In twenty years of campaigning for tribal peoples' rights, nothing prepared me for meeting the Akuntsu.

It was at that first moment, when six solitary figures sitting in a forest clearing grasped our hands, that I fully understood the enormity of this encounter: I was witnessing the extinction of an entire people in my lifetime.

Although nobody has fully mastered the Akuntsu language, it is possible to piece together their story. Konibú, a shaman, listed the names of relatives who had died when they were attacked by ranchers' gunmen, counting them on his fingers and imitating the sound of gunfire. Pupak showed me where he was shot in the back as he fled. Later, when we heard chainsaws echoing through the forest, a collective tremor passed through the group, no doubt reliving harrowing memories of the massacre when their homes were bulldozed and their forest razed to the ground.

Now, with the death of an elderly woman, they have suffered another loss.

I last saw Ururú, who was about 80, a year ago. The Akuntsu were preparing for a ritual, their hair dyed a rich red from urucum (annato paste) and their bodies painted in intricate black patterns from genipapo (black dye from a plant).

There was a quiet, understated dignity in their obvious determination to carry on with life despite the horrors of the past. Before we left, Konibú played his flute – the notes long and tremulous – a haunting and deeply humbling moment. I left wondering if this would be the last time I would meet these remarkable and courageous people.

The future of persecuted tribes like the Akuntsu, who face persecution all over the world, is a question of ethics and justice which goes to the heart of what it means to be human. This persecution is often rooted in greed and racism, but it can be stopped if their right to live their own way on their own land is upheld. It is simply a matter of respect for those who wish to decide themselves how they wish to live. The only real difference between them and us is that we can act to stop genocide, and they can't.

The writer is a Brazil campaigner with Survival International. survivalinternational.org