Face to face with the Stone Age: children from the Karo tribe at the Omo River.

The Omo River empties into Lake Turkana, not far from where Ethiopia, South Sudan and Kenya come together. The river valley is difficult to reach; from Addis Ababa the road trip takes the better part of three days, much of it a jolting endurance trial.

But tourists began to trickle in 20 years ago, and now an estimated 20,000 stream through annually.

In this valley, a UNESCO World Heritage Site, the oldest remains of Homo sapiens have been found, dating back some 195,000 years. DNA evidence suggests that every person on earth today is related to a nameless woman who once lived here. The valley is arguably the true Garden of Eden. Though rugged and wild, it has a green beauty that by itself could draw adventurers.

The most powerful lure, however, is the 16 ethnic groups who make the southern Omo Valley the most genetically and linguistically diverse area on earth. To visit them in some ways feels more like time travel than tourism.

Most of the valley’s inhabitants live in huts built from sticks and mud, a pre-industrial world where electricity and running water seem almost unimaginable. Their scraps of clothing are made from animal skins; children run naked. Some tribes subsist on a handful of crops such as maize and sorghum, supplementing their diets with occasional meat from scrawny goats or cows.

To display their tribal identities and personal status, tribe members engage in ritual scarification, creating elaborate patterns of raised bumps on their flesh, or they use their bodies as canvases for paints that can serve as both creative media and insecticides. Women of the Hamar tribe slather their hair in a mixture of animal fat and red pigment and then shape it into long distinctive tresses.

Perhaps the most famous of these body decorations are the painted clay plates that Mursi women begin wearing when they’re 15 or 16. Into holes cut in their lower lips, they insert a series of larger and larger disks. Somehow the Mursi have come to see this practice as intrinsic to their tribal identity – part of what makes them Mursi and Mursi women beautiful. The plates are heavy, so the women mostly wear them on ceremonial occasions such as weddings and tribal festivals. They also pop them in when the tourists arrive. (The dangling undisked lower lips, if anything, look stranger than the distended ones.)

Interactions between tourists and the Omo Valley villagers add yet another layer of weirdness to the experience. We saw it first in the open country, where prepubescent shepherds turn their backs on their animals and race to the roadside at the first distant cloud of dust suggesting the approach of a Land Cruiser. They perform hip-waggling welcome dances and hope that someone will toss out a used plastic water bottle or stop to pay for a photo.

Most vehicles speed by. But in the villages, demands for payment cannot be ignored. Elders collect admission fees, and every click of the shutter has a price, typically 2-3 birr (11-17 cents). During our recent travels, our guide had advised my husband and me to stock up on candies, pens and razor blades. We traded these too for the tribe members’ images.

Such transactions are one sign of the modern world’s intrusion into this Stone Age society. Others abound. The second-largest hydroelectric project in Africa, located 300 miles up the river, is scheduled for completion in 2013; its impact on the tribes that depend on annual flooding is yet unclear. People talk about a paved road being built by the Chinese to Lake Turkana; it reportedly will run through Turumi, the heart of Hamer territory.

Within the last year or two, cell phone towers have begun sprouting along the dirt roads. At least some of the villagers are acquiring phones too.

We learned this in a tiny encampment of Bodi people. The local chief stretched out in the sun, naked except for a strategically positioned shawl. Then an otherworldly sound intruded. The chief fished a cell phone out of his wrap and stood to take the call.

We couldn’t understand a word of the conversation, but we recognized the sound of imminent change.