How dogs and humans became best friends

In the evolutionary journey of species survival, people and dogs have travelled very separate paths and that begs some questions about how we've become such faithful friends.

What caused humans and canines to start bonding? And why have we remained close companions for so long?

Today, dogs protect families and property, help herd livestock, hunt and retrieve game, work with police, find trapped survivors after natural disasters, sniff out bombs, and even act as early-warning systems for health issues like cancer, seizures or diabetic shock.

They also capture our hearts, sleep in our beds and when we speak to them gaze lovingly into our eyes.

This last behaviour may be the key to it all.

• Related: Who's a better bedmate: cats or dogs?

It turns out that when we look at our dogs and they look at us, we're both releasing a powerful hormone oxytocin that has been associated with bonding.

TheScience of Friendship, a documentary for The Nature of Things, unlocks the physiological mechanics behind this millennia-old love that has transformed two very different species.

First encounters

The relationship between dogs and humans likely involved two basic stages. First, some groups of wolves discovered a unique benefit of living near humans: the easy source of food generated by their waste. Then, new forms of co-operation started to develop.

Researchers believe oxytocin played a role in both phases. At first, oxytocin may have inhibited fear and aggression among wolves living near human populations, which in turn encouraged some of the animals to get closer and interact. Later, the hormone likely triggered prosocial behaviour that encouraged bonds between humans and wolves.

"Our friendship with dogs started deep in the last ice age when we started making friends with certain groups of wolves who wanted to scavenge on our leftovers," behavioural scientist Clive Wynne said in The Science of Friendship. "And that was the beginning of this beautiful friendship."

Given that humans are primates and dogs are carnivores, this bond occurs "across an immense species boundary," Wynne said. Even chimpanzees our closest relatives aren't as skilled as dogs in using human communicative behaviour, such as the mutual gaze.

As one expert described it, "Dogs have hijacked the human bonding system."

Putting love to the test

The big question, though, is why?

"A lot of people have a feeling that their dog loves them," Wynne said, "but they also have this feeling that maybe their dog only loves them because they provide the food."

Wynne's team at the Arizona State University's Canine Science Collaboratory set out to develop experiments to measure the bonds between people and their pups. One was a simple test compelling a dog to choose between a food treat or their human.

"A typical pet dog, eight times out of 10, they'll choose their human in preference to the food," he said of the results.

Would your dog rescue you if you were locked in this box? | The Secrets of Friendship

2 years ago

Duration 3:08

Clive Wynnes team at the Arizona State University's Canine Science Collaboratory set out to develop experiments to measure the bonds between people and their pups. One was a simple test compelling a dog to choose between a food treat or their human. Another experiment placed people in a large box on the floor and sent their dog to the rescue as the trapped person cried out for help.

Another experiment placed people in a large box on the floor and sent their dog to the "rescue" as the trapped person cried out for help.

Only about one in three dogs could open the box to free their human, but Wynne says the low success rate was mainly because the dogs struggled with the boxes.

All the animals were clearly upset.

"We see that in their behaviour," Wynne said. "They look and they act distressed."

Sometimes called the "love hormone," oxytocin is released during sexual activity, childbirth and breastfeeding, and is linked to our well-being and ability to manage stress.

It has been credited in humans with mother-child bonding, sexual attachments and feelings of happiness from social connections. It's why we love to be touched. And it's why we feel good as a result of a positive interaction with another human or animal.

For humans and dogs, it wasn't love at first sight, but a love that grew over thousands of years as we worked together, evolved together, and flooded each other's bodies with a rush of emotion.

"That's why there are so many dogs on this planet today," Wynne said.

"And they bring such tremendous joy to our lives. It's such an enriching friendship to have."