AP PHOTO In this February 1993 photo provided by the San Diego Natural History Museum, San Diego Natural History Museum paleontologists at the Cerutti Mastodon site in San Diego, Calif. In a report released on Wednesday, researchers say the site shows evidence of human-like behavior from about 130,000 years ago.

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 Wednesday 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.