They emerge from the jungle half-naked, with pet monkeys wrapped about their heads, then fall in love with cellular phones and TV. They swap their loincloths for pants, baseball caps and sunglasses, settle down in frontier towns and get handouts from the government. They are the Nukak-Makú, an ancient, nomadic group of hunter-gatherers. Now, after thousands of years wandering in the rainforests, they are in deep trouble.

In recent years, the exodus of hundreds of nomadic Nukak-Makú from rainforests here has been viewed by journalists and townspeople in Colombia’s departments of Meta and Guaviare like the landing of extraterrestrials. The Indians’ surreal encounters with modernity—with such wonders as talking boxes and large metal birds that make trails through the sky—have fascinated Colombians.

But for scientists, the Nukak-Makú’s transition signals destruction of the rainforests lying between the Orinoco and the Amazon, and of a people that since time immemorial has lived in harmony with them.