How Dogs Have Uniquely Co-Evolved With Humans Like No Other Species

The connection between human and dog runs deep. Early signs of domestication date back to 33,000 years ago and unambiguously domesticated dogs are common in the archaeological record beginning 15,000 years ago. The pairing makes for a striking case in coevolution — no other species has been so thoroughly integrated into human society. Dogs are our sentinels and shepherds, hunting partners and cancer detectors. And more importantly, to those of us who have had dogs in our lives, they are our dearest friends.

Though in many ways we take their presence for granted, the story of this unprecedented interspecies alliance is complex. In recent decades, we have brought the full force of the scientific method to bear on the origins of our beloved companions. Insights from disciplines as diverse as psychology and archaeology, genetics and biology have filled out the pencil sketch of our shared history brushstroke by brushstroke, resulting in a portrait both surprising and familiar.

The Tenderness of Wolves: Wolf to Dog Evolution

Until relatively recently, the tale of how dogs and humans came to be so intimately acquainted took the form of a parable: Early hunter-gatherers adopted wild wolf pups, abducting them from their dens or perhaps fostering them after killing their parents. Raised by humans and selectively bred over generations for docility and tractability, these lupine changelings morphed into something close to the canines we know today. Like most just-so stories, this appealing etiology has disintegrated under scrutiny.

For one, modern studies of wild wolf pups raised in captivity demonstrate that this would have almost certainly been impractical — the hardscrabble lifestyle of early humans was tough enough. Regardless of how much attention, training and affection are lavished on captive-raised wolf puppies, they remain wolves. They don’t take to training well and are in constant contest with their trainers for dominance.

“I don't know that many hunter gatherers would have had the time or patience to deal with a wolf pup and I don't know why they would want to,” says archaeologist Angela Perri of the University of Nevada, Las Vegas.

Unraveling the Mystery of Dog Domestication

More likely, domestication happened slowly, in fits and starts. “This symbiotic or commensal relationship,” says Robert Quinlan, professor of anthropology at Washington State University, “probably initially happened accidentally.”

Wolves more likely became acclimated to humans while scavenging the remains of their kills — they essentially kicked off the domestication process themselves. “Wolves on their way to becoming dogs were a great alarm system,” Quinlan says. Domestic dogs are in fact more vigilant than wolves. The off-spring of these more-tolerant wolves were likely then selected for other useful skills, such as hunting.

The relationship may have been reinforced by the innate human attraction to pedomorphic, or infantile, features like wide eyes and shortened faces, seen in wolf puppies and exaggerated in domestic dogs, even in adulthood. An ethnographic study co-authored by Quinlan found that, contrary to the stereotyped image of a man and his dog forging their way through the wilderness, women’s perceptions of dogs were positively correlated to both their utility in a given society and their status as persons. It may have been women who consolidated this millennia-long friendship.

Whatever the reasons for the initial attraction, dog domestication was not a singular event, but rather constituted a multitude of events, spaced across geography and time. And it wasn’t a one-way street.

How Did Canines Evolve?

Semi-domesticated animals frequently returned to the wild and interbred with wolves. In a vivid illustration of that fact, it was recently determined that black wolves carry a small amount of dog DNA. Others persisted in an intermediate state, as in the case of the New Guinea highland wild dog and its likely descendant, the dingo. The New Guinea highland wild dog is thought to have left mainland Asia in the company of humans and then returned to the wild before trekking across a land bridge to Australia and there evolving into the dingo.

Perri cautions that it can be nearly impossible to tell if early canine remains were dogs or wolves. “Neither their morphology nor their genetics are smoking guns when it comes to domestication,” she says.

Dog Burials: Evidence of Human and Canine Bonds

At the end of Wilson Rawls’ 1961 classic Where the Red Fern Grows, the adolescent narrator buries his two canine companions with a solemnity all too familiar to those of us who have endured a similar loss. To cynics, this may seem like puerile anthropomorphism — romanticizing a relationship with an animal.

This dismissal, however, ignores thousands of years of similar practices. The careful interment of dogs predates even the rise of agriculture, poignantly reinforcing the strength of the bond between our species. Anthropological analysis finds that dogs have been accorded human-like burial rites for thousands of years.

“It’s ubiquitous,” says Petra Cunningham-Smith, a doctoral candidate at the University of Florida who studies the role of dogs in ancient Mesoamerica. “Dog burials are found all over the world.”

To be sure, some prehistoric pups were sent to doggy heaven in less wholesome contexts. Excavations have turned up plenty of dogs that were sacrificed in religious rituals and others that were more feast than friend, scarred with knife marks indicating slaughter for food. And some inclusions of canine remains in human burials suggest a symbolic rather than companionable relationship, with many graves including single elements such as jaw bones.

“Around 12,000 to 14,000 years ago is where we start seeing morphological and genetic differences in early remains that are so different from those of wolves that we can confidently say we’re seeing domesticated dogs,” says Perri. Skulls with shorter faces and wider eye sockets become more frequent in the archaeological record beginning in this period.

Examples of Differences in Early Domesticated Dogs

One found in Germany, the Bonn-Oberkassel dog, shows signs of physical deterioration due to advanced distemper, suggesting that it was cared for during its illness. A specimen found in a Roman burial in Tunisia, estimated to have been 18 years old at the time of its death, was severely arthritic and missing teeth, meaning that it was almost certainly a treasured pet.

Among the most poignant is a grave found in Israel containing an adult woman with her head nestled against a dog or wolf pup dating to about 12,000 years ago. If not necessarily a definitive case of domestication, the grave does indicate the growing importance of canines in human life.

In Mesoamerica, Cunningham-Smith notes, dogs served as guides to the afterlife. “There were certain requirements,” she says. “For instance, you’d have to have a yellow dog or a red dog. It couldn't be a black dog or a white dog.”

The positioning of dogs during burial and the inclusion of grave goods often mirrors the treatment of human bodies when they are interred. Dog corpses were sometimes curled up, as if sleeping. And the objects with which they were buried signified their societal value. A grave in Skateholm, Sweden included flint blades and deer antlers — also common in nearby male human graves. And Australian rock burials of dingoes, which may have been kept as pets and guard dogs, sometimes feature the same paperbark wrappings used in human mortuary preparation.

Canine Facial Expressions

Many if not most of us have strong reactions when we look at a dog’s face. As it turns out, it’s mutual. The tendency of both humans and wolves to use facial cues in social communication may have been one of the factors that helped to solidify the bond between our two species.

In fact, the ability to communicate with and read facial cues is highly enhanced in domestic dogs as compared to their wild ancestors.

Dogs have actually developed new facial musculature in the domestication process — notably, a muscle responsible for furrowing the brow. The resulting “puppy dog” eyes may have helped to strengthen the caretaking response that probably played into our initial desire to associate with canines. Observations at animal shelters have found that dogs who make an archetypically worried or sad expression are adopted more quickly, indicating a probable selective advantage.

Our pups seem to know that we respond to their expressions, too — they make more of them when we’re paying attention than when we’re not.

The Power of Eye Contact With Dogs

They also make extensive use of eye contact in order to communicate with us. “We found that the dingoes are much more likely to make eye contact than wolves,” says Angie Johnston, primary investigator at the Center for Canine Cognition and Social Learning Laboratory at Boston College.

Dingoes are roughly intermediate in the domestication process — not quite dogs, not quite wolves. Her lab’s studies have shown that dogs make even more eye contact than dingoes, suggesting that this communicative ability has been enhanced during the domestication process.

Domestic dogs are also skilled at using “gaze alternation” to direct their human companions’ attention toward problems that they are unable to solve, such as extracting a hidden treat from a container. “We're not sure yet exactly why they're looking back and making eye contact. But it could be that they're seeking help on these tasks,” Johnston says. And they are able to use human eye movements, nods and finger pointing to locate an object. Wolves, and surprisingly even highly intelligent apes, perform poorly on tests of their ability to understand such gestures.

This ability appears to be innate — even young puppies understand facial and gestural cues. Intriguingly, an experiment in which red foxes were domesticated over a scant 30 generations found that these canines, too, had an instinctive understanding of these signals.

Do Dogs Love Us?

The ability of dogs to communicate with us is empirically provable. But can we say definitively whether our emotional attachments to them are requited? We can’t ask them outright, but some studies offer support for the assumption that they love us back.

One investigation found that dogs were able to visualize their owner’s face when they heard their voice. And another discovered that dogs were more strongly drawn to images of their owner’s smiling faces, as opposed to images depicting a neutral expression.

In an fMRI study, the caudate nucleus — the brain’s reward center — showed the most activation when the dog was presented with an object that smelled of its owner, as opposed to other olfactory stimuli. While tantalizing, these findings might have other explanations. Humans are, after all, responsible for feeding their pets. Could these responses simply indicate anticipation of food or other caretaking needs?

More compellingly, it has been shown that dogs are drawn to people who are pretending to cry, even when they are strangers and their owners are present. This is at least suggestive of an empathetic instinct. Similarly, another fMRI study demonstrated that the caudate nucleus showed greater activation in response to praise than it did in response to food.

Perhaps the most convincing evidence of canine love comes from, of all sources, urine — specifically its oxytocin content. Analyses of dog and human urine show that both species release oxytocin when they look at each other. Oxytocin has been referred to as the “love hormone” and is also stimulated in humans while holding infants, hugging and engaging in sexual activity. The next time your dog has an accident, consider this: As messy as it is, maybe there’s a little bit of love soaking into the carpet.

An Evolving Relationship

The road connecting human and dog still has plenty of bumps in it. We still wrestle with questions of animal welfare — many purebred dogs suffer from congenital disorders and animal abandonments, while declining, are still a major issue. Domestication, even at this advanced stage, is still not set in stone. More than three-quarters of dogs worldwide are free-ranging animals — existing in a liminal state between wildness and domesticity.

At the same time, there is a suite of evidence showing the increasing value placed on this interspecies relationship. More than half of millennials own dogs per some estimates — and the vast majority of dog owners consider their pets to be members of the family. Even in China, where dogs have been served as food to some people, pet ownership is growing exponentially.

Some Chinese cities have even moved to end dog slaughter. Postponement of child-rearing — or deciding not to have children at all — may in part account for these trends, and in doing so provides further support to the notion that dogs have co-opted our caretaking impulses. Interestingly, women are the primary caretakers of dogs in more than 70 percent of households.

We are far more likely to anthropomorphize our dogs than any other pet animal. And both men and women speak to dogs in what is called “doggerel.” It is very similar to “motherese” or infant-directed speech — the baby talk we use on human infants.

“I may be wrong, but I call it love — the deepest kind of love,” opines a character in Where the Red Fern Grows. Whether or not this is ultimately true in a provable sense remains to be seen. Initiatives like the ManyDogs project, a consortium of researchers, hope to replicate the wide ranging and often contradictory experiments that have sought to explain the dog-human connection.

“The field is new,” says Johnston, who contributes to the project. “Be patient with us as we try to gather evidence.”