In a small region of South America there are 125 villages of the Yanomamo Indians. They are all at war with each other. Yet although a third of all adult males die in these wars and many of the children are killed by their parents at birth, this culture has survived for a thousand years.
Violence is a way of life yet they have means of containing it. Their technology is Stone Age yet their political abilities would do credit to a modern statesman.
For the last 10 years a team has been studying the tribe. They have looked at the way in which warfare has affected its evolution, and in doing so have gained some insight into our development.
Tonight's Horizon traces the events as one village tries to form an alliance with an old enemy. Feelings ran high and the feast erupted into violence - but then the Yanomamo have a way of dealing with it.