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Critics have similar doubts about charcoal Guidon believes came from ancient fireplaces. "Radiocarbon dating is tried and true," explains archaeologist David Meltzer of Southern Methodist University. "The problem is linking the dating of objects to human occupation. How do you know it was a piece of charcoal touched by human hands and not just a piece of burned tree?" Brian Fagan of the University of California at Santa Barbara is a bit more blunt: "I think Pedra Furada is absolute horse manure."
That kind of derision doesn't faze the feisty Guidon. On the charcoal deposits, she argues, "If they had been left by forest fires, carbon deposits would have been found scattered across a wide area." They are not. In many cases, the charcoal is ringed by stones, says Guidon, which is strong supporting evidence that these were man-made hearths, not natural formations. Besides, the area was a humid, tropical rain forest 30,000 years ago, and natural fires would have had a hard time getting started.
The artifacts in deep layers don't trouble her either. They couldn't have been washed in from elsewhere and mixed, she says, because the rock shelter where they were found is more than 60 ft. above the surrounding terrain. Nor < could the objects have tumbled down from higher up on the cliff, says Guidon, since the cave is protected by a massive rock overhang that would have kept out both falling rock and flowing water.
Those who remain skeptical of pre-Clovis findings are most troubled by the ambiguous nature of many of the artifacts. To make a convincing case for a pre-Clovis culture, says Cornell archaeologist Thomas Lynch, "recognizable artifacts from that period must be dispersed over a broad area, reflecting the movement of primitive peoples from place to place. A Clovis point is just as recognizable as the tail fin on a 1952 Cadillac."
Like its more conventional counterpart, the pre-Clovis theory has some logistical problems. If humans got to South America by 13,000 years ago, they would have had to cross the Bering land bridge many thousands of years earlier. That would have been no problem, but heading south from there would have been tough: ice sheets -- or the inhospitable terrain they left behind -- cut off virtually all access to the bulk of North America from Alaska between 20,000 and 14,000 years ago. Guidon's rather controversial answer: maybe the immigrants came over to South America in boats directly from Asia.
Despite all the doubts and unanswered questions, the case for pre-Clovis Americans is gaining ground. Even if the discoveries at Pedra Furada fail to satisfy the critics, sites such as Monte Verde and Meadowcroft are powerful testimony that early migrations did take place. However the first immigrants got to the New World, and whatever the reason why they left behind so little physical evidence, it has become difficult to deny their existence -- and increasingly likely that earliest American history will have to be rewritten.