Dr. Trumble was trained as an anthropologist, and his field — evolutionary medicine — taught him to see our surroundings as a blip in the timeline of human history. He thinks it’s a problem that medical research focuses almost exclusively on “people who live in cities like New York or L.A.” Scientists often refer to these places as “Weird” — Western, educated, industrialized, rich and democratic — and point out that our bodies are still designed for the not-Weird environment in which our species evolved. Yet we know almost nothing about how dementia affected humans during the 50,000 years before developments like antibiotics and mechanized farming. Studying the Tsimane, Dr. Trumble believes, could shed light on this modern plague.

The Tsimane suffer from high infant-mortality rates, but those who reach adulthood live about as long as most other people, making it possible to measure their health outcomes up to age 90 and beyond. The Tsimane Project researchers have spent more than 15 years following their volunteers and providing medical treatment. They’ve found that Tsimane differ from the rest of us in many ways. For example, they have the cleanest arteries of any population that has ever been studied, meaning that they may be largely immune to heart disease.

Dr. Trumble was not the first member of the Tsimane Project to wonder about dementia in this population. In 2002, one of the group’s founders, Michael Gurven, began testing mental fitness by asking older people to do puzzles. This and other cognitive-performance data piled up until 2015 — the year that Dr. Trumble’s uncle died. That was when Dr. Trumble, Dr. Gurven and other researchers decided to dive into it.

Dr. Trumble was particularly interested in the ApoE4 gene, often called the Alzheimer’s gene. Americans who carry two copies of the gene are more than 10 times as likely to develop the late-onset form of the disease. Dr. Trumble found something startling when he looked into the Tsimane data: Many of those with a copy of the gene seemed to perform better on the cognitive tests.

He mulled this paradox in his sunny lab back at Arizona State University. He had just returned from another trip to the Tsimane settlements, and a bit of Bolivia had come with him: an intestinal infection from the campylobacter bacteria and two nasty species of E. coli. “I got so sick that I almost missed my wedding,” he said. This was not his first encounter with tropical parasites. Years before he had noticed what looked like a zit on his nose. When it kept growing, he realized it was a flesh-eating parasite called leishmania. Chemotherapy saved his nose, and perhaps his life.