The ancient tools and bone are 14,550 years old, they reported Friday in the journal Science Advances, making them the most ancient human remnants ever found in the southeastern United States. The researchers say the find is unequivocal proof that people were in Florida more than 1,000 years earlier than anyone had imagined — a discovery that could help rewrite the history of humans on the continent.

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The new study comes as something of a vindication for the swampy site in the Florida panhandle, named Page-Ladson for the diver who discovered it and the family whose land it is on. In the 1980s, archaeologist James Dunbar and paleontologist David Webb dug up the knife-scarred mastodon tusk that had been left there and estimated it to be more than 14,400 years old.

But the anthropological community was quick to cast doubt on that date. For half a century it had been assumed that the Clovis people — skilled hunters famous for their distinctive fluted spear points — were the first to migrate from Asia and then down through Canada after the glaciers began to melt at the end of the last ice age, roughly 13,000 years ago.

The age given for the tusk didn't fit that paradigm, other scientists said — the ice-free corridor wouldn't even have been open yet. Something must have gone wrong with the dig or the radiocarbon dating, or perhaps the marks on the tusk were caused by something other than a human. Even Dunbar and Webb expressed some misgivings about their results.

"I always felt that Dunbar and Webb had been kind of maligned," said Mike Waters, the director of the Center for the Study of the First Americans at Texas A&M and a principal investigator on the latest Page-Ladson report. "So when I was given the chance to go back there, I jumped at it."

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This time, Waters and his colleagues were armed with dating techniques orders of magnitude more precise than their predecessors'. They also had an increasingly compelling case for "pre-Clovis" occupation of the Americas: genetic analyses showing that Native Americans' ancestors arrived here some 16,000 years ago and archaeological sites as far-flung as Oregon and Chile bearing evidence of human presence long before Clovis.

"What we tried to do at Page-Ladson is make a really strong case that would be unassailable ... that these artifacts are man-made and they're exactly where those people left them 14,550 years ago," Waters said.

The project involved years of painstaking excavation in the Aucilla River, a slow-moving, coffee-colored waterway shaded by cypress trees and inhabited by alligators. Underwater archaeologists dug up and dated layer after layer of sediment from the river bottom, sifting through each patch of dirt for evidence that humans had once been there.

They uncovered what co-author Tom Stafford calls a "chronological layer cake." More than 70 samples of ancient organic material taken from the site and radiocarbon dated at Stafford's lab showed that each layer was slightly older than the one before it. They prove that nothing had disturbed or mixed up the sediments as they were laid down over time.

By the time archaeologists reached the 14,500-year-old stratum, they began to find objects they say could only have come from humans: five sharpened rocks that were carried in from elsewhere in the region, and a double-sided stone knife, or biface, that would have been among the most advanced technologies of the time. The team then re-examined the mastodon tusk found by Webb and Dunbar (who was also part of this excavation) and determined that it was most likely butchered by humans.

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"It's really exciting," said Jessi Halligan, an archaeologist at Florida State University and Waters's fellow principal investigator. "We have these unambiguous cultural artifacts found in an intact geological stratum that dates to more than 1,500 years older than Clovis. That's why it's a big deal. That's why we have to revisit our theory for how the Americas were colonized."

Donald Grayson, an anthropologist at the University of Washington who specializes in American prehistory, noted that there's some doubt about radiocarbon dates coming out of the Aucilla River. Ancient carbon dissolved in the water can contaminate samples, causing them to appear older than they really are.

Halligan countered that the dates coming out of Stafford's lab match what is known about environmental changes at the time. For example, a layer of rapidly deposited dirt was estimated to have been laid down between 14,500 and 14,000 calendar years before present — at exactly the time that rising sea levels would have caused a huge influx of sediment. If the samples were contaminated, that wouldn't be the case.

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"These are the most precise ages we can get," she said.

The discovery also jibes with other pre-Clovis archaeological finds across the Americas, including the more than 14,300-year-old Paisley Caves site in south-central Oregon. Dennis Jenkins, an archaeologist for the Museum of Natural and Cultural History at the University of Oregon who co-led excavations of the caves, said that report offered "yet another data point" in favor of an increasingly popular new theory about America's first humans.

"I believe that the majority of professional archaeologists have reached a point that, yes, they agree there was something here a minimum of 1000 years before Clovis ... and since the ice free corridor wasn't open yet, obviously there are a lot implications for getting people down from the interior of Alaska," Jenkins said.

It seems likely that the first Americans sailed down the Pacific coast, where pockets of land and seal-rich seas would have sustained them as they migrated south to places like Paisley Caves and Monte Verde in Chile. From there, they may have followed America's river systems to the other side of the continent, or trekked across Central America at its narrowest point and sailed up into the Gulf of Mexico.

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The Page-Ladson find also challenges another piece of anthropology orthodoxy: Scientists have long-believed that the arrival of human hunters in the Americas precipitated a "blitzkrieg" extinction of the region's megafauna — mammoths, giant bison, ground sloths, and others — because Clovis points appear at exactly the moment in the archaeological record where giant mammal fossils vanish. But the discovery of tools and a butchered mastodon bone suggest that humans and these large animals co-existed for at least 1,500 years.

So, how did the Page-Ladson people get to Florida? And what happened when they arrived?

"We just don't know yet," Waters said. "But what we do know now is that there were people at Page-Ladson 14,550 years ago ... and I'm hoping that it will blow the fence sitters off on to the pre-Clovis side and maybe it will open the eyes of the Clovis First proponents."

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"And then," he added, "we can start looking for answers to all those other questions."