Scholars say that the caves of Carajás, which archaeologists began exploring in the 1980s, offer coveted insight into what may be the earliest known stages of human settlement in the world’s largest tropical rain forest, helping to piece together the puzzle of how the Americas came to be inhabited.

Pieces of ceramic vessels and tools made of amethyst and quartz are among the signs of human occupation from thousands of years ago. Such artifacts, along with the abundance of the caves and rock shelters themselves, make Carajás one of the Amazon’s most important places for the study of prehistoric humans.

The Amazon is already a hotbed of archaeological investigation, as researchers find evidence that far more people might have lived in the region than once considered possible. While the Amazon was once thought incapable of supporting large, sophisticated societies, researchers now contend that the region might have been home to thriving urban centers before the arrival of Columbus.

Before those cities were carved out of the forest, people lived in the Amazon’s caves. At Pedra Pintada, a cave that, like those in Carajás, is also in Pará, Anna C. Roosevelt, an American archaeologist, has shown that hunter-gatherers moved to the region 10,900 to 11,200 years ago, far earlier than once thought, about the same time people in North American were hunting mammoths.

Outside the Amazon, remarkable discoveries have been announced in recent months at other Brazilian sites. At Lapa do Santo, a rock shelter near the city of Belo Horizonte, archaeologists said this year that they had found the New World’s oldest known figurative petroglyph. The rock art, a drawing of a man with an oversize phallus, is thought to have been made 10,500 to 12,000 years ago.

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To reach the caves of Carajás, researchers must drive hours along washboard roads cut through the jungle, before scaling escarpments with spectacular views of the Carajás Mountains, a range of canopied peaks rising out of the forest. Macaws fly overhead and bats swirl inside the earth cavities in which hunting tribes once found shelter.

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Some of the caves, substantially cooler inside their openings than the surrounding forest, are large enough for more than a dozen people; others might have provided just enough space for two or three people.

Vale, then a state-owned company, began developing the iron ore deposits here after they were discovered in 1967 by a Brazilian geologist on assignment to find manganese for the U.S. Steel Corporation. Vale has since been privatized, but the government still controls big equity stakes.

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Thanks largely to its Carajás complex, where thousands of workers labor 24 hours a day amid the clamor of digging machines, Vale accounts for 16 percent of Brazil’s total exports. As Vale grapples with a sharp decline in profits this year and delays at projects outside Brazil, Carajás is expected to become more important.

Vale has said it plans to create 30,000 jobs in the expansion of iron-ore mining at Carajás, a $20 billion project called Serra Sul, which is already luring thousands of migrants from around Brazil to this frenetic part of the Amazon.

To comply with regulations governing archaeological sites, Vale executives said, the company hired archaeologists and a team of speleologists, or cavers, to survey the caves, which are clustered around the open-pit Carajás mine. Vale also adapted its construction proposal to preserve some caves while planning to destroy dozens of others. While Vale acknowledged that at least 24 of the caves to be destroyed are of “high relevance,” it said it would also preserve caves in another part of Pará to compensate for their loss.

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“For us there is just one procedure, and that is being transparent,” said Gleuza Josué, Vale’s environmental director. Describing the expansion of Carajás as a project of “paramount importance,” she said that Vale had rigorously complied with environmental and archaeological legislation in order to move forward with its plans.

Regulatory officials said they had won concessions from Vale but had not been able to stop the mine expansion. Despite archaeological concerns, the government granted the company a crucial environmental license in June, allowing the expansion to move forward.

The company still needs another installation license, expected to be granted in 2013, to go ahead with Serra Sul. Archaeologists and cavers familiar with Carajás seem resigned to the possibility that Vale will get its way.

Frederico Drumond Martins, a government biologist who oversees the Carajás National Forest, said he remained concerned that mine expansions here in the decades ahead could eventually destroy every last cave in Carajás.

Renato Kipnis, a respected archaeologist in São Paulo whom Vale hired to survey the caves of Carajás, said that Vale had prohibited him from discussing their archaeological significance, because of a confidentiality agreement Vale had required him to sign. Later, a Vale spokeswoman allowed Mr. Kipnis to be interviewed by e-mail, but only if the company was allowed to vet his replies.

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In written replies screened by Vale, he marveled at the importance of the caves.

“The great challenge,” he said, “is finding middle ground between preservation and development.”