wooly mammoth & pre-clovis man
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Archaeologists have unearthed thousands of ancient stone tools which they say could push back the theory of human presence in North America by as much as 2,500 years.

A team of researchers from the Texas A&M University in the US discovered the ancient artefacts from an archaeological site near Texas which predate the technology widely assumed to have been carried by the first settlers.

The trove of 15,528 artefacts, including chipping debris from working stones and 56 tools like blades, scrapers and choppers, was found buried in sediments believed to be between 13,200 and 15,500 years old, the Daily Mail reported.

The find was located five feet below materials left by the well-known Clovis culture, which was once thought to have been the first American settlers around 13,000 years ago, the researchers said.

"It''s like finding the Holy Grail," Professor Michael Waters, who led the discovery, was quoted as saying.

He said: "This discovery challenges us to rethink the early colonisation of the Americas.

"There''s no doubt these tools and weapons are human-made and they date to about 15,500 years ago, making them the oldest artefacts found both in Texas and North America."

According to Professor Waters, the small tools found from the site were "a mobile tool kit" and of the type that could have led to the later development of the fluted points that trademark Clovis technology.

While there are other pre-Clovis sites across the country, the new find included significantly more artifacts than the others, he added.

Anthropologist Tom 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".

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

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

Dr 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.

Steven Forman, of the University of Illinois, who is a co-author of the paper, said the team used luminescence dating -- which can determine when the material was last exposed to light.

They took samples by hammering black, sealed copper pipe into the layers, the researchers reported in the journal Science.