Traci Watson

Special for USA TODAY

In a provocative and controversial claim, scientists say a scattering of bones and stones suggests ancestral humans reached the New World more than 100,000 years earlier than previously thought.

Most genetic and archaeological evidence shows humans first entered the Americas some 15,000 years ago. But a study nearly 25 years in the making in this week’s Nature finds that the 130,000-year-old bones of a mastodon, an extinct relative of the mammoth, unearthed in California were split open with blows from rocks. Rocks discovered near the bones bear the hallmarks of use as hammers, the scientists report.

The smashed bones may have been the handiwork of a Neanderthal, the scientists say, or the more ancient human relative called Homo erectus, or even our own species, Homo sapiens.

“We are making a claim that’s kind of out there,” acknowledges study co-author Daniel Fisher of the University of Michigan. “We have had to toil over years to make sure we have considered every angle.”

Experts not involved in the study, however, question the claim, in part because of the total lack of other evidence of a human presence so long ago.

“The potential implications (are) staggering,” says Jon Erlandson of the University of Oregon, who was not involved in the study. But “broken bones and stones alone do not make a credible archaeological site.”

Researchers spent almost a quarter of a century building a case. Back in 1992, a backhoe revealed mastodon remains during construction along State Route 54 in San Diego. The location is now called the Cerutti Mastodon Site, after discoverer Richard Cerutti of the San Diego Natural History Museum.

Digging revealed two large, heavy stones whose sharp edges and cracks pointed to use as anvils. Each anvil stone was surrounded by mastodon-bone fragments and flakes of stone, as if someone had been crushing bone on the anvil.

The bone fragments, some from a mastodon’s stout thigh, were scarred with marks and notches that are hallmarks of a violent blow on fresh bone. Old bone, the scientists say, shows different fracture patterns. Near the anvils the researchers found three bulky “hammer stones” apparently used to bash the bones.

The scientists tested their interpretations on skeletons from modern-day elephants, going so far as to dig up buried bones that were “extremely fresh and smelled extremely bad,” says study co-author Steven Holen of the Center for American Paleolithic Research. When the scientists slammed rocks into the elephant bones, they re-created the damage seen on the mastodon bones.

Aside from ancient humans, none of the predators of 130,000 years ago could break open a mastodon thighbone, say the researchers. They also ruled out damage by water and by animals, such as mastodons, stepping on the bones, which would have created random scratches rather than the spiraling fractures seen at Cerutti.

The scientists don’t know what kind of human pulverized the bones, presumably to craft bone tools or to eat the nutritious marrow. Perhaps Neanderthals or other ancient humans crossed the sea between Asia and the Americas aboard crude boats. Or groups walked across the now-submerged land bridge between the continents.

Whomever they were, “there is a mass of evidence that, put together, should overcome any skepticism,” says study co-author Richard Fullagar of Australia’s University of Wollongong.

“Should,” maybe, but it hasn’t.

“To demonstrate such early occupation of the Americas requires the presence of unequivocal stone artifacts,” says Michael Waters of Texas A&M University. “There are no unequivocal stone tools associated with the bones at the Cerutti Mastodon site.”

“If this site is as reported, then it would indicate that there was an ancient human dispersal to the Americas that left no trace in the fossil record or the genome of ancestral Native Americans,” says John Shea of Stony Brook University, who adds, "Nature can do some odd things to rocks and bones, creating objects that look very similar to artifacts."

Expecting such sentiments, the researchers are already using new methods to analyze the bones. The team feels “a responsibility to continue to check this through new ways of testing,” Fisher says. “I don’t think this is the last word.”