As it turns out, the same thing happened to humans as they came out of the forest, invented agriculture and settled into diets rich in grains.
“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 wolves were captured by hunter-gatherers, who tamed, bred and eventually settled down with 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 many as 33,000 years ago. Archaeological remains reveal dogs and humans 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.