YAEDA VALLEY IN TANZANIA— As we hike down a rocky slope, through thorny acacias that snag our clothes and past the emaciated carcass of a cow, we hear people singing. We are approaching a small camp of Hadza hunter-gatherers, and our Tanzanian guide thinks they must be celebrating something.

But as we near a few huts made of branches and draped with mosquito netting, a slender woman in a worn T-shirt and sari totters toward us. "She is drunk," says Killerai Munka, our guide.

The woman calls her children, and as she puts their small hands inside ours we get a sour whiff of diarrhea. That's when she tells Munka that her youngest child—a baby boy—died the night before. "He wanted to sleep some more and didn't wake up," Munka translates from Swahili.

A couple of pastoralist men, probably members of the local Datoga tribe, are also visiting. They carry wooden staffs, wear brass hoop earrings, and have brought a bottle of homemade alcohol. They have traded that bottle, and likely others, for honey gathered by the Hadza, who by now have had too much to drink.

Times are hard for the Hadza, who include some of the last people on the planet to live as nomadic hunter-gatherers.

Their way of life has been a magnet for researchers for 60 years, and the subject of hundreds of scholarly papers, because it may offer the closest analog to the way our African ancestors lived. The iconic lifestyle persists: Just that morning in another Hadza camp called Sengele, an hour's walk away, women and children were digging tuberous roots for food. Men were gathering honey by smoking out bees from baobab trees. But that lifestyle is quickly disappearing.

Today, of roughly 1000 Hadza living in the dry hills here between salty Lake Eyasi and the Rift Valley highlands, only about 100 to 300 still hunt and gather most of their food. Most of the others do forage—but they also buy, trade, or are given food, and sometimes alcohol and marijuana. Many live part of the year in larger semipermanent camps in the sprawling settlement of Mangola, where they depend on income from tourism and occasional jobs on farms or as guards.

Most Hadza now go to school for a few years, speak Swahili in addition to their own click language, and wear donated Western clothes. Some carry cellphones. But, "They are not integrating into a normal rural Tanzanian life," says evolutionary anthropologist Colette Berbesque of the University of Roehampton in London, who has studied the Hadza since 2007. Instead, she says, they are "transitioning to a life where they're at the absolute bottom of the barrel."

It is a tragic story that has played out many times before as hunter-gatherers around the world have been displaced by more politically powerful settlers. Although the Hadza have proved resilient in the past, researchers warn that they now face a daunting convergence of threats.

Their Brooklyn-size territory is being encroached on by pastoralists whose cattle drink their water and graze on their grasslands, farmers clearing woodlands to grow crops, and climate change that dries up rivers and stunts grass. All those pressures drive away the antelope, buffalo, and other wildlife the Hadza hunt. "If there are no animals, how are we going to feed our people?" asks Shani Msafir Sigwazi, a Hadza who is a law student at Tumaini University Makumira in Arusha, Tanzania. "How are we going to protect our life in the bush?"

"The last 5 years have drastically altered the landscape politically, socially, and ecologically," says human behavioral ecologist Alyssa Crittenden of the University of Nevada in Las Vegas, who has studied the Hadza since 2004. "It's clear to anyone who goes out to see the Hadza that we're dealing with small populations being pinched on all sides."

Worried about the Hadza's plight, researchers wonder about their responsibilities to the people they have studied intensively for decades. Many researchers are seeking ways to help, even as they vie to study the few Hadza who still hunt and gather full time. But some researchers have stopped fieldwork altogether, saying the Hadza lifestyle has changed too much. "The narrative that they are perfect hunter-gatherers has been eroding since the first researchers have worked with them," says paleobiologist Amanda Henry of Leiden University in the Netherlands, who has studied Hadza gut bacteria and diet; her team is not returning.

From the very first, researchers who studied the Hadza realized they were walking a tightrope—studying a traditional way of life that their very presence risked altering. James Woodburn was a 23-year-old graduate student in 1957, when he became the first anthropologist to study the Hadza. He quickly realized that the tire tracks of his Land Rover created new paths for the Hadza, so he sold it and walked everywhere with them instead. "I was most anxious not to affect their nomadic movements," says Woodburn, now retired from the London School of Economics.

All the Hadza he saw then were nomadic hunter-gatherers who ranged across 1000 square kilometers of bush, an area 20% larger than New York City. Yet even then, they were losing their traditional lands at a great rate, Woodburn says, and had less than half the 2500 square kilometers they inhabited when German geographer Erich Obst met them in 1911.

Still, Woodburn recalls an "exceptional abundance of game" in the 1960s, including "a herd of 400 elephants, also lots of rhino, hyenas, lions, and many, many other animals." At the time, he found, the Hadza were healthier than farmers and herders, as he reported at the famous "Man the Hunter" symposium in Chicago, Illinois, in 1966. And although the Hadza traded with their agricultural neighbors, exchanging meat and skins for beads, pots, and iron knives, few people from other tribes had settled on their land. They did not intermarry much and kept to themselves.

The Hadza also resisted many attempts by governments and missionaries to move them into settlements to become farmers. So many Hadza died of infectious diseases in camps in the 1960s that Woodburn worried they would be wiped out. But survivors always left the camps to return to the bush.

Woodburn realized that farming was antithetical to the Hadza's egalitarian values, as he described in a landmark paper in 1982 in the journal Man . He noted that they were vigilant in preventing any single person from acquiring assets or wealth, or asserting power or status over others. They shared the food they hunted and gathered the same day or soon after in an "immediate return" system. Woodburn contrasted that approach with "delayed return" societies, in which individuals invest in building personal assets that pay off later—for example, spending perhaps weeks crafting a boat and then storing caught fish for many months. Such societies, he argued, more readily adopt farming or herding, which allow individuals to acquire power, rank, and wealth.

The Hadza are not living fossils "lost at the bottom of the Rift Valley for thousands of years," says Nicholas Blurton-Jones, professor emeritus at the University of California, Los Angeles (UCLA), who did fieldwork with the Hadza from 1982 to 2000. They also have evolved over the millennia and long ago adopted new tools, such as metal arrowheads and cooking pots. But in their rich and relatively undisturbed savanna home, the Hadza have offered a steady stream of researchers a unique view of the way of life and selection pressures that "many have suggested brought our species into being," he says.

The Hadza are just a touchstone for so much.

Over the years, studies of the Hadza have revealed that grandmothers' food production boosts child survival so mothers can bear more children; that men prefer to hunt large game because having reputations as good meat providers makes them desirable mates and allies; and that hunter-gatherer children forage for enough food that they are "cheap" to raise, boosting fertility and population. "The Hadza are just a touchstone for so much," says anthropologist Kristen Hawkes of The University of Utah in Salt Lake City, who did fieldwork with the Hadza from 1984 to the early 1990s.

Today, at least a dozen research groups from around the world have permits to study the Hadza. One is led by Jeff Leach, a visiting research fellow at King's College London, who helped show that the Hadza have more diverse gut bacteria than people on a Western diet do. "East Africa is ground zero for the human microbiome," he says. "With the Hadza, who are exposed to the urine, blood, and feces of every animal they hunt, you can get a picture of all the microbes on that landscape."

Other studies focus on their lifestyle. Crittenden recently found that Hadza men who switched to an agricultural diet suffered less dental decay (probably because they ate less honey), but that women and children ended up with more cavities . A team led by UCLA biological anthropologist Brian Wood, who has studied the Hadza since 2004, learned that they use only as much energy every day as sedentary Westerners, suggesting that hunting and gathering can be remarkably efficient; and that the Hadza sleep less than recommended in Western guidelines.

Even as studies proceed, the Hadza's future is darkening. The biggest threat comes from farmers and pastoralists and their cattle encroaching on Hadza land. In 2011, after years of negotiation between a local nongovernmental organization (NGO) and government officials, the Tanzanian commissioner for lands gave the Hadza rights to a 230-square-kilometer area. That was a major victory, but the egalitarian Hadza have lacked the leadership or organization to protect their land.

"When you look at the Hadza, we have no leaders to represent us in government," Sigwazi says. Local governments enforce land and grazing rights, and the Hadza have far fewer representatives on village councils than the Datoga or Iraqw farmers who live nearby. As a result, the Hadza have had to agree to give away grazing rights on their land in the dry season. The laws do prevent the free-for-all hunting on Hadza land that happened in the mid-1980s when many elephants were poached, says Daudi Peterson, co-founder of Dorobo Safaris and the Dorobo Fund, which uses fees from research and sustainable tourism to protect wildlife and fund health care and education for the Hadza and other groups. ( Science paid fees to the fund to visit Hadza land.) However, he adds, "Flagrant abuse of the laws" by herders has taken place.

The Hadza are particularly concerned about Datoga pastoralists who let their cattle graze on grass and drink from water holes on Hadza land year-round. In one Hadza camp, a woman named Tutu pointed to her people's huts. Their tree-branch frames were draped with clothes and bark instead of the traditional grass thatch. "The cows eat all the grass," she explained.

The Datoga are also moving in, building bomas —mud-walled huts encircled by acacia-thorn fences that contain livestock at night—near water sources. The settlements keep the nonconfrontational Hadza and their prey away from the water. "You can see from Google Earth where Datoga bomas are and how the Hadza—especially the women—adjust their spatial behavior to avoid them," Wood says.

"The Datoga come here and take over the area—they put in their permanent houses," said a Hadza man named Shakwa. "Our land is getting smaller and smaller. It is not like a human being that gets pregnant and can give us more and more land."

The incursions, with cattle grazing deep in the bush, have worsened in the past 3 years because of climate change, which has displaced the Datoga and other herders from lands outside the district, says Partala Dismas Meitaya, who works for the Ujamaa Community Resource Team in Arusha, the local NGO that negotiated the land rights. Half the Datoga's cattle died on their own grazing lands during the last rainy season from November 2017 to mid-January, which was unseasonably hot and dry. Their hardship makes them resent the rights deeded to the Hadza. "People ask, ‘Why are the Hadza—a small number of people—taking a big part of the land?’" Meitaya says. "‘Why don't they share the land?’"

A few signs of cooperation have emerged. Three Datoga are working with seven Hadza youth to patrol grazing on Hadza land. "They are cooperating in a peaceful way to make sure there isn't another fight between the Hadza and Datoga," Meitaya says.

But the threat from cattle is not the only force driving the Hadza from their ancestral land. Marina Butovskaya, a physical anthropologist from the Russian Academy of Sciences in Moscow, is stunned at how quickly woodland is being cleared for farming at the edges of Hadza land. "When we arrived there, in 2003, there was only bush, and there were plenty of wild animals," she recalls. "Now, along the road to Mangola, it's fields, fields, fields."

In her 5 months in the Mangola area, between September 2017 and February 2018, new power lines (which allow irrigation equipment) attracted an influx of farmers. They used tractors to clear a swath of land 10 kilometers closer to Hadza land. "You cannot imagine how fast it's going," Butovskaya says.

When the land is cleared, wild animals lose habitat, leaving fewer to hunt. The farmers also cut down wild fruit trees on which the Hadza depend, they told Wood recently. To survive, some Hadza take handouts of maize flour from missionaries or trade meat and honey for flour to make porridge. Or they head to one of a dozen "tourist camps" in the Mangola region, where they earn money by re-enacting their traditional ways. Thanks to a newly improved road, tourists from Ngorongoro Conservation Area, which draws 400,000 people a year, can "bomb down" to see the Hadza in Mangola in 1.5 hours, Peterson says.

Researchers are well aware of the irony that their research, which made the Hadza famous, also draws tourists, which in turn encourages the Tanzanian government to build roads. "If we never studied the Hadza, would they have been better off?" Hawkes wonders.

The tourism has a toxic impact. In the roughly 3 weeks that ecological anthropologist Haruna Yatsuka of Nihon University in Mishima, Japan, was in a tourist camp in Mangola in 2013, 40 tourist parties came from 19 nations. The tourists began arriving at 6 a.m. and watched the Hadza hunt (for show—they seldom got meat when with tourists), dig up tubers, or perform dances. In one camp, Hadza wore baboon skins, which is not their traditional dress but fits tourist expectations, Leach says. The Hadza also got money by selling souvenirs such as bead bracelets, or from tips. "Tourism now brings income to the Hadza and has had a tremendous effect on their livelihood, diet, residence, and nomadic patterns," Yatsuka says.

She observed the most destructive impact as soon as the tourists left in midafternoon, when the Hadza used their earnings to buy alcohol. "Everybody drinks: pregnant women, breastfeeding women, the men," says Monika Abels, a developmental psychologist at Tilburg University in the Netherlands, who compared child development between a tourist camp and Hadza bush camps. Sometimes the drinking starts early in the day, the children don't get fed, and drunk men beat women, Abels says.

Blurton-Jones has noted higher rates of alcoholism, disease, and early death for Hadza living in Mangola than in the bush. The Hadza themselves recognize that trend, and complain about being "tired" in camp, Yatsuka says. Turnover is high, as Hadza go into the bush to recover. Yatsuka is now studying how competition to sell souvenirs affects the Hadza's egalitarian culture. What happens when one Hadza woman makes money but another doesn't?

All those changes also affect research. Leach and others must stop data collection when missionaries give Hadza grain or antibiotics. "I think the way some of the recent papers report the situation they're studying is bordering on not very honest," Blurton-Jones says. "They need to tell us how much maize they get, how often do they get alcohol, how often do tourists come."

Others agree: "In my tenure, I've seen dramatic, dramatic change," Berbesque says. "There are Hadza keeping chickens; they have cellphones. It's not necessarily bad … but they are not pristine hunter-gatherers anymore." She has cut back her study of dietary preferences and will not take new students to study the Hadza until more protections are in place. Abels, too, probably will not return.

Some researchers think scientists have asked too much of the Hadza. "A woman said to me, ‘My body is tired,’" Crittenden says. "‘I'm tired giving my hair, my poop, my spit, my urine.’" Crittenden believes researchers now have a duty to their longtime subjects. "The Hadza have been desperately asking researchers to help them," she says, noting that Hadza have approached her at least a dozen times in the past few years for help with political advocacy, land rights, health care, and education.

Most researchers do step up. "You end up doing humanitarian work," Leach says. "I'm buying school clothes for 100 kids."

The top priority is to stop incursions on Hadza land so people who want to hunt and gather can continue to do so. One approach is to engage with local government and others on the Hadza's behalf. For example, Wood spoke with missionaries in 2014 who wanted to drill a well in an area that was "basically the last stand for the Hadza" who live in the bush. He told them a well would draw Datoga to water their cattle and thus harm the Hadza. But intervening carries risks, Wood warns: Evicting the Datoga and others from Hadza land could trigger a backlash.

Wood and other researchers are taking steps to respond to Hadza who increasingly want more say in who studies them and what kinds of studies are done. "What advantage do we get from your study?" Sigwazi asks. "I want to know the results of my poo. Tell us your important results."

Crittenden and Berbesque hope to help the Hadza develop a code of ethics like one unveiled last year by the San people of southern Africa, another intensively studied group. That code requires that the San Council approve and manage research protocols, says Bob Hitchcock, an anthropologist at The University of New Mexico in Albuquerque, who helped the San draft it. But Hitchcock foresees a challenge with the Hadza, who "don't have the same level of representation, the coordinated body" to do this, he says.

Researchers are sharply divided over a code, in part because many think scientists do more good than harm. They note that in 2007, scientists helped organize protests when the Tanzanian government evicted the Hadza from some of their land, proposing to turn it into a private hunting park for the United Arab Emirates's royal family. They also disagree that the Hadza are overstudied, arguing that many teams are there only for a month or so and don't overlap much. "I'm the only researcher in the field right now," Wood says.

As researchers, Hadza, and others consider how best to move forward, they agree on one thing: "It is important that every Hadza individual has the opportunity to choose a lifestyle for themselves," says Woodburn, who at age 84 still returns to camp with Hadza friends every few years. Sigwazi says: "I want to protect the culture of my people so the Hadza can enjoy their life—so they can wake up in the morning and hunt in the bush. It's a simple life, but a kind of wonderful life."