After more than a century, scientists finally are beginning to understand the dog's long journey from wolf to woof.
"Everybody has wanted to know about the dog's origins. But there's been very little facts," says evolutionary biologist Peter Savolainen of Sweden's Royal Institute of Technology in Stockholm.
Now, two new genetic studies published today in the journal Science offer a clearer picture of where and when wolves became domesticated and turned into man's best friend. A third study in the journal, meanwhile, suggests this relationship has had at least one unexpected consequence: Dogs have become more adept than any other animal at reading human cues.
Scientists, dating back to at least Charles Darwin, have puzzled over the origins of Canis familiaris. In recent years, studies have established that dogs evolved from wolves, possibly in the Middle East. Estimates of when that occurred have ranged from 8,000 to more than 100,000 years ago.
But the new studies suggest that the animals originated in East Asia, perhaps modern-day China, about 15,000 years ago and then trotted alongside humans through Europe and across the Bering Strait to North America.
To pinpoint the dog's origins, Savolainen and his colleagues visited dog shows and city markets around the world, eventually collecting DNA from 654 dogs and 38 wolves. The researchers noticed the greatest genetic variation among samples taken from East Asia, suggesting that that is where dogs had the most time to breed and diversify, just as human genetic diversity is greatest in Africa, where homo sapiens is believed to have emerged.
In addition, the scientists determined that the pooches could be divided into about five genetic groups, meaning that all dogs - from the tiny Chihuahua to the heavyweight St. Bernard - have evolved from as few as five female wolves, says Savolainen.
He acknowledged that DNA offers only a rough estimate of when dog and wolf parted ways, so he also considered archaeological evidence. The oldest dog bone is widely believed to be a 14,000-year-old jaw remnant found in Germany. An archaeological dig in Israel turned up a 12,000-year-old grave containing a woman cradling what many believe is a puppy in her hands.
Some scientists are not convinced that dogs evolved that early. Raymond Coppinger, a biologist at Hampshire College in Massachusetts, says the clearest evidence is bones dating to 8,000 years ago found with primitive collars and other ornamentation.
It is not clear how dogs developed. One possibility is that early hunters snatched wolf puppies and raised them as pets or hunting dogs, breeding only the most docile of the litter.
But scientists who have studied wolves dispute this. Coppinger notes that wolves raised by humans never completely lose their wild ways.
"Have you ever seen a wolf act at the circus?" he asks. "They're not very trainable."
Coppinger argues that rather than humans approaching wolves, it was the other way around: Certain wolves unafraid of humans began to hang around early villages to scavenge. Over time and many generations, he says, they became less fearful and more friendly. That's when they became dogs.
The dog's role as town mooch also seems to gibe with early names for the animal. Ancient Romans dubbed the dog canis, which Latin dictionaries define as "parasite, hanger-on," notes Stephen Budiansky in The Truth About Dogs.
A second study in Science offers an explanation for another longstanding canine conundrum. Archaeologists have shown that dogs arrived in North America long before Christopher Columbus and other European visitors. How did that happen?
Some scientists had argued that the dogs evolved from North American wolves.
But using ancient dog DNA samples recovered from the Alaskan tundra to Peruvian mountaintops, Jennifer Leonard at the National Museum of Natural History and colleagues drew a different conclusion: Dogs originated in Asia and eventually trotted across the Bering Strait land bridge at the end of the last ice age, probably alongside early humans.
There's no shortage of pet theories to explain why settlers brought dogs with them. One obvious possibility is protection, says Robert Wayne, an evolutionary geneticist at the University of California, Los Angeles who worked with Leonard on the study.
"The New World was a scary place," he said. Saber-toothed cats with 7-inch fangs and other now-extinct beasts roamed the continent. Dogs might have been used to sound the alarm.
Thousands of years before man tamed the horse, dogs were likely hauling people and their possessions across the frozen tundra. And in a pinch they may have served as dinner, just as they still do in some parts of the world.
While mysteries about dogs' origins remain, it is becoming clear that spending so much time around humans has changed dogs in other ways. They seem to be more savvy at reading human social cues than wolves or even chimpanzees, generally thought to be the sharpest of our animal cousins.
To test this, Harvard anthropologist Brian Hare and his colleagues hid food under one of two containers. Then the researchers offered the animals a hint - a subtle nod, glance or tap - to indicate where the food was hidden. To rule out superior smell, the researchers made sure that both containers reeked of food.
While chimps have shown that they can follow a human gaze, they proved to be relatively clueless in catching the researchers' hints, picking the right container only slightly more often than they would have according to chance. Human-reared wolves did better in the test. But dogs, the researchers found, did best of all.