The recent discovery of ancient tools in a Texas creek bed shows human settlers arrived in North America about 2,500 years earlier than originally believed, say archeologists.

"We have found evidence of an early human occupation … 2,500 years older than Clovis," Michael Waters from Texas A&M University said in a release.

The Clovis people — once thought to be the continent's oldest human culture — go back to about 13,000 years ago, which would make these newly discovered artifacts about 15,500 years old.

Over the past few years, scattered evidence has hinted at earlier cultures. But such evidence has been disputed in part because so few artifacts have actually been recovered — until now.

The excavation at the Debra L. Friedkin Site in Texas, where tools dating back 15,500 years have been found. Courtesy of Michael R. Waters

Details of the excavation are published in Thursday's issue of the journal Science.

"This site contains some of the best evidence for early human occupation on the North American continent," said Steven Forman, an earth and environmental sciences professor at the University of Illinois at Chicago.

"What's really special about this site is we found artifacts below the well-known Clovis horizon," he said in a podcast released by the university.

The Clovis horizon refers to the layer of earth where their distinctive flute-shaped pointed tools have been found.

For many years, the Clovis people were thought to have arrived here from Northeast Asia by crossing the Bering Land Bridge, which once connected Asia and North America.

From there, archeologists had believed they spread out across the continent and eventually made their way down to South America.

Poking holes in Clovis theory

But recently, archeologists have highlighted various problems with this theory, starting with the fact that no Clovis technology has been found in Northeast Asia. And while Clovis-type tools were discovered in Alaska, they are all too young to be Clovis.

Meanwhile, the Texas excavation has revealed blades, scrapers and choppers in the 20-centimetre layer of earth below where Clovis artifacts had previously been found.

Anthropologist Tom D. Dillehay of Vanderbilt University, who was not part of the research team, said he is concerned that the separation of layers at the site "appears not to be as clear as the authors would have us believe."

University of Oregon archaeologist Dennis L. Jenkins said he was also initially skeptical of the find, commenting, "it would have been a hard sell" from many other researchers.

Jenkins, who three years ago reported discovery of 14,000-year-old evidence of human DNA in a cave in Oregon, said he was concerned that settling or rodents had mixed up the specimens in Texas.

But, he said, Waters' team had done "incredible, meticulous scientific work."

"I believe he's made the case," he said. Jenkins said he would have preferred carbon-dating of the specimens, but that couldn't be done because there was no organic material to be tested in the newly found layer.

Instead, the team used a new dating technique called luminescence dating, which measures light energy trapped in minerals such as feldspar and quartz formed centuries ago.

The archeologists now suggest that Clovis tools could have evolved from the tools found in the creek bed. That would mean the Clovis culture was actually homegrown and did not come from Asia.

"This discovery provides ample time for Clovis to develop," said Waters.

"People [who lived at the site] could have experimented with stone and invented the weapons and tools that we now recognize as Clovis … In short, it is now time to abandon once and for all the 'Clovis First' model and develop a new model for the peopling of the Americas."

With files from The Associated Press