The Tsimane get most of their calories from complex carbohydrates high in fiber like plantain, corn, cassava, rice and bananas, supplemented with wild game and fish. Dr. Gurven has published detailed studies showing that they have exceptional cardiovascular health and almost no diabetes. Yet Dr. Gurven has seen several cases of Tsimane people developing and dying from Type 2 diabetes after leaving their villages and moving to the nearby town of San Borja, where they took sedentary office jobs and gave up their traditional diet.

“They changed from their traditional diet to eating in town where everything is fried,” he said. “They started eating fried chicken and rice and drinking Coca-Cola. Some of these folks can see a pretty rapid change in health.”

For the new study, Dr. Pontzer and his colleagues analyzed data on hunter-gatherers and other small-scale societies across the globe, from South America to Africa and Australia. They looked at detailed dietary assessments of fossil and archaeological records to get a sense of what early humans ate. And they included new data collected from the Hadza, a community of people who spend their days hunting and foraging in northern Tanzania, much as their ancestors have for tens of thousands of years. The Hadza consume what some call “the oldest diet.” Dr. Pontzer has spent time with them and long studied their health.

In a typical day, the Hadza set out in groups early in the morning to hunt and forage in the savanna. The women traverse hilly terrain to collect wild berries and dig up tubers resembling fibrous sweet potatoes. Getting them is not easy, Dr. Pontzer said: The women use sticks to dig up the tubers, in some cases while carrying infants on their backs. The men head out to hunt animals, often killing small ones but, about once a month, something big like a zebra, warthog or gazelle. On days when their hunts come up short, they head over to beehives and collect honey, which is one of their favorite foods, accounting for at least 15 percent of the calories in their diet.

“On any given day in a Hadza camp, there is almost always honey, a little meat and tubers,” Dr. Pontzer said.

The amount of daily calories the Hadza consume is similar to that of the average American. But they rely on a fairly small number of foods. And notably they do not have potato chips, candy bars, ice cream and other ultra-processed foods that combine large amounts of fat and simple carbs — foods that are engineered to be irresistible even when we are not hungry.

The lack of novelty and variety in hunter-gatherer diets may be part of the reason they do not overeat and become obese. Studies show, for example, that the greater the variety of food choices in front of us, the longer it takes to feel full, a phenomenon known as sensory specific satiety.

“It’s the reason you always have room for dessert at a restaurant even when you’re full,” Dr. Pontzer said. “Even though you’ve had a savory meal and you can’t eat one more bite of steak, you’re still interested in the cheesecake because it’s sweet and that button hasn’t been worn out in your brain yet.”