It was once thought that all Yanomami Indians of the Brazilian Amazon had been contacted during the latter half of the 20th century, after which almost a fifth of their people died from diseases carried by gold-miners. Now, days after aerial photographs and footage of a threatened tribe living in the Brazilian rainforest on the border with Peru were published by Survival International, the organisation has revealed that there is a group of uncontacted Yanomami still living in Roraima state, northern Brazil.

The Moxatetéu people live in an area with the highest concentration of illegal gold miners. If the miners are not expelled urgently, they could come into contact with the Yanomami and pose a threat to their lives. "There are many uncontacted Indians," Davi Kopenawa, spokesperson of the contacted Yanomami people told Survival recently. "I want to help my uncontacted relatives, who have the same blood as us. They have never seen the white man's world."

An international campaign resulted in the creation of the Yanomami Park in 1992. José Carlos Meirelles, who works for Funai, Brazil's Indian Affairs Department, told Survival, "These people, the Moxatetéu really exist. Those empty spaces of the Yanomami Park are not as empty as people think. I'd go as far as saying that there is not just one uncontacted group in this area."

The Moxatetéu are one of more than 100 peoples worldwide who live and thrive in isolation from society. Exact numbers are unknown: there could be up to 70 isolated peoples in the Brazilian rainforest, according to Funai, and an estimated 15 uncontacted tribes in Peru. Gauging how many such tribes there are in West Papua is difficult, because its treacherous terrain, and militarisation of some areas, prevents easy access.

Little is known about uncontacted tribes, but it is easy to imagine what they don't know: of cars or banks or telephones, the concept of government, the internet or the administration of Barack Obama, or why loggers would want to denude their forests. "We didn't know about deforestation," said a man from Brazil's Enawene Nawe tribe, after they were first contacted in 1974. "We didn't know that tractors existed and we didn't know about chain saws that cut down trees." Everything else about their lives – their languages, their names, what they hope for and to whom they pray – is mere speculation. But photographs such as those published last week put an end to speculation about their existence.

"The images show irrefutably that they are alive and healthy," said Fiona Watson, field and research director at Survival International. "And they negate the notion put forward by some that they have been 'invented' by environmentalists opposed to oil exploration in the Amazon." The images will help those campaigning to ensure that the limits of uncontacted territories are defined and protected under international law.

Stephen Corry, director of Survival, believes that a change in attitude will help tribes sustain their lifestyles. "So often, uncontacted tribes are referred to as 'backward' or 'uncivilised' because they choose to live in a different way from industrialised societies," he says. "It's the idea that's backward, not them."

Wade Davis is an American ethno-botanist and explorer-in-residence at National Geographic. "Just to know that such cultures exist is to remember that the human imagination is vast, fluid and infinite in its capacity for social and spiritual invention," he says.

Diversity has never much mattered to those determined to get their hands on lands rich in resources. When Christopher Columbus arrived in 1492, it is thought there were several million tribal people living in Brazil; today, there are approximately 650,000. Most died from diseases to which they had little or no immunity; it is still common for at least half of a tribe to die after first contact.