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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 dog'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's 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 co-evolution," 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 be guards, workers and companions.
Another theory is that dogs were descended from wolves captured by hunter-gatherers, who tamed and bred them.
Dog evolution is a contentious subject, and the new findings are unlikely to settle the debate. Among the uncertainties is when some wolves began to evolve 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 years ago. That was at the dawn of agriculture; the two species appear to have been at least acquaintances by then.
"Pretty much everyone without an agenda agrees that we don't really have a good handle about why wolves domesticated into dogs when they did," said Adam Boyko, a geneticist at Cornell University who studies dog evolution and was not involved in the new research. "But it does seem reasonable, and in agreement with the fossil and genetic record, that it could have predated agriculture somewhat."
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, an emeritus professor of biology and 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."
Accompanying the dietary change-and probably evolving along with it-were behavior changes that allowed dogs to tolerate living near people and ultimately being adopted by them. The Swedish researchers found strong evidence of genetic differences in brain function-and particularly brain development - between wolves and dogs, which they have not yet analyzed in detail.
In the new study, Axelsson and his colleagues examined DNA from 12 wolves and 60 dogs. The wolf samples were from animals from the United States, Sweden, Russia, Canada and several other northern countries. The dogs were from 14 different 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 there 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; the third is the absorption of those molecules in the intestine.
"It is such a strong signal that it makes us convinced that being able to digest starch efficiently was crucial to dogs. It must have been something that determined whether you were a successful dog or not," Axelsson said.
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 from 4 to 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. People also produce the enzyme in saliva, which allows the first steps of digestion to occur while the 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.