The first known Americans may have arrived more than 100,000 years earlier than scientists first thought and they might have been Neanderthals.
Researchers say a 130,000 year old site in Southern California where bones and teeth of an elephant-like mastodon were smashed with rocks is evidence of human-like behaviour.
If true, the finding would far surpass the widely accepted date of about 15,000 years ago.
But the earlier date means the bone-smashers were not necessarily members of our own species, Homo sapiens.
Researchers speculate these early Californians could have instead been Neanderthals, a little-known group called Denisovans, or another human forerunner named Homo erectus.
"The very honest answer is, we don't know," Steven Holen, lead author of the paper released on Wednesday by the journal Nature, said.
Holen, who is director of the Centre for American Paleolithic Research, says they could have arrived by land or sea.
Potential paths could have been via the Beringea land bridge that used to connect Siberia to Alaska, by watercraft along the Beringea coast or across open water to North America.
The site was first unearthed in the winter of 1992-93 in a routine dig by researchers during a freeway expansion project in San Diego.
The Nature analysis focuses on remains from a single mastodon and five stones found nearby.
It says the mastodon's bones and teeth were placed on two stones used as anvils and smashed with three stone hammers to reach nutritious marrow and create raw material for tools.
But the report has been met by skepticism from experts who don't think there is enough proof.
"If the results stand up to further scrutiny, this does indeed change everything we thought we knew," Chris Stringer of London's Natural History Museum said.
"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.
The stones were up to 30 centimetres long and weighed as much as 14.5 kilograms but weren't hand-crafted tools, another of the paper's authors and curator of palaeontology at the San Diego Natural History Museum Tom Demere said.
"We expected skepticism because of the extremely old age of this site," Holen said.
"I think we made a very good case."
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