The science behind why your dog copies your behavior and tears up when they're reunited with you

• Dogs evolved over centuries from ancient wild wolves into loyal partners to humans.
• A growing body of research explains how dogs' distinctive features and skills strengthened the human-canine bond.

Dogs have long been considered man's best friend, and for good reason. Over centuries, our four-legged canine companions evolved to become loyal partners.

A growing body of scientific research suggests our furry friends have distinctive features and skills that allow them to perceive, understand, communicate, and show affection toward humans.

From sniffing out when we're stressed to tearing up when reunited with their owners, here is what science reveals about how humans' special relationship with dogs developed over time.

Scientists generally agree that dogs evolved from wolves to become the furry, fetch-loving domestic animals they are today roughly 15,000 years ago. But scientists have different theories on how exactly the split from wolves happened.

In a study published in the journal Nature in June, geneticists at the Francis Crick Institute analyzed the genomes — or all of the genetic information — of 72 ancient wolves from Europe, Siberia, and North America spanning 100,000 years. Then they compared the wolf DNA, which makes up the genome, to DNA of modern and ancient dogs.

By analyzing the DNA, scientists found early and modern dogs are most similar to gray wolves in Siberia about 13,000 to 23,000 years ago, suggesting domestication took place somewhere in Asia.

"That's consistent with a wolf population from Central Asia leading to the origin of dogs," Adam Boyko, a canine geneticist at Cornell University, who was not involved in the study, previously told Insider.

That might not be the end of the story, Boyko added. The findings don't rule out the possibility that dogs may have been domesticated multiple times, as ancient dogs in the Middle East, Africa, and Southern Europe also show ancestry from wolves in the Middle East.

Though their wild ancestors are known for their ferocious nature, dogs evolved to have bigger and more baby-like eyes over time.

In a study published in the Proceedings of the National Academy of Science in 2019, researchers discovered that dogs have muscles around their eyes that help them make puppy dog eyes — that soft squinting look that melts your heart. Wolves don't have these muscles, which suggests dogs' adorable expression is an evolutionary trait that helped them get on better with humans.

Dogs may be able to smell when you're stressed, according to a study published in Plos One in September. Stress responses trigger physiological changes in humans' sweat and breath that dogs can detect.

The study involved collecting breath and sweat samples from human participants before and after they completed a "stress-inducing" task. Then dogs trained in identifying odors had to choose between a sample from an unstressed human and a stressed one. The dogs correctly identified the stress sample 94% of the time.

"This study demonstrates that dogs can discriminate between the breath and sweat taken from humans before and after a stress-inducing task," the study authors wrote, meaning they were able to identify human odors associated with stress.

Dogs even adopt their owners' personalities. Research published in the Journal of Research in Personality in 2019 surveyed more than 1,600 dog owners, representing about 50 different breeds. The owners were asked to evaluate their dogs' personalities and answer questions about their dogs' behavioral histories. The owners were also asked questions about their own personalities.

Extroverted owners rated their dogs as more active and playful, while owners of more fearful dogs reported more negative emotions.

And while several studies support the notion that dogs and owners often look alike, that's largely because owners tend to pick dog breeds based on the breed's resemblance to themselves.

Research suggests dogs are hardwired to communicate with and understand people in unique ways.

"Dogs are very responsive to the way that we talk to them," John Bradshaw, anthrozoologist and author of "The Animals Among Us," previously told Insider. "It tricks many owners into thinking they literally understand every word."

But while they might not understand the words you're saying, Bradshaw added, dogs are very good at learning to respond to their owner's tone of voice in specific ways. "It's almost true that the dog is responding to every word they say," Bradshaw said.

A study published in the journal Scientific Reports in 2021 found that dogs can understand the difference between humans' accidental and deliberate actions — or, in other words, when we screw up.

The experiment involved 51 dogs who were taught they'd get tasty treats from a human through a gap in a glass partition. Then researchers looked at how dogs reacted when food rewards were withheld from them.

In the study, dogs waited much longer to retrieve the treat when the researcher purposefully withheld it than when the researcher dropped it or couldn't get it through the glass partition. That finding suggests dogs can distinguish between humans' intentional actions and unintentional ones.

"Dogs' communicative skills uniquely position them to fill the niche that they do alongside humans," Emily Bray, a canine-cognition researcher at the University of Arizona, Tucson, previously told Insider in an email. "Many of the tasks that they perform for us, now and in the past (i.e. herding, hunting, detecting, acting as service dogs), are facilitated by their ability to understand our cues."

It's common to see dogs overjoyed to be reunited with their owners.

In a study published in Current Biology in August, researchers recruited 22 dogs and their owners. First, researchers measured tear volume in dogs' eyes during a normal interaction with their owners, as a baseline. Then, following five to seven hours spent away from their owner, researchers measured tear volume on the surface of the dogs' eyes each time they were reunited.

After a long period of separation, the dogs' eyes were brimming with tears when they saw their owner.

"We found that dogs shed tears associated with positive emotions," Takefumi Kikusui, the study's lead author and a veterinary researcher at Azabu University in Japan, said in a press release.

This behavior may be related to dogs producing more oxytocin, commonly called the "love hormone," when they see their owners. Oxytocin also plays a role in emotional bonding in humans.