Dogs may have become man's best friend thanks, in part, to their ability to stomach a starchy diet.
According to new genetic research, domestic dogs' genomes better equip them to handle starches than wolves. Domestic dogs also show differences from wolves in portions of the genome linked to brain development, perhaps hinting at behavioral changes that occurred as canines became less wild.
The findings are particularly fascinating given that humans who live off farmed foods show similar genetic changes as dogs compared with humans who survive mostly by hunting and gathering, said study researcher Erik Axelsson of the department of medical biochemistry and microbiology at Uppsala University in Sweden.
"It's cool that we've shared an environment for such a long time and we've eaten the same kind of food for such a long time, that we have started to become more similar in that way," Axelsson told LiveScience. [10 Things You Didn't Know About Dogs]
The DNA of domestication
Dogs have been intertwined with humans for thousands of years, but no one is sure how far back the bond stretches. Humans were buried with dogs some time between about 11,000 and 12,000 years ago in Israel, perhaps the oldest agreed-upon archaeological evidence for domestication, though the remains of a possible domestic dog dating back 33,000 years were uncovered in 2012 in a cave in Siberia.
Understanding domestication is interesting in its own right, Axelsson said, but comparing wild and domesticated animals can also help researchers track down the functions of individual genes that change during the domestication process. The results may even affect research on human health. In the case of dog diet, for example, canines might be a good model for human diabetes. Dogs are already treated for cancer with experimental drugs that might someday help humans.
Axelsson and his colleagues analyzed the entire genetic codes of 12 wolves from across the globe, as well as the genomes of 60 individual domestic dogs from 14 different breeds. They pooled the domestic pups' results so that the genetic traits of individual breeds wouldn't skew the findings and then compared the pet dogs to the wolves, looking for places where the genomes diverged.
This game of "spot the differences" led the scientists to focus on 36 different regions. They found that 19 of these regions contained genes crucial for brain functioning, including eight important for the development of the nervous system.
It was no surprise to see differences in brain genetics, Axelsson said, given that dogs had to modify their behavior to fit into human society. What did surprise the researchers, however, were 10 regions held genes involved with diet, specifically the breakdown of starches. Humans are well-equipped for starchy diets: Human saliva contains an enzyme called amylase, which starts breaking down starches as soon as food hits the mouth. Dog drool doesn't have this advantage, but dogs do excrete amylase from their pancreases, allowing for the digestion of starches in the gut.
The researchers found that dogs have more copies of a gene called AMY2B, crucial for amylase production, than wolves. And in dogs, this gene is 28 times more active in the pancreas than in wolves.
Dogs also showed changes in specific genes that allow for the breakdown of maltose into glucose, another key starch digestion step, and in genes allowing for the body to make use of this glucose.
How did wolves become dogs?
The findings can't pin down exact dates for dog domestication, but they do lend weight to one hypothesis, which is that wolves were drawn to early human settlements in order to scavenge at waste dumps, Axelsson said. Theorists have speculated that wolves that were less shy would have had an advantage, as they wouldn't have run when humans were around. The proximity could have been the first step in domestication.
"We think that our results regarding starch digestion fit really neatly with that idea," Axelsson said. "Being an efficient scavenger didn't only take a special type of behavior but also a digestive system that could cope with the food that was present at the dump."
The researchers are trying to pinpoint in more detail when the starch gene changes occurred. They're also taking a closer look at the behavioral genes that differ between dogs and wolves.
"Now we're also trying to take the behavioral side of the story further to try to pinpoint the genes, the individual mutations, to understand exactly how they might have changed the dog brain and dog behavior," Axelsson said.
The researchers report their results Thursday (Jan. 24) in the journal Nature.