NEW YORK >> A startling new report asserts that the first known Americans arrived much, much earlier than scientists thought — more than 100,000 years ago —— and maybe they were Neanderthals.

If true, the finding would far surpass the widely accepted date of about 15,000 years ago.

Researchers say a site in Southern California shows evidence of humanlike behavior from about 130,000 years ago, when bones and teeth of an elephantlike mastodon were evidently smashed with rocks.

The earlier date means the bone-smashers were not necessarily members of our own species, Homo sapiens. The researchers speculate that these early Californians could have instead been species known only from fossils in Europe, Africa and Asia: Neanderthals, a little-known group called Denisovans, or another human forerunner named Homo erectus.

“The very honest answer is, we don’t know,” said Steven Holen, lead author of the paper and director of the nonprofit Center for American Paleolithic Research in Hot Springs, South Dakota. No remains of any individuals were found.

Whoever they were, they could have arrived by land or sea. They might have come from Asia via the Beringea land bridge that used to connect Siberia to Alaska, or maybe come across by watercraft along the Beringea coast or across open water to North America, before turning southward to California, Holen said in a telephone interview.

Holen and others present their evidence in a paper released today by the journal Nature . Not surprisingly, the report was met by skepticism from other experts who don’t think there is enough proof.

The research dates back to the winter of 1992-3. The site was unearthed during a routine dig by researchers during a freeway expansion project in San Diego. Analysis of the find was delayed to assemble the right expertise, said Tom Demere, curator of paleontology at the San Diego Natural History Museum, another author of the paper.

The Nature analysis focuses on remains from a single mastodon, and five stones found nearby. The mastodon’s bones and teeth were evidently placed on two stones used as anvils and smashed with three stone hammers, to get at nutritious marrow and create raw material for tools.

Patterns of damage on the limb bones looked like what happened in experiments when elephant bones were smashed with rocks. And the bones and stones were found in two areas, each roughly centered on what’s thought to be an anvil.

The stones measured about 8 inches to 12 inches long and weighed up to 32 pounds. They weren’t hand-crafted tools, Demere said. The users evidently found them and brought them to the site.

The excavation also found a mastodon tusk in a vertical position, extending down into older layers, which may indicate it had been jammed into the ground as a marker or to create a platform, Demere said.

The fate of the visitors is not clear. Maybe they died out without leaving any descendants, he said.

Experts not connected with the study provided a range of reactions.

“If the results stand up to further scrutiny, this does indeed change everything we thought we knew,” said Chris Stringer of the Natural History Museum in London. Neanderthals and Denisovans are the most likely identities of the visitors, he said. Denisovans, more closely related to Neanderthals than to us, are known from fossils found in a Siberian cave.

But “many of us will want to see supporting evidence of this ancient occupation from other sites, before we abandon the conventional model of a first arrival by modern humans within the last 15,000 years,” he wrote in an email.

Erella Hovers of the Hebrew University of Jerusalem and the Institute of Human Origins at Arizona State University in Tempe, who wrote a commentary accompanying the work, said in an email that the archaeological interpretation seemed convincing. Some other experts said the age estimate appears sound.

But some were skeptical that the rocks were really used as tools. Vance Holliday of the University of Arizona in Tucson said the paper shows the bones could have been broken the way the authors assert, but they haven’t demonstrated that’s the only way.

Richard Potts of the Smithsonian Institution’s National Museum of Natural History, said he doesn’t reject the paper’s claims outright, but he finds the evidence “not yet solid.” For one thing, the dig turned up no basic stone cutting tools or evidence of butchery or the use of fire, as one might expect from Homo sapiens or our close evolutionary relatives.

The lead author, Holen, told reporters Tuesday that he and co-authors were ready for such criticism.

“We expected skepticism because of the extremely old age of this site,” he said. “I think we made a very good case.”