The bond between dogs and humans is ancient and enduring. Dogs snuggle up to us at night, gambol by our side during daily walks, and flop adoringly at our feet when we crash on our couches. But new research shows that the connection runs deeper than you might think. It is embedded in our genes.
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 disease—have been evolving in parallel for thousands of years.
This parallel evolution was likely driven by the shared environments of humans and dogs, wrote the authors in a study published May 14 in the journal Nature Communications.
"As domestication is often associated with large increases in population density and crowded living conditions, these 'unfavorable' environments might be the selective pressure that drove the rewiring of both species," the authors surmise.
For example, living in crowded conditions with humans may have conferred an advantage on less aggressive dogs, leading to more submissive canines and eventually to the pets whose puppy-dog eyes gaze at us with unconditional affection. (Related: "Opinion: We Didn't Domesticate Dogs. They Domesticated Us.")
The study authors suggest that dogs were domesticated 32,000 years ago; that's much earlier than current estimates, which place domestication at around 15,000 to 16,000 years ago. (Related: "Ancient Dog Skull Shows Early Pet Domestication.")
"Thirty-two thousand is a little bit old," said Bob Wayne, an evolutionary biologist at the University of California, Los Angeles. Although he does acknowledge that the timing of a split between wolves and dogs has varied widely—ranging between 6,000 and 120,000 years ago.
The study authors also proposed that dog domestication originated in Southeast Asia, rather than the Middle East, as others have suggested.
A Canine "Missing Link"?
The scientists involved in the study sequenced the genomes of four gray wolves from Russia and China, three Chinese street dogs, and three domesticated breeds—including a German shepherd, a Belgian malinois, and a Tibetan mastiff.
They were then able to figure out which genes were associated with domestication and how far back that shift may have occurred. The team also looked at the dog genes selected for during domestication and compared them with human genes.
"The history of dog domestication is often depicted as a two-stage process," wrote Weiwei Zhai, a genetics researcher at the Chinese Academy of Sciences in Beijing and a study co-author, in an email. "The first stage is from wolves to dogs. The second stage is from dogs to breeds." (Read "How to Build a Dog" in National Geographic magazine.)
Southeastern Asia street dogs, including the Chinese street dogs in the study, may be an evolutionary bridge between wolves and purebred dogs due to their greater genetic diversity when compared with other street dogs from around the world, Zhai explained. This would make the Chinese street dogs a kind of "missing link" among canines.
When Zhai and colleagues took their canine sequences and compared them with the human genome, the team found that sequences for things such as the transport of neurotransmitters like serotonin, cholesterol processing, and cancer have been selected for in both humans and dogs.
Though selection in the same gene in two different species, known as convergent evolution, is rare in nature, said Zhai, their results weren't too surprising. After all, humans and dogs have shared the same living environment for years.
In addition to sharing genes that deal with diet and behavior, dogs and humans also share diseases, including obesity, obsessive-compulsive disorder, epilepsy, and some cancers including breast cancer, wrote Ya-ping Zhang, a professor at the Chinese Academy of Sciences in Kunming (map), in an email.
This might be due to the fact that genes often have multiple effects, explained Zhai. "Some of the effects will be beneficial, while others can be deleterious. When the selective advantage outweighs the deleterious cost, the gene can still be selected [for]."
The cancer-related genes the research team found evolving together in both dogs and humans could be the result of processes like this, said Zhai.
Far From Over
"This is nice that [their study is] based on complete genome data," said UCLA's Wayne, who provided reference data the study authors compared with their genetic sequences. Other studies have used only snippets, such as mitochondrial DNA.
But he cautioned that comparing human and canine genomes can be tricky, adding that the evaluation of canine sequences from other places in addition to China and Russia would have helped in the dating of domestication and in establishing its location.
Furthermore, Wayne said, without further comparisons between humans and other domestic animals like goats or horses, it's hard to know whether the parallel evolution in the genomes of humans and dogs is unique or not.
Even so, he added, the study adds another chapter to the story of dog domestication—a story far from over.