Margarita Mbywangy has spent her life fighting for the right to exist. At the age of five, she was kidnapped and sold into domestic slavery, removed from her family and the hunter-gather way of life that her Ache tribe had practiced in eastern Paraguay for millennia.

Ms Mbywangy spent the next 13 years known only as Margarita – the name chosen by her new "mother" who insisted she was her daughter, but never hugged her, didn't send her to school and made her cook and clean for the family. She looked and felt different; people in the street called her "Indo" – a derogatory term used to insult Paraguay's indigenous people – but she had no identity papers, just a name.

This part of her story is by no means extraordinary. In the 1960s and 1970s many indigenous children in Paraguay were kidnapped and their parents killed by government forces and farmers who wanted to develop the acres of forest, their ancestral land, where they lived a nomadic life, trying to avoid the threats of the "civilised" world.

By 1976, all the Ache had been forcibly resettled on small areas of designated land where they had to swap hunter-gathering for agriculture in order to survive. Many died trying to defend themselves and the forest; many more died from new diseases such as flu because they had no immunity to these common conditions. The land was sold to farmers, roads were built and the valuable timber harvested. Only 36 families survived the slaughter. The government was accused of genocide.

"When we were taken out of the forest and forced to live in communities, we were left without medicine or doctors, and many, many more people died than even in the fighting. That was really the end of our way of life," she says. Ms Mbywangy, 49, cannot remember those early years, and perhaps would never have known her story had she not found her family at the age of 18. For two years she tried to find out who she really was with the help of a priest and missionaries – whose predecessors had been responsible for brutal civilisation programmes centuries before.

"My people cry when they are sad and when they are happy, so when they saw me after so many years they started crying. But it was difficult, I was so desperate to know my mother and father but they were already dead, and I couldn't speak Ache, there were many mixed emotions."

From her siblings she learnt that her father had died from a snake bite; her mother from flu. She had been captured by farmers on horseback along with two other children when trying to escape with her clan. Ms Mbywangy learnt her forgotten language, and reassimilated with every tradition that her people still practised as best they could. They have been "given" a small forest where they can hunt monkeys and rodents and collect wild fruits, but they also cultivate maize, sweet potatoes, peanuts and rice.

The Ache tribe is now the second smallest, but fastest growing, indigenous group in Paraguay, with about 1,200 people in six communities, each with different customs. Their ancestral forest, and with it their old way of life, has been largely destroyed. Across the world there are more than 150 million tribal people in 60 countries, but only 100 truly uncontacted tribes are known to still exist.

More than half these tribes are in the Brazilian Amazon basin, 15 in Peru and one in Bolivia. Outside Latin America there are uncontacted groups in India, Bangladesh, Sri Lanka, Australia and Russia. Only one, part of the Ayoreo nomadic tribe, is known still to exist in Paraguay. The threats they all face are simple: diseases introduced by outsiders and deforestation, for logging, farming, mining and oil and gas exploration.

The rights organisation Survival International last week released the first- ever images of the nomadic Mashco-Piro Indians in Peru to pressure the Peruvian government into protecting the tribe's land from loggers and other outsiders. About 70 per cent of land in the Peruvian Amazon has already been sold off to oil and gas companies.

Paraguay, like many of its neighbours, has signed International Labour Organisation 169, a law which protects the land rights of indigenous people. It also has strong national laws which guarantee lands to tribal groups. But an unwelcome throwback to the country's violent past means there is no public land in Paraguay, as it was all sold to raise money for the government in the early 20th century. Since then there has been no land reform in Paraguay and the government cannot afford to buy back the land.

Apart from a few hard-earned victories, the vast majority of indigenous groups are still struggling to retrieve even small parts of their ancestral land. Many work in slave labour conditions or rely on food handouts.

Ms Mbywangy eventually became a tribal chief and, in an extraordinary twist, she was approached in 2008 by the newly elected leftist president, Fernando Lugo, after she became a tribal activist. Despite her lack of political experience she was appointed Minister for Indigenous Affairs – the first woman and first indigenous person to hold the job. "I accepted because I thought it would be a great opportunity for not just my people, but for all the indigenous people, to fight for what we need, for the forest," she says.

The tenure was bittersweet, and she stood down at the end of last year amid internal and external opposition. "I would never disappoint my people, never let them down. So I left." She is clearly angry and disillusioned.

Ms Mbywangy was in the UK speaking at a conference held by the World Land Trust (WLT), which helps NGOs in 20 countries buy land to protect the rapidly disappearing flora and fauna.

John Burton, chief executive of WLT, said: "Conservationists like us need to save big areas of land to protect the wildlife. Groups like the Ache also need big areas of land, but in order to live their lives as hunter-gatherers. These are not necessarily incompatible, but there is potential for conflict so we have to learn to work together. Establishing trust with indigenous people, who have suffered such terrible abuses from outsiders, is the most difficult thing."

Ms Mbywangy says: "The small areas of forest we have left are crying out asking to be saved. It is very important to protect the birds and animals, but also, organisations like the Wildlife Land Trust must realise that indigenous people are part of the forest too... so we will work with those NGOs trying to preserve the forests, because we are the forest. We cannot survive without it."

First contact: The hazards

Most invasions of areas which are home to uncontacted tribes are prompted by the desire of loggers, miners, oil companies and cattle ranchers to seize lands and resources. But well-intentioned non-governmental organisations, missionaries, tourists and even locals who try to make contact can prove dangerous.

One of the reasons for trying to avoid contact, such as in the case of the people of the Mashco-Piro tribe who were chanced upon by Spanish archeaologist Diego Cortijo recently in a remote part of Peru, is the risk of passing on diseases to which they have no immunity. "First contact" usually results in 50-80 per cent of the tribe dying of imported sicknesses.

The danger of forcing contact on isolated nomadic tribes was reaffirmed by the recent death of Nicolas "Shaco" Flores (right), who was shot by an uncontacted tribe's arrow near Manu National Park in Peru. He had been leaving food and gifts for Mashco-Piro Indians for 20 years and thought he was helping, but even he became viewed as a threat by some in the group.

In July 2011, a Brazilian tribe who lived near the Peruvian border is believed to have been massacred by drug traffickers. A few months later an eight-year-old girl from the Awa tribe was burnt to death by loggers in the north-east.

About 450 tribespeople were murdered in Brazil between 2003 and 2010, according to the Catholic Indigenous Missionary Council.