Twenty years ago I participated in the Earth Summit in Rio. I remember the hope and elation among hundreds of indigenous peoples from all four corners of the earth who had come together to push for their rights on an international stage. Far from the official summit from which they had been barred, one of the defining moments was the signing of the Kari Oca declaration in which they reaffirmed their 'inherent rights to self-determination' while walking 'to the future in the footprints of our ancestors'.

Since then there have been advances, for example the approval of the UN Declaration on Indigenous Peoples. There have also been set backs not least in Brazil, host for Rio+20, and where there are powerful moves in Congress to undermine hard won indigenous rights.

20 years on tribal peoples are facing unprecedented pressures on their territories and natural resources. None more so than the Awá, one of South America's last remaining nomadic hunter-gatherer tribes whose forests are engulfed by loggers and cattle ranchers and disappearing faster than in any other indigenous territory in the Amazon. The uncontacted Awá are on the run, surviving in tiny fragments of forest, but for how much longer?

I first met the Awá just after the Earth Summit in 1992. They recounted in graphic detail how their relatives died in brutal massacres and from epidemics of the 'whiteman's diseases'. The few hundred survivors were hanging on, but only just.

Ten years later with the BBC's Mike Donkin, I returned. The situation was even worse - more forest had gone and the Awá were angry but defiant. To'o Awá told us "We're going to fight for our land. We're not going to let the whites in. We're not going to let them finish our land. If we let them in we'll soon be worse off than them. We want to raise our children here."

The forest means everything to the Awá as To'o explained, "We love the forest because we were born here and we know how to live off the forest. We depend on the forest. Without the forest we'll be gone, we'll be extinct."

The Awá will not be at Rio but their shadow will be there. On Tuesday Amiri Awá will launch a video appeal to the Brazilian government with this message: "We are people too, you cannot abandon us. We are people, we're not dogs who can be left on the side. We are people, just like you, and you can help us. You can help us to evict those people, the loggers. You can remove them and make them leave our land."

If we cannot ensure the survival of the Amazon's last nomads who probably more than any people, define what it is to live sustainably, and who ask for nothing other than respect for their land and way of life, what will that say about the path humanity is heading down?

Please join Survival's campaign for 'Earth's Most Threatened Tribe'