One spring morning in 1963, a Soviet scientist named Lyudmila Trut was making the rounds at a commercial fox farm, visiting several litters of three-week-old fox pups. As she approached one cage, a fuzzy male pup named Ember began to wag his tail. This simple, back-and-forth movement was a startling sight. Several years earlier, Trut and another scientist had launched an audacious experiment to solve the mysteries surrounding dog domestication by trying to replicate the process in foxes. Ember’s restless tail was the best sign yet that they were succeeding.

BOOK REVIEW — “How to Tame a Fox (and Build a Dog): Visionary Scientists and a Siberian Tale of Jump-Started Evolution,” by Lee Alan Dugatkin and Lyudmila Trut (University of Chicago, 240 pages).

A six-decade project that challenged conventional wisdom about domestication and evolution and is still yielding new scientific insights.

“Wagging their tails in response to humans is one of the signature behaviors of dogs, and until that day, they were the only animals observed to do so,” Trut and the biologist Lee Alan Dugatkin write in their new book, “How to Tame a Fox (and Build a Dog).” And yet, here was Ember, who “appeared to be wagging his tail due to a new emotional response to people, and if other pups also began to do so, that might prove to be a big step in the process of domestication.” This comprehensive book provides an inside look at one of the most remarkable and longest-running experiments in science. It’s a rich and fascinating story of a six-decade project that challenged conventional wisdom about domestication and evolution and is still yielding new scientific insights today.

The fox experiment was the brainchild of Dmitri Belyaev, a geneticist who worked at Moscow’s Central Research Laboratory on Fur Breeding Animals, where he was tasked with helping fox breeders produce animals that would earn more money for the Soviet Union’s lucrative fur industry. But as he worked with the often-ferocious foxes that lived on Soviet fur farms, he began to wonder how humans had managed to tame the wolf — a close relative of the fox — into the docile domestic dog. Fossil evidence provided snapshots of how wild animals had changed over the course of domestication, but a major riddle remained unsolved: How had the process begun in the first place? As Dugatkin and Trut put it, “How had fierce wild animals, intensely averse to human contact, become docile enough for our human ancestors to have started breeding them?”

Belyaev had a theory. In his own work, he had noticed that while most foxes were aggressive or agitated around people, a few seemed to have an innate calmness. Perhaps, he speculated, all our ancestors had done was breed the wild wolves that seemed to be the most naturally docile, exhibiting the least fear of and aggression toward humans. “And over evolutionary time, as our early ancestors had begun raising them and selecting for this innate tameness, the animals became more and more docile,” Dugatkin and Trut write. “He thought that all of the other changes involved in domestication had been triggered by this change in the behavioral selection pressure for tameness.”

Belyaev decided to test his theory by putting it into action. He would start with wild foxes, breeding the tamest ones he could find over the course of many generations. “If he could basically turn a fox into a dog-like animal, he might solve the longstanding riddle of how domestication comes about,” the authors write.

The idea wasn’t just scientifically bold — it was politically risky. Stalin’s government had banned genetics research in 1948, calling it a “bourgeois perversion,” and many leading geneticists had been fired, arrested, imprisoned, and even executed. (Belyaev’s older brother, a prominent geneticist, was among those killed.) So Belyaev would have to be discreet about the real purpose of his experiment, spinning it as physiological, rather than genetic, research.

In 1958, he recruited Trut, a young animal behaviorist, to run the experiment. She almost immediately began to have doubts about the endeavor. “Having had no prior experience with foxes, Lyudmila was taken aback at first by how aggressive they were,” Dugatkin and Trut write. “Becoming acquainted with these ‘fire-breathing dragons,’ as she called them, snarling and lunging at her when she approached their cages, she found it hard to believe that they could ever be tamed.” Still, she would try. Each morning, she donned a pair of thick gloves and began visiting each fox, carefully observing its reaction as she approached, opened its cage, and slid a stick inside. She selected the calmest foxes, bred them together, and then selected the tamest of the pups to parent the next generation.

It didn’t take long for dog-like traits to emerge. By the fourth generation — and just the fourth year of the experiment — Ember was wagging his tail. By the sixth, about 2 percent of the pups would lick Trut’s hand, roll over for belly rubs, and cry when their human caretakers walked away. By the following generation, 10 percent of the pups were displaying these behaviors. “There seemed to be no doubt at all that these pups, from as early as they could walk, eagerly sought contact with humans,” Dugatkin and Trut write. These tame foxes also seemed to have extended puppyhoods, remaining playful and curious well past the age that wild fox pups typically mature. Their bodies changed, too; the tame foxes developed curly tails, floppy ears, and piebald coats.

Maybe it wasn’t the foxes’ underlying genetic code that was changing, but how the genes were regulated or expressed. The idea was wildly ahead of its time.

These new traits had appeared “mind-bogglingly fast,” over far fewer years and generations than evolution was thought to occur. The speed and nature of the changes led Belyaev to propose a radical theory. “Belyaev had realized that most of the changes they’d seen in the foxes involved changes in the timing of when traits turn on and off,” Dugatkin and Trut write. “Many of the changes they were observing in the tamer foxes involved retaining a juvenile trait longer than normal. The whimpering was a youthful behavior that normally stopped as foxes matured. So was calmness; fox pups are serenely calm when they’re first born, but as they age, foxes typically become quite high-strung.” It occurred to Belyaev that maybe it wasn’t the foxes’ underlying genetic code that was changing from one generation to the next, but how the animals’ genes were regulated or expressed; certain genes that were already present in wild foxes might have become more or less active in the tame ones, or have turned on or off at different stages of development.

The idea was wildly ahead of its time, and it would be decades before research would bear it out. In the meantime, Belyaev and Trut kept breeding foxes. They built their own experimental fox farm in Siberia, and Trut moved into a nearby house with some of the tamest foxes, which quickly adopted behaviors common in pet dogs. (A visiting researcher later demonstrated that the tame foxes had the same high level of social intelligence that dogs did — and better social cognition than the wild foxes.) Belyaev died in 1985, but two decades later, researchers finally validated his hypothesis, documenting differences in gene expression between tame and wild or aggressive foxes. (Gene expression isn’t the entire story — researchers have also found changes in gene sequence in the tame foxes — but it’s clearly an important part of it.)

Dugatkin and Trut deftly synthesize scientific findings from fields ranging from genetics to animal cognition and openly grapple with some provocative unanswered questions: How much further can scientists push these foxes? What do the foxes tell us about the domestication of more distant species, such as cows and pigs? And might they teach us something about our own evolution? (Belyaev proposed that as we organized ourselves into ever-larger social groups, there would have been a selective advantage for individuals who were calm and comfortable around others, rather than aggressive and fearful. “Essentially, we are domesticated, but in our case self-domesticated, primates,” Dugatkin and Trut write.) The answers to these questions won’t come easy, but the experiment is still running; considering what scientists have learned so far, there’s no telling what evolutionary insights might emerge if they keep Belyaev’s legacy — and his line of tame foxes — alive for another 60 years.

Emily Anthes, who has written for Undark, The New York Times, The New Yorker, Wired, and Scientific American, among other publications, is the author of “Frankenstein’s Cat: Cuddling Up to Biotech’s Brave New Beasts.”