AS the 500th anniversary of Europe's discovery and colonization of the New World approaches, the treatment of the Americas' native peoples is likely to get increased attention. And that attention could center on Brazil, where the last relatively untouched tribes cling to their traditions in an Amazon region beset by development, modernization, and greed.Brazil's government has an ambivalent relationship with the country's Indians. The Brazilian Constitution gives the tribes legal sanction, requiring that their lands be delineated and protected. In practice, however, the agency charged with guarding the Indians' rights has been chronically underfunded. Powerful local and regional interests, intent on allowing the encroachment of ranchers and prospectors on tribal lands, have tended to steamroll any opposition. The arm of the national government rarely extends to Brazil's wild, western expanses of jungles and rivers. Environmental laws, as well as the laws protecting the Indians, were trampled in recent years by a rush of people seeking gold and land. That could be changing. President Fernando Collor de Mello came to office vowing to preserve the Amazon and last year sent police units into areas inhabited by the Amazon's Yanomami Indians. The police drove out thousands of illegal gold miners and destroyed landing strips. But follow-through was weak, according to critics of the government. During his June trip to the United States, Mr. Collor was confronted with charges that the miners had reinvaded the Yanomami lands. Since then, the Brazilian president has announced a number of new policies aimed at protecting Amazonia, including $100 million in debt-for-nature swaps - a marked reversal from Brazil's past determination to resist any foreign involvement in how it uses its lands and resources. Brazil also put a $1.5 billion plan for preserving its threatened rain forests before the recent Group of Seven economic summit in London. Collor is clearly interested in improving Brazil's international standing as an environmental good citizen - not least, perhaps, because his country will host a United Nations conference on the environment and development next year. And any steps that protect the Amazon environment help the native peoples too. The question remains whether help can come fast enough. Some experts believe that at the current mortality rate - caused primarily by diseases introduced by the hordes of miners - the Yanomami, for example, could disappear in 10 years. That can't be allowed to happen. It's already too late for the Yanomami and other tribes to reclaim a state of complete isolation from the outside world. But as modern influences like Western clothing and better tools come in, their irreplaceable culture and their knowledge of the ways of the rain forest can't be allowed to go permanently out, as has too often happened in the past.