaves the question of what splintered the bones. “It is one thing to show that broken bones and modified rocks could have been produced by people, which [this paper does],” says Don Grayson, an archaeologist at the University of Washington in Seattle. “It is quite another to show that people alone could have produced those modifications.” And Gary Haynes, an archaeologist at the University of Nevada in Reno, notes that “the features of the broken bones … are also produced when heavy construction equipment crushes buried bones.”

Author and site excavator Thomas Deméré, a paleontologist at the San Diego Natural History Museum, says the team carefully excavated 50 square meters beginning in 1992, and the bones described in the paper were “deeply buried. … There was no equipment damage to the heart of the site.”

The lack of actual shaped stone tools, a “universal” at Old World sites, troubles archaeologist John Shea of the State University of New York in Stony Brook. Even the archaic humans that might have been present then, like Homo erectus or the mysterious Denisovans, left such flakes. “This evidence is well documented, but it’s not sufficient to close the case,” Shea says.