Domesticated dogs, these creatures that fetch sticks and sit at our command, seem so common and normal that we take them for granted. They are so much a part of human life, both past and present, that it's hard to imagine a world without them.
But dogs haven't always been around. Part of the Canidae family that includes wolves and coyotes and jackals, domesticated dogs are rather new to this planet and what they've accomplished since teaming up with humans is miraculous.
In the space of just a few thousand years, dogs have changed their shape and behaviors to fit into almost every known human environment and endeavor, from Huskies pulling sleds in the Arctic to Border Collies herding sheep in Scotland and Pekinese warming laps in midtown Manhattan.
"Dogs may well display the greatest range of shapes of any mammal that has ever existed," note biologists Raymond and Lorna Coppinger. "As reproductive adults, they may have a greater range of sizes and shapes than any vertebrate species that ever lived."
And yet, at the molecular level not much has changed since dogs branched off from the family of wolves. The DNA makeup of wolves and dogs is almost identical.
Dogs are not wolves. They have smaller heads, smaller brains and smaller teeth. Dogs do not think or behave like wolves. They eat with humans, which is something wolves would never do, and they can be trained to do perform almost any feasible task. Just try to train a wolf to sit or roll over or retrieve a ball...
Some researchers studying the evolution of dogs argue that Neanderthal humans started the process of domestication by adopting and taming wolf pups as early as 135,000 years ago. Natural selection would have favored the pups that were less wild and aggressive and better at begging for food.
But other researchers, like the Coppingers, have trouble believing that early man was capable or interested in domesticating wolf pups. They believe dogs are descendants of a wolf-like adaptation that specialized in scavenging from human refuse dumps. These animals domesticated themselves by becoming comfortable with people and increasingly tame.
"We descended from apes, but we don't behave like them and we don't think like they do. We are a much different animal than the apes in spite of our common genetic ancestry. The same is true of the dog and its ancestor," the Coppingers explain in their book, Dogs: A New Understanding of Canine Origin, Behavior and Evolution.
There's not much evidence available to prove the case either way. The earliest fossil remains of dogs and people together is from a 12,000-year-old grave in Israel where a human skeleton was found holding the remains of a pup.
Swedish researchers Peter Savolainen and Carles Vilà recently analyzed DNA samples taken from dogs in Asia, Europe, Africa, and arctic America. They found that, while most dogs shared a common gene pool, genetic diversity was highest in East Asia, suggesting that dogs may have been domesticated there the longest.
Vilà compared DNA sequences of Old World dogs with those found in the Americas, including some Latin America and Alaskan dogs that were on this continent before the first European explorers. Similarities among the sequences indicated that all the dogs shared a common ancestor.
A certain cluster of sequences from ancient Latin American dogs didn't match any from modern dogs. The researchers concluded, therefore, that those breeds died out and the ones we know today all came from Old World breeds that followed early humans into the Americas.
Both researchers rely on genetic sequences from the dogs' mitochondrial DNA, which, unlike the DNA in the cell nucleus, is inherited directly from the mother. Computer programs grouped certain sequences inherited together, or "haplotypes," according to their similarities. The haplotypes formed four main "clades." Ninety-five percent of the Old World dogs belonged to three of the major clades at similar rates in all regions. Thus, the major present-day dog populations at some point had a common origin from a single gene pool containing the three clades, the authors suggested.
The Native American dog sequences also clustered in several subgroups, whose haplotypes were similar or identical to those of Old World dogs. These subgroups likely represented the multiple lineages that crossed the Bering Strait along with the first humans to do so, approximately 12,000-14,000 years ago, Vilà and his colleagues concluded in two reports published in the journal Science.
Whoever their descendants are and no matter how or why they became domesticated, dogs have been incredibly successful biologically. Their worldwide population today is 10 times the combined population of all the earth's wolves and coyotes and jackals. Domestication served them well, whether they were coerced into it by early humans or chose it for themselves.