Dogs are not wolves. If you let all the different breeds mingle, what you get is not a wolf but a mongrel dog with a rounded snout and short brown hair, the sort of beast that lurks in packs around the edges of cities across Africa and south Asia.
One obvious question is, what are the genetic differences that underlie this contrast? A Swedish group has just published the most detailed study yet of the differences between the wolf and dog genomes. The differences that jumped out at them were not in the genes that control hair type or facial structure, but in the ones that control digestion. Dogs have a whole panoply of adaptions that allow them to better digest starch, which allows them to live on wheat, rice, potatoes, and dog food in a way that wolves simply cannot.
This is another sign of the intense evolutionary relationship between dogs and humans. No other animal shows such an intuitive understanding of human signals, or responds as readily to human commands. Dogs simply pay more attention to people -- to our gestures, our voices, our facial expressions -- than other animals do. This is because dogs have evolved to understand us; over the millennia, the dogs that understood better when to obey and when to run left more descendants than those that tried to ignore us. Dogs have also evolved ways of communicating their own feelings to us, especially their expressive faces and human-like eyebrows.
We don't really understand how this relationship began. Some prefer a model of human action, in which hunter-gatherers captured wolves or wolf pups and raised and bred them to be hunting guides. Others think some groups of wolves took to hanging around human camps or villages to eat our trash, and gradually evolved more tolerance and understanding of people, that is, that the wolves took the first steps themselves. The new evidence of digestive genes is being touted in some quarters as evidence for the second model; if the biggest differences are in the digestion, the argument goes, then maybe adaptation to eating stealing human garbage was the first and most crucial change.
I think this is a weak argument; a tolerance for bread-eating could equally well have evolved much later in the dog-human relationship. The timing is also questionable, since many archaeologists think wolves were domesticated 30,000 years ago, long before agriculture made grains our most important food source. But the salience of digestion in the differences between wolves and dogs show how strongly and deeply dogs have adapted to life among humans.