In the growing debate about when people first appeared on this continent, a leading archaeologist said Wednesday he has discovered what could be sooty evidence of human occupation in North America tens of thousands of years earlier than is commonly believed.

University of South Carolina archaeologist Al Goodyear (search) said he has uncovered a layer of charcoal from a possible hearth or fire pit at a site near the Savannah River.

Samples from the layer have been laboratory-dated to more than 50,000 years old. Yet Goodyear stopped short of declaring it proof of the continent's earliest human occupation.

"It does look like a hearth," he said, "and the material that was dated has been burned."

Since the 1960s, anthropologists have generally accepted that hunters migrated to North America about 13,000 years ago over a land bridge into Alaska following the retreat of Ice Age glaciers.

But other sites, including the Topper dig in South Carolina, have yielded rough stone tools and other artifacts suggesting that humans lived in North America thousands of years earlier when the climate was much colder. While there is no ironclad proof that an older culture existed, scientists are increasingly open to the idea that humans arrived from many other directions besides the northwest, perhaps even sailing across oceans.

But a 50,000-year-old fire pit would scorch the prevailing occupation theory.

Goodyear's evidence was examined by other scientists, who performed radiocarbon tests on samples to determine their age. However, he made his initial case for the fire pit Wednesday in a news conference rather publishing data in a scientific journal edited by other researchers.

Goodyear, who has worked the Topper site since 1981, discovered the charcoal layer in May.

Thomas Stafford, director of Stafford Laboratories (search) in Boulder, Colo., then took samples of the substance for tests at the University of California at Irvine.

The results showed that wood varieties — oak, pine, red cherry and buckeye — had been burned in a low-temperature fire at least 50,300 years ago, he said.

Stafford described the burnt layer as measuring 2 or 3 inches thick and about 2 feet wide. Rather than a simple black band in the soil, Stafford said the layer had the "shape of a very shallow plate."

He said it could have been the result of a fire tended by humans, or the ashes could have been deposited by wind, rain or flooding.

Other researchers were more skeptical of Goodyear's discovery, noting that previous claims of very old occupation at other sites never have been verified.

"We still need to be cautious," said Vanderbilt University anthropologist Tom Dillehay (search). "I would not yet rewrite the books. The find is very significant and shows that there is much we don't understand and can't easily reject or accept."

Other scientists were blunter.

"I think it's a 50,000-year-old geologic deposit," said University of Texas archaeologist Mike Collins. "It has almost nothing to do with the story of the peopling of North America."

Modern humans are believed to have emerged from Africa 100,000 years ago and spread around the world, elbowing out less capable human cousins like Homo erectus and Neanderthals.