South of Tallahassee, deep in a sinkhole under the dark Aucilla River, archeologists have found a small stone knife that they date to some 14,550 years ago - to a time when ice thousands of feet thick covered much of North America, mastodons and mammoths roamed, and menacing dire wolves would have been a deadly threat to the human who made the tiny tool.

That knife, the scientists say, is part of some "unassailable" evidence that humans lived in North America well more than 1,000 years earlier than long thought.

That's not just a matter of dates on a page.

Indeed, it forces a rethinking, they said, of the long-held theory of how the Americas' first settlers came. To arrive that early, they couldn't have walked across a land bridge from Asia and down an ice-free corridor in Canada, as previously thought - the glaciers had not yet retreated, and they would have met a cul-de-sac of ice.

Instead, they must have gone in boats and made their way down the coast, eventually spreading across the continents. Perhaps they went all the way around the tip of South America, then north. Perhaps they crossed in Central America. Perhaps they went into the mighty Columbia River, then headed east.

However they got there, it's clear that people more than 14,000 years ago gathered around a small pond near what's now the Gulf of Mexico. Back then it was 130 miles from the coast, part of a much larger Florida. As seas rose, that pond was covered and became a sinkhole under the Aucilla, and the evidence of their lives was covered up. Until now.

For most of the past century, archaeologists believed that the first people in the Americas arrived about 13,000 years ago. They were dubbed the Clovis culture, after the distinctive stone tools first found near Clovis, N.M.

In recent decades though, evidence found at sites in Chile, Texas, Oregon, Pennsylvania and Wisconsin suggested that people lived in the Americas before the Clovis period.

"Slowly but surely we've been getting little glimpses of these people," said Michael Waters, director of the Center for the Study of the First Americans at Texas A&M University and part of the team that explored the sinkhole.

There had always been questions or doubts about the evidence at the other sites, he said, but not at the sinkhole, named Page-Ladson after the diver who found the first artifacts and the family that owns the land.

"It's the breakthrough site," Waters said. "The evidence is unassailable: We have everything, all the evidence that scientists would want to see that tells us people were here in North America prior to the Clovis period."

Jessi Halligan, a Florida State University archaeologist, has been diving at Page-Ladson for years, and has been busy taking phone calls since the group published its results last week in the journal Science Advances.

"I guess flabbergasted is the right word in this case to describe how excited people are about this," she said. "I'm a nerdy academic, and I'm thrilled to find that people think what I do is exciting."

The stone tool, called a biface, can't be radiocarbon-dated because it's not of organic material. But there are a couple of reasons to believe it's as old as the Page-Ladson team believes.

For one, it doesn't have the telltale markings of a Clovis artifact. For another, it was found under four meters of sediment, in a layer of organic material - mostly preserved mastodon dung - that was radiocarbon-dated to 14,550 years ago. And the material was in orderly layers, not mixed up, with newer layers on top, older ones below.

Halligan said the team of archaeologists was building on earlier work at Page-Ladson. A diver, Buddy Page, first found artifacts and bones there in the 1980s, alerting scientists, who continued to search. They found some stone tools, the bones of extinct animals and a giant mastodon tusk, but many archaeologists remained skeptical: The evidence didn't seem strong enough.

But the discovery of the knife - and further examination of the mastodon tusk, dated to more than 14,000 years and showing obvious signs of cutting on it - has caused a stir in the archaeology world.

"I think it's legitimate," said Keith Ashley, an archaeologist at the University of North Florida who was not involved in the research. "I think it's great stuff they're finding there. It's a really productive site."

Halligan began diving in the sinkhole in 2007. It's challenging work. It's cold - the scientists wear lots of neoprene - and pitch black without the lights they carry down. She said she will continue research there, as well as up and down the river, looking for more evidence of the elusive early Americans.

She was at the site on the day when two divers, working with a trowel, uncovered the knife, bringing up something hadn't been touched by another human for millennia.

"It's crazy exciting as an archaeologist," Halligan said. "It meant that we could actually answer our questions at this site. It went from maybe it was, maybe there were people living there, to oh-my-freaking-God there people there, there were really people there 14,550 year ago."

The scientific team acknowledged there isn't a lot of physical evidence to go on - some stone tools, some butchered bones. It's incredibly difficult to find and then date artifacts from long ago. But even those skimpy clues can help get at some long-buried truths.

"The plan is just to learn as much as we can about these early people as we rewrite the story of the first Americans," Waters said. "Now we need to get the data, the hard data, to try to piece it all together. It's an exciting time right now to be in this field."

Matt Soergel: (904) 359-4082