WASHINGTON – You know that dog biscuit shaped like a bone but made mostly of wheat? The fact that your dog is satisfied with it instead of going for a piece of your thigh may be one of the big reasons why its ancestors evolved from wolves to house pets.
A team of Swedish researchers has compared the genomes of wolves and dogs and found that a big difference between the two is a canine’s ability to easily digest starch. On its way from pack-hunting carnivore to fireside companion, dogs learned to love — or at least live on — wheat, rice, barley, corn and potatoes.
As it turns out, that is also a change that human beings underwent as they came out of the forest, built permanent settlements and began to grow grain.
“I think it is a striking case of coevolution,” said Erik Axelsson, a geneticist at Uppsala University. “The fact that we shared a similar environment in the last 10,000 years caused a similar adaptation. And the big change in the environment was the development of agriculture.”
The findings, published Wednesday in the journal Nature, support the hypothesis that dogs evolved from wolves who found a new food source in refuse on the outskirts of human settlements. Eventually they came to tolerate human contact and were brought into the household to serve as guards, workers and companions.
Another theory is that dogs are descended from wolves captured by hunter-gatherers, who tamed and bred them.
Canine evolution is a contentious subject, and the new findings are unlikely to settle the debate. Among the uncertainties is when some wolves began to turn into dogs. Human-tolerant — if not fully domesticated — canids may have existed as long as 33,000 years ago. Archaeological remains reveal dogs and human beings sharing the same graves 11,000 ago, at the dawn of agriculture — the two species appear to have been at least acquaintances by then.
The evidence of natural selection in the number and efficiency of key digestive enzymes supports the hypothesis that dogs may have domesticated themselves as a way to exploit the garbage of permanent human settlements.
“Humans had nothing to do with it,” said Raymond Coppinger, a biology professor and an expert on dog evolution at Hampshire College in Massachusetts. “There was a new niche that was all of a sudden available for somebody to move into. Dogs are selected to scavenge off people.”
In the new study, Axelsson and his colleagues examined DNA from 12 wolves and 60 dogs. The wolf samples came from animals in the U.S., Sweden, Russia, Canada and several other countries. The dogs were from 14 breeds. The researchers compared the DNA sequences of the wolves and dogs — which are subspecies of the same species, Canis lupus — and identified 36 regions in which they found differences suggestive of recent natural selection in dogs.
In particular, dogs show changes in genes governing three key steps in the digestion of starch. The first is the breakdown of large carbohydrate molecules into smaller pieces; the second is the chopping of those pieces into sugar molecules; and the third is the absorption of those molecules in the intestine. The change is at least partly the consequence of dogs having multiple copies of a gene for amylase, an enzyme made by the pancreas that is involved in the first step of starch digestion. Wolves have two copies, dogs have between four and 30.
As it happens, amylase “gene duplication” is also a feature of human evolution. Human beings carry more copies of the amylase gene than their primate ancestors and also produce the enzyme in saliva, which allows the first steps of digestion to occur while food is still in the mouth. That, in turn, rewards chewing and increase’s the palatability of food.
In dogs, however, the increased amylase activity occurs only in the pancreas. The enzyme isn’t at work in their mouths, probably because the food doesn’t stay there long enough. Dogs may be able to eat human food, but they still wolf it down.