How Wild Wolves Became Domestic Dogs

How Wild Wolves Became Domestic Dogs

Jeffrey Cohn

1997

Abstract

A controversial new study by environmental biologist Robert Wayne places the domestic dog's origins at about 135,000 years ago, rather than the 10,000-14,000 years previously thought. Whenever it started, interaction with humans wrought fundamental morphological and behavioral changes in wolves. The debate continues as to the point of origin, with anthropologist Stanley Olsen believing that dogs evolved from different wolf populations in different places at different times.

"The poor dog," wrote poet Lord Byron in a flight of emotion, "in life the firmest friend, The first to welcome, foremost to defend." And certainly, few animal lovers would care to differ. The dog, after all, is commonly referred to as man's best friend, and unquestionably serves a wide range of human purposes. Thanks to artificial selection, there are dogs that guard houses and dogs that herd livestock, dogs that locate game birds for shooting and dogs that retrieve game birds that have been shot, dogs that pull sleds and dogs that sit languidly in human laps.

Clearly, the relationship between dog and human runs deep in our culture and our psyches. No surprise, then, that the origin of the domestic dog has long been a matter for speculation and inquiry. But now, new techniques of molecular biology are allowing researchers to trace dog ancestry and to compare species and even breeds in ways previously unavailable to traditional wildlife biologists, taxonomists, and archeologists. Investigators are making great strides in understanding the origin of the domestic dog, even though results are often subject to dispute and controversy, as might be expected of research on a creature that is genetically complex.

"No other species is so diverse," says Robert Wayne, a University of California-Los Angeles evolutionary biologist who has just completed the largest study ever on dog genetics and evolution. "Dogs are a model for how rapid morphological change might take place in a natural population."....

One of the key questions of dog evolution focuses on the source: From what wild creature did the domestic dog arise? Charles Darwin suggested that the close relationship between wolves, coyotes, and jackals -- all of which can interbreed -- so muddies questions of which species yielded the dog that "we shall probably never be able to ascertain [the dog's] origins with any certainty." Austrian behaviorist Konrad Lorenz added fuel to the fire in the 1950s by suggesting that some dog breeds may derive from jackals, others from wolves. Other biologists have proposed that dogs sprang from coyotes....

New genetic evidence marshaled by Wayne and his colleagues lends strong support to the wolf advocates. As Wayne's team reported in the 13 June Science, they analyzed the monochondrial DNA from 140 domestic dogs representing 67 breeds and five crossbreeds, then compared the dogs' sequences with DNA from 162 wolves collected at 27 localities worldwide as well as with DNA from five coyotes and eight Simien, two golden, and two black-backed jackals.

"The genetic data strongly suggests that the wolf is the progenitor of the domestic dog," Wayne says. Dog gene sequences differ from those of wolves by at most 12 nucleotide substitutions, whereas dog sequences differ from coyote and jackal sequences by at least 20 substitutions and two insertions. Coyotes and jackals are thus "very different [genetically] from wolves and dogs," Wayne says....

Based on studies of canid bones found at human archeological sites, researchers have traditionally placed the domestic dog's origins at about 10,000-14,000 years ago. As discussed in the Science article, Wayne and his colleagues' molecular data indicates that the dog actually is much older....Wayne calculates that the genetic difference between wolf and dog suggests that they separated about 135,000 years ago....

Such a surprising assertion has inevitably spawned controversy. If wolves and dogs diverged when Wayne suggests, some experts ask, then why does the archeological record fail to show morphological differences between wolf and dog fossils until about 14,000 years ago? Wayne guesses that a phenotypic divergence between the two animals began only after humanity converted from hunter-gatherer cultures to more agricultural societies about 10,00-15,000 years ago, imposing new selective regimes on dogs.

Darcy Morey, an adjunct assistant professor at the University of Tennessee-Knoxville whose doctoral dissertation focused on the evolution of humankind's relationship with the dog, disagrees with Wayne's interpretation of the evidence. "How could so fundamental an ecological change occur between wild and domestic populations without altering the animals' size and form?" Morey asks....

Wayne...admits that his study may inflate the date of origin for the domestic dog, but he contends that his evidence is nevertheless correct in indicating that the dog did arise long before the date ascribed to it by archeological evidence. Wayne plans to test micro-satellites, a set of fast-evolving nuclear genes, to confirm the mitochondrial DNA results.

Regardless of when wolves came into the human domain, the relationship wrought fundamental changes on the wolf, remolding the wild animal. Most notably, dog skulls, teeth, and brains are smaller than those of wolves....

Dogs and wolves differ in their behavior as well. For example, female dogs usually come into heat twice yearly, but wolves only once. Moreover, many adult dogs beg for food, a behavior typical of wolf puppies but not of adults. Dogs greet and lick their human masters the way wolf pups do their elders.

Some of the physical traits characteristic of certain dog breeds, such as floppy ears and rounded profiles, do appear in wolves, but only as pups. This appearance of youthful wolf traits in adult domestic dogs suggests that dogs are neotenic, forever immature.

Morey suggests that retention of juvenile morphological and behavioral traits by adult dogs was due to natural, rather than artificial, selection. Presumably, dog domestication began when humans captured wolf pups and raised them as pets. In the wild, mature wolves leave the natal pack to seek mates and start their own packs, or they challenge the dominant animals in their pack and take over. Animals that did this to human masters would likely be killed, giving them little opportunity to contribute to the gene pool of the domestic dog.

The wolves that survived in the human environment and gave rise to dogs probably were individuals that preserved into adulthood the submission that wolf pups demonstrate toward adult wolves....

The subject of dog evolution is rich with unanswered questions, a garden of inquiry for the evolutionary biologist....

One expert contends that no single point of origin exists. Stanley Olsen, a retired anthropologist at the University of Arizona in Tucson and author of the 1985 book Origins of the Domestic Dog, says that fossil evidence from hundreds of human archeological sites in Europe, the Near East, and Asia suggests that dogs evolved from different wolf populations in different places at different times. Olsen believes that large dogs may have derived from the large wolves of northern Europe, whereas small ones came from Asian and Near Eastern wolves....

Wayne's studies suggest that the dog's complicated evolutionary history has yielded an animal of great genetic diversity. Even recognized dog breeds show remarkable genetic variation. Part of this diversity, Wayne thinks, stems from intermittent breeding that occurred between dogs and wolves even after domestication, providing raw material for artificial selection under human control and giving the dog great evolutionary plasticity....

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