Ancient DNA reveals that canine and human genomes evolved in tandem, unlike the feline genome, which remained largely unchanged for thousands of years after the cats entered human’s life
The bond between dogs and humans is ancient and enduring. Dogs are our sentinels and shepherds, hunting partners, and cancer detectors. They also are our dearest friends. Unlike dogs, nowadays the cats contribute virtually nothing in terms of sustenance or work to our endeavor. They are by turns cold and affectionate, serene and savage, endearing and exasperating. And yet the house cat is the most popular pet in the world. Cats have not been domesticated in the same way dogs, cows, pigs, and goats were. They have lived among us, but they pretty much domesticated themselves. It wasn’t until very recently that we began to change them.
Early signs of dogs’ domestication date back to 33,000 years ago and unambiguously domesticated dogs are common in the archaeological record beginning 15,000 years ago. Dogs’ domestication happened slowly, in fits and starts, and most likely started accidentally. As for cats, humans have lived with them for at least 10,000 years – there’s a 9,500-year-old grave in Cyprus with a cat buried alongside its human and ancient Egyptian art has a popular motif showing house cats eating fish under chairs.
Greger Larson, an evolutionary biologist at the University of Oxford, and Pontus Skoglund, a paleogenomicist at the Ancient Genomics laboratory of the Francis Crick Institute, teamed up to study dogs’ domestication[1]. Thus, they unveiled a few surprises in the dog DNA alone. Thus, they found out that all dogs share a common ancestry distinct from present-day wolves, with limited gene flow from wolves since domestication, but substantial dog-to-wolf gene flow. They also discovered that, as early as 11,000 years ago, there were five distinct genetic clusters, or lineages, of dogs, which gave rise to canines in the Near East, northern Europe, Siberia, New Guinea, and the Americas. Because dogs had already diversified so much by that time, “domestication had to occur long before then,” Skoglund says. That fits with archaeological evidence: the oldest definitive dog remains come from Germany about 15,000 – 16,000 years ago.
In many places, the team found a strong overlap between human and dog genomes. For example, farmers and their pups in Sweden about 5,000 years ago both trace their ancestry to the Near East. This suggests early farmers took their dogs with them as agriculture spread throughout the continent. But sometimes the stories didn’t match up. For example, the dogs of the farmers in Germany about 7,000 years ago, who also came from the Near East and lived with dogs, seem more like hunter-gatherer pups, similar to the ones who came from Siberia and Europe. That suggests many early migrants adopted local dogs that were better adapted to their new environment.
Then the scientists brought the human DNA into the mix. They selected human DNA samples from the same places and eras they had ancient canine DNA for and traced the genetic history of each. They made an amazing discovery: the human-dog connection is a truly genomic love story. “It’s like you have an ancient text in two different languages, and you’re looking to see how both languages have changed over time,” Skoglund says. Furthermore, a team of researchers from the University of Chicago and several international institutions found that several groups of genes in humans and dogs – including those related to diet and digestion, neurological processes, and some diseases – have been evolving in parallel for thousands of years[2].
During the roughly 30,000 years they have lived with humans, the dogs’ bodies and temperaments have transformed radically. The dogs’ ability to communicate with humans and read facial cues is highly enhanced compared to their wild ancestors. Dogs have actually developed new facial musculature[3] 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 humans’ initial desire to associate with canines. Observations at animal shelters have found dogs that make that archetypically worried or sad expression are adopted more quickly[4], indicating a probable selective advantage. The pups seem to know that humans respond to their expressions, too – they make more of them[5] when people are paying attention than when they don’t. They also make extensive use of eye contact in order to communicate with people. Domestic dogs are also skilled at using “gaze alternation”[6] 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,” says Angie Johnston, primary investigator at the Center for Canine Cognition and Social Learning Laboratory at Boston College. Dogs are able to use human eye movements, nods, and finger pointing to locate an object. This ability appears to be innate – even young puppies[7] understand facial and gestural cues.
Humans not only communicate pretty well with dogs, but some of them truly love their canine companions. But what about the dogs’ feelings toward humans? There are some studies that offer support for the assumption that dogs love their owners back. One investigation found that dogs were able to visualize their owner’s face when they heard their voice[8]. And another discovered that dogs were more strongly drawn to images of their owners’ smiling faces[9], as opposed to images depicting a neutral expression. In a functional MRI (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[10], as opposed to other olfactory stimuli. While there are other possible explanations for these findings, like the dog’s anticipation of food or other caretaking needs, the possibility that the dogs have real feelings for their owners cannot be ignored. More compellingly, it has been shown that dogs are drawn to people who are pretending to cry[11] even when they are strangers and their owners are present. This is at least suggestive of an empathetic instinct. 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’s urine[12] show that both species release oxytocin when they look at each other[13]. 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.