Like many breakthroughs in science, Dmitri Belyaev’s silver fox domestication experiment began with a thunderbolt: one simple, powerful, new idea. Born of a parish priest in early 20th century Russia, the geneticist proposed that all domestic animals were tamed through a generations-long process in which our distant ancestors repeatedly chose the calmest animals — those that were friendliest to people — for breeding. Whether horses for transport, dogs for protection, pigs for food, or oxen for labor, the essential trait was that the animals not try to bite the hand that fed them. Belyaev went on to speculate that all of the other characteristics we tend to see in domesticated species — their curly tails, floppy ears, juvenile facial, and body features — were somehow byproducts of this selection for the friendliest of the friendly.

As a test, Belyaev decided that he would build a dog out of a fox, in real time, to understand how man’s best friend came to be. No one had ever attempted anything like it. No matter, he would try. At the time, in Stalinist Russia, the idea was considered radical and out of line with State orthodoxy. There were men who might very well have thrown the scientist in prison for what he was dreaming. But he would perform his magic in a far off, frozen land: The Siberian town of Novosibirsk, where winter temperatures can plummet to a bone-chilling -50 degrees Fahrenheit.

Some 60 years later, his experiment is still going. It is one of the longest running science experiments ever, having outlived even its creator. And after all this time, it is still shaping the way we think about fundamental questions in biology — and even influencing the way we understand our own evolutionary trajectory.

At the helm with Belyaev since day one of the experiment has been Lyudmila Trut, the Jane Goodall of the fox world. The two researchers began the experiment at a perilous time in Soviet science. A malevolent charlatan named Trofim Lysenko had conned Stalin into believing that the sort of genetics Belyaev and Trut were doing was bourgeoise science promulgated by wreckers and saboteurs. Thousands of geneticists in the Soviet Union had already been purged from their labs. Their crime? Studying genetics. Many ended up in prison; a few were murdered.

But for Belyaev and Trut, the science was too important to leave undone. Although working in Siberia helped them fly under Lysenko’s radar, it was still a dangerous business. The very year they began, Nikita Khrushchev, under Lysenko’s spell, visited the institute where the experiment was housed and came within a hair’s breadth of shutting it down.

But on the experiment went. Since its inception in 1959, Trut, Belyaev, and their team of Russian biologists have raised tens of thousands of foxes, breeding approximately the sweetest and calmest 10 percent of each generation. (Belyaev died in 1985, but Trut, who recently turned 85 years old, continues to lead the experiment to this day.) Within six years — six fox generations — they had gone from wild animals that fled from humans, attacked when cornered, or both to foxes that begged for belly rubs, wagged their tails when Trut approached, and whined when she left. A few years later, there were foxes with floppy ears, curly tails, and mutt-like fur. Next came rounded, short, dog-like snouts. The foxes became more carefree with each generation, the result of more serotonin, a “happiness chemical,” coursing through their systems. They also had markedly reduced stress hormone levels than their cousins in the wild. They even followed the gaze of people — which is almost unheard for animals, except dogs.

As Belyaev predicted, all these changes are the result of selection for friendliness and friendliness alone. Today, molecular and developmental biologists, working together with evolutionary biologists, are piecing together how this hodgepodge of changes to behavior and appearance is connected to tameness per se. In a recent paper in Nature: Ecology and Evolution, Anna Kukekova, Trut, and their team dove deep into the genome of the silver fox, hunting for clues about the process of domestication. On fox chromosome number 15, they found a hotspot for genetic changes associated with domestication. One of the genes in this hotspot, SorCS, is thought to be linked to memory and learning. Indeed, the domesticated foxes are especially smart when it comes to solving problems that rely on social cognition.

There has also been a spate of technical papers on the role that gene expression — the switching of genes between “on” and “off” states — plays in the domestication of foxes and, in all likelihood, other animals. It is all too easy to forget that Belyaev predicted the link between domestication and what we now call gene expression patterns before the field of molecular genetics ever existed.

To this day, no study has taught us more about domestication, the very process responsible for our pets and farm animals (and crop plants), than Belyaev’s fox experiment. The research, in conjunction with the human fossil record, even led anthropologist Richard Wrangham and others to argue that humans self-domesticated themselves by choosing the behaviorally tamest mates and groupmates, and that over the last 30,000 years — or perhaps longer, according to Wrangham — this has led to changes in our body and facial characteristics, stress hormones levels, and other traits.

The fox experiment has shed light not only on domestication but on the entire process of evolution itself. Before this experiment, the notion that natural selection based on behavior, and only behavior, could influence what an organism looks like, how often it reproduces, which hormones it produces, and how smart it is was the stuff of stories.

Belyaev would have been proud of how his experiment stood the test of time. And Trut, in her own understated way, is proud as well. When, as a newly minted graduate from Moscow State University in 1959, she paired up with Belyaev to test his audacious ideas, she took to heart what the fox told the Little Prince in Antoine de Saint-Exupéry’s classic children’s tale: “You become responsible forever for what you have tamed.” For Trut, this meant that if the experiment worked — if they actually tamed the foxes — she could never leave them.

It did work, and she never has.

Lee Alan Dugatkin, Ph.D. is an evolutionary biologist and historian of science in the Department of Biology at the University of Louisville. He and Lyudmila Trut are the authors of “How to Tame a Fox and Build a Dog” (University of Chicago Press, 2017).

UPDATE: An earlier version of this article incorrectly implied that anthropologist Richard Wrangham believes human domestication happened over a period of 30,000 years. Wrangham believes the process took longer.