In recent years Nukak clans in the jungle have also had some contact with missionaries and with farmers and sedentary indigenous groups, who trade their crops for meat hunted by the Nukak, who picked up at least the notion of agriculture.

Though it is unclear how big the Nukak population once was, anthropologists believe that what little contact the Nukak have had with outsiders has most likely left them reduced by Western diseases, including influenza and the common cold, to which they have no natural defenses.

Janet Chernela, an anthropologist who has worked with the Nukak, said a study she had conducted showed that Nukak who abandon their nomadic lives and settle down, even temporarily, become susceptible to illnesses, including soil-transmitted diseases.

What little is known about this latest group is that it abandoned the Nukak National Park, which is nearly half the size of New Jersey, in the state of Guaviare. Belisario — who knows several of the towns outside the reserve, having been reared for part of his childhood by settlers who encroached on the jungle — led the way.

It was no easy journey, the Nukak told Dr. Maldonado, spanning nearly 200 miles from the eastern end of their reserve to this town, known locally as San José. They arrived in the central plaza malnourished and exhausted, as astonished by this world of low-slung jungle buildings, jeeps and paved roads as the townspeople here were astonished by them.

Image A Nukak man hunting for monkeys outside San José, using a blowgun and darts tipped with curare. Credit... Luca Zanetti for The New York Times

"When I've had some time to talk to them and asked where they came from, they just say 'the bush,' " said Xismena Martínez, who oversees aid to the Nukak for San José. "But that could mean anywhere."