SAN MARTIN DE SAMIRIA, PERU - To the untrained eye, all evidence here in the heart of the Amazon signals virgin forest, untouched by man for time immemorial - from the ubiquitous fruit palms to the cry of howler monkeys, from the air thick with mosquitoes to the unruly tangle of jungle vines.

Archaeologists, many of them Americans, say the opposite is true: This patch of forest, and many others across the Amazon, was instead home to an advanced, even spectacular civilization that managed the forest and enriched infertile soil to feed thousands.

The findings are discrediting a once-bedrock theory of archaeology that long held that the Amazon, unlike much of the Americas, was a historical black hole, its environment too hostile and its earth too poor to have ever sustained big, sedentary societies. Only small and primitive hunter-gatherer tribes, the assumption went, could ever have eked out a living in an unforgiving environment.

But scientists now believe that instead of stone-age tribes, like the groups that occasionally emerge from the forest today, the Indians who inhabited the Amazon centuries ago numbered as many as 20 million, far more people than live here today.

"There is a gigantic footprint in the forest," said Augusto Oyuela-Caycedo, 49, a Colombian-born professor at the University of Florida who is working this swath in northeast Peru.

Stooping over a man-made Indian mound on a recent day, he picked up shards of ceramics and dark, nutrient-rich earth made fertile hundreds of years ago by human hands. "All you can see is an artifact of the past," he said. "It's a product of human actions," he said.

The evidence is not just here outside tiny San Martin de Samiria, an indigenous hamlet hours by speed boat from the jungle city of Iquitos. It is found across Amazonia.

Outside Manaus, Brazil, Eduardo Neves, a renowned Brazilian archaeologist, and American scientists have found huge swaths of "terra preta," so-called Indian dark earth, land made fertile by mixing charcoal, human waste and other organic matter with soil. In 15 years of work they have also found vast orchards of semi-domesticated fruit trees, though they appear like forest untrammeled by man.

Along the Xingu, an Amazon tributary in Brazil, Michael Heckenberger of the University of Florida has found moats, causeways, canals, the networks of a stratified civilization that, he says, existed as early as A.D. 800. In Bolivia, American, German and Finnish archaeologists have been studying how pre-Columbian Indians moved tons of soil and diverted rivers, major projects of a society that existed long before the birth of Christ.

Many of these ongoing excavations follow the work of Anna C. Roosevelt. In the 1980s on Marajo Island, at the mouth of the Amazon, she turned up house foundations, elaborate pottery and evidence of an agriculture so advanced she believes the society there possibly had well over 100,000 inhabitants.

Her initial conclusions, published in 1991, helped redirect scientific thinking about Amazonia, with younger archaeologists who followed buttressing and building upon her findings.

"I think we're humanizing the history of the Amazon," said Neves, 44, a professor at the University of Sao Paulo. "We're not looking at the Amazon anymore as a black box. We're seeing that these people were just like anywhere else in the world. We're giving them a sense of history."