'We said to them, 'Come closer' but they said to us, 'Go further back''

"First just one came out, then two, then three, four, five, six, seven, but there were more than that in total. We had a dozen machetes, a dozen knives and some axes and pots with us. We gave these to them. Not by hand, but by leaving them on the beach. We said to them, 'Come closer' but they didn't want to. They said to us, 'Go further back, further back,' so we did."

The encounter between José, a Peruvian from the Las Piedras river area near the border with Brazil, and members of the large isolated Mashco-Piro tribe living in the deep Amazonian rainforest, took place this year and was described to the anthropologist Richard Hill, of Survival, the international campaign for tribal peoples.

Following a series of similar encounters and incidents, such as one this week when a Peruvian government team photographed a group of 21 Indians from the air, Mr Hill and other anthropologists are reassessing how many tribes there may be left who have chosen to shun the 21st century.

"Only 30 or so years ago, it was believed there were just 12," said Stephen Corry, the director of Survival. "Now we think there are 107 living in isolation. As more and more incursions are made into the forest, more and more groups are being found. The more people look, the more are being found," he said.

Some tribes who shun contact have a fair idea of life outside the forest, according to Mr Corry, and may have machetes which they could have acquired from contact with other groups. "Others may have had contact with outsiders generations ago, before they retreated deeper into forests because of incursions by westerners. Others may have no idea of country, other languages, or money, and no one has got close to them".

This year the Brazilian government increased its estimate of the number of isolated tribes in its part of the Amazon from 40 to 67. But it acknowledged some were reduced to a few individuals. One tribe is believed to be down to one man, known as the Man of the Hole, who digs holes in the forest to catch animals and fires arrows at anyone who comes near.

There is another large group of uncontacted tribes in eastern Peru, where the government has licensed 70% of the forest to oil and logging companies. These companies are coming into close contact with groups that were suspected but not encountered. Peruvian officials have tried to deny their presence, but the evidence is now incontrovertible. "We think there are 15 groups," said Mr Hill. "Many are the descendants of tribes contacted over 100 years ago, during the rubber boom, who fled the prospect of enslavement and decimation by new diseases."

The other concentration of groups is in West Papua, where vast areas of forest and mountain have been barely explored and access is particularly difficult because of the Indonesian military. Little research has been done, but occasional sightings of tribes by missionaries in aeroplanes suggest there could be as many as 40. At least 16 isolated groups are thought to live in the vast, mostly untouched Mamberamo river basin, an area almost the size of Britain.

Elsewhere, there are three known isolated groups in the Andaman islands of India, five in Bolivia, possibly one or two in Colombia and Suriname, one in Paraguay, and maybe one or two bushmen groups in southern Africa. In 1984, the Pintupi, semi-nomadic people, came out of the Australian desert.

Sydney Possuelo, the director of the Instituto Indigenista Interamericano and former head of the Brazilian government's department of unknown tribes, has led many expeditions into the Amazon to try to protect such groups. "What is happening is that groups are constantly moving around because of the pressure from the development activities of white people," he said.

He said the greatest concentration might be in the protected Vale do Javari indigenous area, with as many as 1,350 uncontacted people. Most, he said, probably fled there after contact with Europeans searching for timber or gold many years ago.

All known isolated groups are thought to be in danger. "The greatest threats come from our permanent need for growth, in the search for minerals, energy, timber and agriculture," said Mr Possuelo. "The losses really start a long time before contact. The reduction in size of the territories of uncontacted peoples because of the constant expansion of our extractive industries and consequent reduction of the areas where they move around hunting and fishing, jeopardises their sources of food and so reduces their ability to survive."

Mr Corry said there was also a very significant threat of disease. "More than 20% of the Yanamami Indians died in the 1980s and 90s because of contact with gold-miners who brought in illnesses. Following exploration on their land in the 1980s, more than 50% of the previously uncontacted Nahua tribe died in Peru. Ninety percent of Indians in the Javari valley, including six uncontacted tribes, suffered from malaria or hepatitis brought into the area in 2006.

"They remain in isolation because they choose to, and because encounters with the outside world have brought them only violence, disease and murder. They are among the most vulnerable peoples on earth, and could be wiped out within the next 20 years unless their land rights are recognised and upheld. Surely the world is big enough for all of us, including those whose way of life is most different to ours."

Killer diseases

Isolated tribes have little or no immunity to the diseases brought in by outsiders. Colds and flu become killers, and 50-90% of tribe members commonly die from first contact with outsiders. The result is that entire cultures that have taken centuries to evolve can be being wiped out in days as disease invades a population. Epidemics of measles, smallpox, yellow fever, whooping cough, influenza and later malaria have all had a devastating effect on indigenous peoples in the Amazon and elsewhere. Anthropologists now take precautions including wearing masks to avoid accidentally passing on diseases.