When we domesticated animals, we changed their behavior and their appearance. Why do so many domesticated animals share physical characteristics? During evolution, did we also domesticate ourselves?
In 1959, researcher Dimitry Belyayev (Беля́ев) travelled to Novosibirsk, the USSR, to begin an experiment that continues to this day. The question he sought to answer was, what happens to animals when we domesticate them?
Unlike taming, in which a particular animal is taught to obey commands or to behave in a way that is desirable to humans, domestication is a process that applies to an entire species, rather than to an individual animal. A domesticated species is genetically altered, and consequently differs externally and usually also behaviorally from the wild species from which it evolved. Hundreds and thousands of years of breeding have changed domesticated species and adapted them to human needs. The modern cow produces more milk, the chickens lay more eggs, and the horses today are larger and run faster than their ancestors, the wild horses. Dogs are more obedient, with an innate tendency toward attraction to humans, and various breeds were cultivated for designated tasks: Shepherd dogs, hunting dogs, sled dogs – each breed with the physical and character traits that assist it in its mission, to a point of harm to the dogs, in extreme cases.
While each species was domesticated for a different purpose, domesticated animals exhibit certain recurring characteristics. One example is the drooping ears in many domesticated species, in contrast to their wild ancestors’ upright ears. Some have smaller skulls; shorter snouts; short, curly tails; small teeth; patterned fur – as opposed to the monochromatic coat common among their wild ancestors; and so on. And it doesn’t end with their appearance: Domesticated animals often have a longer or more frequent estrous (heat), i.e., several times a year instead of once, and they play and behave "childishly", as pups would, even as they grow older. This set of features shared by numerous domesticated animals is referred to as the “domestication syndrome.”
Belyayev first came across this idea when he was reading Charles Darwin’s The Variation of Animals and Plants under Domestication, which triggered his curiosity. Since different animals were domesticated for different purposes, how is it possible that the same traits appear again and again across many species?
In an attempt to answer this question, Belyayev first tried to figure what all domesticated species share. He realized that they are all calmer than their wild ancestors. A basic requirement of any animal we want to raise, regardless of its purpose, is that it will refrain from attacking humans or run away when approached; that it would not be overly afraid of humans and consider them an enemy to be killed. Therefore, Belyayev said, the first stage in domesticating each species was choosing the calmer individuals, those who are friendlier towards humans. All other traits of the domestication syndrome, he argued, should be linked in some way to this primary feature. It is the main goal, while the other traits are, to some extent, side effects.
Patterned fur, drooping ears, playfulness, and friendly behavior. Shepherd Dog |Shutterstock
A dog from a fox
This hypothesis sounds reasonable, but how can it be tested? Belyayev had an idea: He will take a wild animal, the silver fox, a subspecies of the red fox (Vulpes vulpes), and try to domesticate it. “He said: I want to build a dog from a fox,” said Lyudmila Trut, a research student who accompanied him from day one, in an article for Scientific American.
The researchers created a measure that determines how calm and social to humans the animal is, and in each generation, bred only the ten percent scoring highest in this index. Thus, if they saw changes in foxes throughout the generations, they could trace them to the selection of this fundamental character trait. Belyayev died in 1985, but his experiment is still ongoing, and Trott, 86, is still the lead investigator.
In the first generation, Trut said, “Almost all the foxes looked less like dogs and more like fire-breathing dragons.” When the researchers approached the cage and placed a wooden stick inside it, many of the foxes reacted aggressively, while others cowered in fear at the rear of their cages. Neither response was what the researchers were looking for. They focused on the small number of foxes that remained calm throughout; they were not happy to see the humans, but didn’t panic or try to attack them. These were the foxes chosen to become the parents for the next generation.
Thus, in subsequent generations, there were more calm foxes, which observed without responding as humans approached. By the fourth and fifth generations, some of the pups looked forward for the researchers to approach and wagged their tails in anticipation. Then, in the sixth generation – that is, after six years – a new type of foxes appeared.
A few pups in this generation, about two percent, not only tolerated the presence of humans but were eager for contact with them. They wagged their tails as the researchers approached, licked their hands, and allowed the caretakers to pick them up and pet them, just like dog puppies. When the researchers walked away, these pups would whine in distress. In each generation, the proportion of these foxes, dubbed the “elite,” increased until, after tens of generations, about 70% of the foxes were “elite.”
Yet the more intriguing traits were those that were not selected for but appeared spontaneously, with the selection for sociability. Within a decade, some of the elite foxes had drooping ears and a curly tail, and a little later, there came foxes with a coat of patterned fur, instead of the uniform color of the original silver foxes. Their snouts became more rounded and shorter, and their periods of estrous grew longer. Their levels of glucocorticoid, hormones associated with stress, were about half of that in wild foxes, and their juvenile behavior persisted well into adulthood. All of these traits are typical of the domestication syndrome, and they appeared in the foxes in a surprisingly short time.
Not only did they tolerate the presence of humans, but they were eager for contact. Belyayev plays with the friendly foxes SPL, Sputnik
Domestication in the embryonic stage
Belyayev proved that the domestication syndrome does indeed develop – quickly – in animals that were domesticated solely on the basis of their sociability and calmness, but had yet to explain how this happens. The assumption is that when Belyayev and the research team bred the calmest foxes, specific genes that contribute to this calmness, became more prevalent in the population. But why would these genes also affect other syndrome features?
In recent years, researchers led by Richard Wrangham have tried to explain this using the neural crest theory. The neural crest is a group of cells that develop in the early fetal stages in all vertebrates. These cells originate in an area near the neural tube, which later becomes the spinal cord, but they migrate to different areas of the body during embryonic development. Neural crest cells give rise to the skin, face, teeth, nervous system, and adrenal gland cells. The latter produces and releases into the blood essential hormones, including stress hormones, whose levels in domesticated animals are low. The adrenal gland, researchers say, is the key to the domestication syndrome.
According to the theory, in the early stages of the domestication process, the animals selected are always the calmer ones, and the genes that propagate in the domesticated population are genes that cause the adrenal gland to be less active. This is because individuals with relatively low levels of stress hormones are less likely to respond aggressively or fearfully. Such low activity of the adrenal gland, researchers say, results from a change in fetal development, a change which directly bears on the neural crest cells. Fewer such cells may be formed, or the cell may migrate later or more slowly – different domesticated species may harbor different mutations that affect these cells in various ways – the result is always a less active adrenal gland.
But this is not the only outcome. Because the mutated genes affect the neural crest cells, they also influence many other areas of the body that these cells migrate to. In domesticated animals, fewer neural crest cells reach the skin and become pigment-producing cells, thus creating patterned fur. Fewer cells build the cartilage in the ears and tail, resulting in drooping ears and a curly tail – and the small rounded snout, small teeth, and small skull. Changes in the nervous system can also affect reproductive hormones and may result in longer periods of estrous. Thus, selecting for one trait, i.e., sociability, indirectly causes the entire domestication syndrome.
An interesting video by NPR about the domestication syndrome and the neural crest theory:
A hippie ape
Is it possible that this process, which domestic animals undergo, could occur without human involvement? Brian Hare of Duke University claims that it can. It might explain the evolution of the bonobo, a close relative of the chimpanzee.
The bonobo’s ancestors separated from the chimpanzees’ ancestors just one or two million years ago – a few millions of years after the lineage that led to them separated from the lineage that led to us. They are similar to chimpanzees in appearance, with several distinguishing characteristics. They usually have red lips and black faces, and often have a light spot on the rear. Their ears are smaller than those of chimpanzees, their forehead is high, and they have a typical “hairstyle”– the hair on their heads is parted in the middle and long. Once called “pygmy chimpanzees,” bonobos are not significantly smaller than their relatives, although males tend to be leaner than the muscular chimpanzees. They have narrower shoulders, a relatively small head, and long legs. “Bonobos,” says primate researcher Frans B. M. De Waal in a Scientific American article, “have more style.”
The most significant difference, however, is in their behavior. While chimpanzees are patriarchal and led by alpha males, among bonobos, the females appear to be the leaders. The chimps, especially the males, are known for their aggressiveness, both within their own group and towards chimps from other groups. They fight each other over group dominance, compete for food, and often threaten and even attack the females. They are highly territorial, and every encounter between two groups is hostile and typically ends violently, with possible fatalities. The males patrol their territory, and if they chance upon a solitary male, they may kill it.
The bonobos are much calmer. While they are no angels – they, too, fight and may injure and possibly kill one another, their levels of violence are much lower in comparison to that of the chimpanzees. Male chimps often form alliances, together ensuring other males don’t gain access to females and force females to mate with them; in bonobos' societies, on the other hand, the common form of alliance is between males and females, especially mothers and sons. The females also form alliances among themselves, and a male who tries to threaten or attack a female will soon find that the other females will not tolerate it. Thus, among bonobos, unlike the chimpanzees, the choice with whom to mate is mainly the females’. The bonobos are also territorial, and when two groups meet, they may threaten each other, but in most cases the threat does not turn into physical violence – one group simply leaves. Occasionally, an encounter between groups can even be friendly, with individuals from the two groups sitting side by side, playing, or mating.
Sexual intercourse is generally an essential part of the bonobo’s social life. They do it often, in various positions, including face-to-face, and in all possible combinations: Not just a female with a male, but also a male with a male and a female with a female. They do not reproduce at a higher rate than the chimps, who engage far less in sexual activity – it seems that, akin to their naked relatives, they do it for pleasure. Along with their calm temperament, this feature gave them the nickname “the Hippie Ape,” which prefers to make love, not war.
When not engaged in sexual intercourse, bonobos spend a great deal of time playing. All monkeys and apes play when they are young, but bonobos do so to a greater extent and continue to play into adulthood. In some respects, their behavior, even as adults, is similar to that of young chimpanzees. Experiments have shown that they happily share food with their friends and even with outsiders, which adult chimps do not. In social situations, the testosterone levels in the blood of male chimpanzees increases, but such an increase is not seen in the bonobo males.
When Brian Hare heard about all these features in a lecture by Harvard University anthropologist Richard Wrangham, he had a sudden realization. “He [Wrangham] was talking about how bonobos are an evolutionary puzzle; they have all these weird traits relative to chimps and we have no idea how to explain them,” Hare recalled in an interview with Scientific American. “I said, 'Oh that's like the silver foxes!'”
Indeed, several distinctive bonobo characteristics are consistent with the domestication syndrome: Smaller heads, ears, and teeth, and of course, their behavior – less aggressive, more tolerant, and with “childish” play into adulthood. Could it be that the bonobos went through domestication, even without human contact?
In a 2012 article, Hare, Wrangham, and an additional researcher elaborated on their hypothesis. Bonobos and chimpanzees diverged from a common ancestor when the Congo River was formed and separated the two ape populations. The population south of the river developed into the bonobos, and the population to the north became the ancestors of the chimps. But these two territories were not identical. In the north, the chimpanzee ancestors lived near gorillas and had to compete with them over food and territories. Because resources in any one area are limited, the chimps live in a smaller, less stable groups compared to bonobos: The group often splits into subgroups, which later regroup and then split again. The limited resources also lead to increased competition within the group, and the individuals who are successful at gaining food and opportunities to mate are the more aggressive and powerful males.
The bonobos, on the other hand, did not encounter direct competition from another ape, and so had greater resources at their disposal. They formed larger, more stable groups that enabled the females to form a coalition among themselves and to repel threats and assaults from the males. Under these conditions, the males that had greater reproductive success were actually the ones who formed alliances with the females – instead of attacking them.
Thus, the difference between the species was created over time, the researchers claim. The bonobos that were most tolerant and calm produced more offspring, passing along these traits. In parallel, other features that resemble those of the domesticated animals also evolved: The natural selection for calm behavior caused the bonobos to “domesticate” themselves.
Not everyone agrees with these conclusions. “I don't buy it,” declared de Waal, who has researched bonobos and chimps for many years. He asserts that we cannot tell if the bonobos have indeed become calmer during evolution: The bonobos’ and chimpanzees’ common ancestor may already have been tolerant and fond of sex, and it is the chimps that have actually changed over the last million years, to become aggressive. In this case, we need to explain what caused the increased aggression of the chimpanzees, not the domestication of the bonobos. Hare admits that this indeed is a problem. “It’s a real challenge,” he said, “especially since we don’t even have a single fossil to point to the evolution of any of these species.”
Less aggressive, more playful - in all kinds of games. Bonobo Washing in the River |Shutterstock
Survival of the friendliest
After presenting their hypothesis about the bonobo self-domestication, Hare and Wrangham turned to another species that they believed had undergone a similar process: Homo sapiens.
When it comes to tolerance or lack of aggression, humans may not be the first species we think of, but we are actually unusual among apes and monkeys in our tendency for cooperation and tolerating the company of strangers. Moreover, “Without our tolerance, we would not have been able to evolve into who we are today,” argues Wrangham. “Reduced reactive aggression must feature alongside intelligence, cooperation, and social learning as a key contributor to the emergence and success of our species,” he wrote in his book, The Goodness Paradox. Our ability to learn from each other and work together, which underlies human culture, could not have developed in a society that has constant and violent competition.
In a 2017 article titled “Survival of the Friendliest,” Hare reviewed the evidence for “self-domestication” of humans. He claims that genetic changes have led to hormonal changes, such as a decrease in testosterone levels and an increase in oxytocin, which in turn led to changes in behavior and in anatomy as well. Fossils show that over the past millennia, our faces became flatter, the brow ridge shrank dramatically, and our skull became more rounded. Behavior cannot be examined in fossils, but in comparison to apes, humans are more sociable, live in larger groups, and also play and enjoy other social activities throughout their lives.
In another article published in 2017, researchers from Europe and the U.S. examined which genes have changed during the evolution of dogs, cats, horses, and cows, as well as humans. They compared the genome of the domesticated animals to that of their wild relatives, and the human genome to that of our extinct relatives, the Neanderthal man and the Denisovan man. They found several genes that have changed both in humans and in domesticated animals, genes that are involved in brain activity and in behavior, as well as in anatomy. “It seems there is a ‘special’ intersection between humans and domesticated species, and we take this to be evidence for self-domestication,” said Cedric Boeckx, head of the research team.
Many of these genes are involved in the fetal development of the neural crest – as would be expect by the domestication syndrome theory. This theory is still in its infancy, and further research is needed to understand what occurs, from the genetic point of view, when an animal becomes domesticated, either on its own or with human help. “In a sense, what we did is narrow down the set of genes to examine experimentally,” Boeckx concludes.
The researchers do not claim that human evolution in its entirety can be explained as a case of domestication – numerous other factors have certainly influenced it. But the theory offers a new perspective on some of our most significant characteristics, and how these evolved. Belyayev probably did not imagine, when he left for Novosibirsk sixty years ago, that his silver fox experiment would reveal something about our own evolution.