The standard myth about the origin of the dog is that man found him to be a useful companion and so took him in. Dogs were sentinels or shepherds or they helped in the hunt. The oldest archaeological evidence of dogs with a morphology distinct from that of wolves is from about 12,000 years ago in the Middle East, suggesting an evolution coinciding with the rise of the first agricultural settlements and permanent villages, and pre-dating the domestication of other animals, including sheep and goats, by a few thousand years.
The view that dogs came along at about the same time as human beings settled down is so widespread and so often repeated in standard texts that it is more than a bit surprising to find genetic evidence flatly contradicting it. The evidence comes from a study by Robert Wayne, an evolutionary biologist at the University of California at Los Angeles, who has applied the modern tools of genetic fingerprinting to dogs, coyotes, wolves, and jackals. He and his colleagues collected blood, tissue, or hair samples from 140 dogs of sixty-seven breeds and 162 wolves from three continents. To gauge how closely related these various canines were and when they might have diverged from a common ancestor, the scientists measured differences in their mitochondrial DNA. Mitochondria are like small cells within the cells of animals; they convert stored food into energy with the assistance of oxygen, and they also have the peculiarity—much cherished by geneticists—of reproducing asexually, independent of the rest of the cell. The regular DNA of an animal cell derives equally from both parents. Mitochondrial DNA, however, comes entirely from the mitochondrial DNA of the mother. In normal sexual reproduction genetic change from one generation to the next is very rapid, as the parental genes are mixed and remixed in new combinations. Mitochondrial DNA, in contrast, can change only by mutation, which takes place quite slowly—at a rate of around one or two percent every 100,000 years.
That means that mitochondrial DNA can be used as an evolutionary chronometer. Wolves and coyotes differ by about six percent in their mitochondrial DNA, and, according to fossil evidence, separated from a common ancestor about a million years ago. Wolves and dogs differ by about one percent; using the wolf-coyote time scale, this suggests that they parted company about 135,000 years ago—a lot earlier than the date implied by the first distinctly non-wolflike dog fossil.
Wayne's study also definitively laid to rest an assertion made by both Charles Darwin and Konrad Lorenz—that more than one wild canid species had to have made an appearance in the dog's recent family tree, given the diversity of physical types and behaviors exhibited across the range of modern dog breeds. In fact, long sequences of dog mitochondrial DNA are similar or identical to those in gray wolves, and analysis of the highly variable markers in the regular DNA of dogs and wolves shows a considerable overlap there as well. Jackals and coyotes, though they can interbreed with dogs and produce fertile offspring, possess quite distinct groups of mitochondrial DNA sequences.
The evolutionary chronometer is a measure of ancient origins—it cannot pick up divergence into separate breeding lines that has occurred in the past few hundred years. The most striking discovery Wayne's team made was that there is almost no correlation between a dog's breed and the mitochondrial DNA sequences it carries. In eight German shepherds the scientists found five distinct sequences; in six golden retrievers they found four. And the same sequences repeatedly showed up in many different, and apparently quite unrelated, breeds. The Mexican hairless, or Xolo, a breed known from historical and archaeological records to have existed more than 2,500 years ago in Aztec Mexico—and which presumably separated from Old World breeds some 12,000 years ago, when the Bering land bridge disappeared—contained representatives of all the major mitochondrial DNA sequences found in dogs throughout the world. (The Xolo sequences also resembled those of Old World wolves much more closely than those of New World wolves.)
The point is, then, that if dogs were indeed domesticated more than 100,000 years ago, as Wayne's data suggest, there wasn't much selective breeding going on for most of those 100,000 years. Rather than diverging into separate lines, the dog gene pool remained a well-mixed soup in a bowl of global dimensions. There was considerable gene flow throughout the population, which would not have been the case had early human beings been trying to direct the breeding of their dogs or to develop special lines with certain selected characteristics. Wayne's study also suggests that for a long time the genetic difference between a dog and a wolf was too small to cause any striking morphological change that would show up in the fossil record.
Even if the step from wolf to dog was a small one, it apparently didn't happen very often. Wayne found that the dog mitochondrial DNA sequences fell into four major groups. If there had been a continual influx of new wolf blood into the dog population (that is, if the dog had been reinvented again and again from wild populations at different times), such distinct grouping would not have occurred. Wayne's conclusion is that the earliest dogs "must have been integrated somehow into human society" to keep them genetically isolated from the surrounding population of wild wolves, and also that the domestication of dogs from wild populations must have been "a rare event"—something that happened only a few times in history.
That it happened at a time when "humans were barely human," as Gregory Acland—a veterinarian who works with Aguirre at Cornell's Center for Canine Genetics and Reproduction—puts it, raises an interesting possibility. It suggests that early man may not have sought to domesticate dogs at all. Rather, proto-dog found it in his interest to hang around people, and somehow persuaded them not to throw rocks at him or eat him.
That is a teleological statement, of course; if this scenario is correct, there was no conscious intent on the part of the dogs. But there was arguably little or no conscious intent on the part of the people, either. The wonder and beauty of natural selection is that it is creative; it crafts solutions that for all intents and purposes seem to reflect intelligence—"unthinking" intelligence, as the philosopher Daniel Dennett aptly put it. The evolutionarily correct way to state all this is that human beings, with their campfires and garbage heaps and hunting practices, but above all with their social interactions, represented an ecological niche ripe for exploitation by wolves. Or at least by those wolves that through some chance modification in their genetic makeup were able to exploit that niche and then prospered to pass on those traits to their offspring. Although wolves today are the most widespread wild land mammal in the world—with a range that extends from North America to Europe to Asia, encompassing everything from semi-desert to tundra to subtropical forest—their total population probably numbers no more than 150,000. In the United States there are about 50 million owned dogs and millions more unowned—eloquent evolutionary testimony to the wisdom of mooching off people rather than fighting it out in the wild.