By Seth Millstein
They were once violent predators. Now they’re companions who sit in our laps, accompany us on walks and stare lovingly into our eyes.
How did this happen? How did man’s best friend come to be?
The origins of dogs are hazy, but researchers believe they share a deep biological connection with humans. They were first domesticated from a wolflike ancestor between 10,000 and 30,000 years ago, and have co-evolved with us.
Around the world, dogs and humans of different breeds acquired traits that helped the two species thrive living alongside each other.
The first proto-dogs likely followed hunter-gatherer humans around in Europe in hopes of eating their leftovers. Due to genetic variation, some were more friendly to humans than others, and tagging along closer gave the friendlier ones better access to food.
Being around these four-legged creatures helped humans survive as well: The proto-dogs could act as guards and sniff out potential prey for humans to hunt.
As a result, both species enjoyed an evolutionary advantage by hanging out with each other and reproduced in greater numbers. By the time humans developed agriculture — around 10,000 years ago — these wolflike predators had become the domesticated dogs we know and love.
DNA evidence indicates that the dog and the modern wolf (above) are both descendants of an extinct wolf-like animal that lived in Europe.
For dogs and humans to live and work together, it helps to understand each other, and it appears that dogs evolved an ability to read human emotions. Humans express emotion more robustly on the right sides of their faces than on the left. Using eye-tracking equipment, researchers found that dogs look at the right side of a human face first, where emotional cues are stronger.
Dogs don’t exhibit this “left gaze bias” when looking at other dogs — or chimpanzees or inanimate objects or anything other than human faces. Dogs evolved to scan people’s faces in a way that’s well-suited to recognizing their emotions.
Another experiment asked human subjects to assess the emotional quality of recorded dog barks. They mostly agreed with one another and accurately read the dogs’ emotional states, for example recognizing through sound only that a dog tied up was in despair. Dog owners and non-dog owners were equally adept at categorizing barks, suggesting that the ability is innate, not a learned one.
The closest living relative to dogs, gray wolves, have only one kind of bark, and they only yelp it in one situation, as a warning. So it appears that dogs evolved an expressive range of barks to communicate with humans, and that humans evolved the ability to distinguish between those barks.
When a mother breastfeeds, her body releases the hormone oxytocin into her bloodstream, generating feelings of happiness and emotional bonding. Researchers believe people developed this trait because it makes them care for their offspring.
A Swedish study measured oxytocin levels of humans before and after they played with dogs. The oxytocin levels went up when they were with dogs, almost exactly the same amount as during breastfeeding. A separate study found that dogs and humans both experience an oxytocin spike when merely gazing into each other’s eyes.
So maybe couples who choose to have dogs rather than kids may not be making such a different choice after all, at least not on a neurochemical level.
But do pets truly enjoy their owners’ company, or are they just in it for the food? Recent evidence suggests that man’s best friend is a true friend indeed, not just a scavenger.
In a 2015 study, dogs were presented with a series of scents: That of a familiar human (their owner or a friend), an unfamiliar human, a familiar dog and an unfamiliar dog. Researchers measured the neural activity in the dogs’ caudate nuclei, a section of the brain dubbed “the neural basis of romantic love.” The caudate area was activated more by the owners’ scents than anybody else’s, even that of the dogs’ handlers in the experiment. Dogs don’t just recognize their owners; they love them.
Originally published October 20, 2015.
