TIWANAKU, BOLIVIA -- By the time the interlopers arrived, this ancient city was already abandoned and largely in ruins. The Inca legions and later the Spanish conquistadors dismantled its monuments to build their own temples to their own gods, leaving behind a jumble of massive, finely cut stone blocks amid a wide valley corrugated with unexplored mounds and furrows.

But gradually Tiwanaku is revealing its past. Here and at other sites along South America's arid Pacific coast and sprinkled throughout its high Andean valleys, the prehistory of the New World is being rewritten.

As Latin America ambivalently prepares for 1992's commemoration of the 500th anniversary of Christopher Columbus's epochal voyage, recent discoveries have offered a far richer, more complex portrait of the civilizations that flourished and faded for thousands of years before Europeans "discovered" their legacy.

This region, in particular, is being seen as one that can be discussed, in terms of antiquity and achievement, along with ancient Egypt, Mesopotamia, China and the Indus Valley as a true cradle of human development.

The finds are provoking debate about history and heritage, about cultural identity, about the nature of the Spanish conquest and about the nature of the people who were conquered.

"Eventually, the mentality will change," said Lupe Andrade, editor of the daily newspaper Ultima Hora in the nearby Bolivian capital of La Paz and a longtime patron of archaeological projects at Tiwanaku.

"We used to think of these relatively simple-minded peasants who had some artistic talents," Andrade said. "Now we begin to think of a highly technological society that was very advanced in medicine, in metallurgy, in agriculture."

The vast Inca empire that conquistador Francisco Pizarro obliterated is now seen as derivative and in some ways a comedown. The Incas were latecomers, having completed their domination of western South America in the late 1400s, less than a century before the Spanish arrived. By then, people had been living in the Tiwanaku valley for nearly 3,000 years.

"There is no question that much of the technological and organizational sophistication that we associate with the Incas was based on patterns established 1,000 years before, 2,000 years before," said Alan Kolata, an anthropologist and archaeologist from the University of Chicago.

Tiwanaku is but one of many sites whose importance is now being understood in a new light. Temples along the Peruvian coast once thought to date to the period just before the birth of Christ, for example, are now believed to have been built 2,000 years earlier. When the Roman Empire was at its height around A.D. 100, a Peruvian civilization called Moche was producing some of the finest gold artifacts ever found in the New World.

Kolata, who has been digging here for 10 years, said that as early as 1000 B.C. the Tiwanaku civilization began a highly complex agricultural project -- a system of canals and raised fields that harnessed both rainfall and the nearby waters of Lake Titicaca to produce astounding crop yields.

Using the canals and fields the way it appears the ancients used them, Kolata's team has helped local villagers produce seven times the amount of food they had been growing with so-called modern techniques. In an area where farmers used to harvest only 1 1/2 tons of potatoes per acre, villagers using the raised fields can grow nearly 10 tons per acre.

Crop yields like these allowed the Tiwanaku valley to eventually support a sophisticated urban population of up to 400,000, Kolata said. At a time when Europe's cities were fetid and full of disease, Tiwanaku's wealthier citizens enjoyed running water and an elaborate sewer system graded to modern tolerances. Tiwanaku's sewers, Kolata said, compare favorably with those of modern-day La Paz. At its height between A.D. 400 and A.D. 1000, Tiwanaku was capital of an empire that covered an area roughly the size of California. There was apparently a distinct social and economic order: Holy men and elites lived in luxurious surroundings with brightly painted walls and friezes, skilled craftsmen in smaller homes, tradesmen and workers in dwellings that were humbler still.

Much of the Tiwanaku stonework was dismantled after the Spaniards came and used to build churches, dwellings -- even, much later, to build culverts for the railroad line that runs past the ruins. That which survives is stunningly precise. The more recent structures were built with blocks hewn to a standard size and fitted so closely that not a razor blade can pass.

A three-tiered pyramid called Pumapunku includes stones weighing up to 160 tons apiece. Hewn from rock miles away, they might have been rolled into the city on logs or round stones, researchers speculate.

A large temple at the center of the complex, reconstructed decades ago by archaeologists, was aligned to mark the summer and winter solstices. An adjacent pyramid -- the most massive structure at the site -- was built with a network of pools and cascades, apparently to represent the Tiwanakans' view of the water cycle.

The Tiwanakans practiced forms of surgery and produced exquisite objects out of precious metals. But they never invented the wheel, nor did they develop a written language or discover the usefulness of smelted iron.

Suddenly, around A.D. 1000 or a little later, the lights went out in Tiwanaku. The city's monumental core fell into disuse and the population dispersed into the hills. Kolata and his team of researchers are trying to find out why.

"There are some indications that there might have been a major drought around that time lasting six or seven decades," he said. "It's easy to see that if something went wrong with those raised fields they had built, they were clearly going to have trouble supporting an urban population. There are Spanish documents that mentioned of small kingdoms in this area before the Incas came. That probably represents the Balkanization of the Tiwanaku empire."

The Incas arrived around 1470 and tried to reestablish central order, in part by building on the mystique of the Tiwanaku empire. One legend has the Incas using Tiwanaku as a model for their capital of Cuzco. Another has an Inca ruler sending his wife to Tiwanaku to give birth to his son and heir -- perhaps as a way of appropriating the legitimacy of the Tiwanaku dynasty.

"It's similar to the way the Romans looked back to Greece, a way of encompassing the past," Kolata said.

Current-day politicians have also begun to look back to Tiwanaku in an attempt to steal a bit of its magic. Bolivian President Jaime Paz Zamora regularly uses the ruins as a backdrop for diplomatic events. A rising populist politician, Carlos Palenque, went there to found his political movement.

For nearly 500 years, the fair-skinned elites of Latin America have looked to Europe as heritage and inspiration. Now it has become more chic than ever before to celebrate indigenous culture -- for some, even to acknowledge or claim to be partly of Indian descent.

Some scholars and theologians see Columbus and those who came after him not as intrepid explorers, but rather as genocidal usurpers. Others note that pre-Columbian societies featured human sacrifice, slavery and a degree of authoritarianism far beyond that of Renaissance Europe.

Meanwhile, for the large and mostly impoverished Indian majorities in countries such as Bolivia and Peru, ruins like those at Tiwanaku are a kind of talisman of past glories. A new debate has been joined over who has the better claim to this legacy -- the archaeologists or the local descendants of the people who built the monuments ages ago.

In Peru, for example, archaeologist Walter Alva is credited with discovering the "Lord of Sipan," a Moche burial site on the northern coast that contained perhaps the richest trove of gold objects ever found in the Americas. But before Alva arrived, grave robbers called "huaqueros" had already delivered some of the choicest finds from Sipan into the hands of collectors like Enrico Poli, who maintains a private museum of pre-Columbian and colonial objects in his Lima home.

"The huaqueros are the real owners," Poli said recently. "The archaeologists have no understanding of the religious significance of the things they find. Who were the Moche? Who were the Incas? You go outside Lima and you find their sons and daughters living today."

Whether the blame belongs to the Spanish, the Incas or the vagaries of nature, Tiwanaku's time as an urban center is long past. A small, poor village huddles next to the ruins against the knifelike winds that slice through the high valley, which hangs at more than 12,000 feet above sea level. A sliver of Lake Titicaca can be seen in the distance nearly 10 miles away.

The village at Tiwanaku and the towns around it are poor farming communities, their prospects now brightened by Kolata's agricultural experiments -- if money can be found to fund a larger-scale project. Among themselves the townspeople speak Aymara, an Indian language that links them with the past.

Cesar Calisaya, 37, was born at the ruins. His father, Andres, worked for many years as a kind of curator of the site, and when he became old and feeble, Cesar moved back from the big city of La Paz to take over. He is now "the key to the whole thing," in Kolata's words -- interpreter, diplomat and repository of institutional knowledge for a succession of archaeological teams that have come to Tiwanaku to stir up the dust of the ages.

Calisaya knows every inch of Tiwanaku. He points out the places where the Incas carved their characteristic curlicue hieroglyphs into the slabs of stone as a kind of calling card, and the places where the Spanish carved crosses into what they must have seen as graven images.

When a group of schoolchildren begins to climb over exposed walls instead of walking around them, Calisaya firmly shoos them away. "The problem is that there is not enough money to excavate properly and not enough money to protect everything," he says.

Calisaya sweeps his hand across the valley. "We know of places out there, entrances, that we don't tell the archaeologists about," he says. "For now, no. When we are sure they can be preserved, then we will tell."