Origin of the domestic dog

A dog in Diego Velázquez's Las Meninas

The origin of the domestic dog (Canis lupus familiaris) began with the domestication of the gray wolf (Canis lupus) several tens of thousands of years ago. Domesticated dogs provided early humans with a guard animal, a source of food, fur, and a beast of burden. The process continues to this day, as the intentional cross-breeding of dogs continues, to create the so called "designer dogs".

Domestication

The earliest fossil carnivores that can be linked with some certainty to canids are the Eocene Miacids some 56 to 38 million years ago. From the miacids evolved the cat-like (Feloidea) and dog-like (Canoidea) carnivores. Most important to the ancestry of the dog was the canoid line, leading from the coyote-sized Mesocyon of the Oligocene (38 to 24 million years ago) to the fox-like Leptocyon and the wolf-like Tomarctus that wandered North America some 10 million years ago. From the time of Tomarctus, dog-like carnivores have expanded throughout the world.^[1]

Possible routes

Comparative illustration of wolf and dog tracks

• Orphaned wolf-cubs: Studies have shown that some wolf pups taken at an early age and reared by humans are easily tamed and socialized.^[2] At least one study has demonstrated that adult wolves can be successfully socialized.^[3] However, according to other researchers attempts to socialize wolves after the pups reach 21 days of age are very time-consuming and seldom practical or reliable in achieving success.^[4] Many scientists believe that humans adopted orphaned wolf cubs and nursed them alongside human babies.^[5][6] Once these early adoptees started breeding among themselves, a new generation of tame "wolf-like" domestic animals would result which would, over generations of time, become more dog-like.^[7]
• The Promise of Food/Self Domestication: Early wolves would, as scavengers, be attracted to the bones and refuse dumps of human campsites. Dr. Raymond Coppinger of Hampshire College (Massachusetts) argues^[8] that those wolves that were more successful at interacting with humans would pass these traits onto their offspring, eventually creating wolves with a greater propensity to be domesticated. The "most social and least fearful" dogs were the ones who were kept around the human living areas, helping to breed those traits that are still recognized in dogs today.^[6] Coppinger believes that a behavioral characteristic called "flight distance" was crucial to the transformation from wild wolf to the ancestors of the modern dog. It represents how close an animal will allow humans (or anything else it perceives as dangerous) to get before it runs away. Animals with shorter flight distances will linger, and feed, when humans are close by; this behavioral trait would have been passed on to successive generations, and amplified, creating animals that are increasingly more comfortable around humans. "My argument is that what domesticated—or tame—means is to be able to eat in the presence of human beings. That is the thing that wild wolves can't do."^[9] Furthermore, selection for domesticity had the side effect of selecting genetically related physical characteristics, and behavior such as barking. Hypothetically, wolves separated into two populations–the village-oriented scavengers and the packs of hunters. The next steps have not been defined, but selective pressure must have been present to sustain the divergence of these populations.
• As a beast of burden: North American Indians used dog-sized travois before adapting the horse for this purpose, and huskies are famous for pulling sleds for Inuit communities. It is very probable that the dog was the original beast of burden before the domestication of the horse or ox.
• Dogs as a source of food and fur: While, currently, most societies have difficulty thinking of dogs (or wolves) as a meat animal, it is conceivable that the human-canine bond was also fostered by humanity's use of dogs as a source of meat and fur.^[10]

Archaeology

Archaeology has placed the earliest known domestication at potentially 30,000 BC,^[11][12] and with certainty at 7,000 BC.^[13] Other evidence suggests that dogs were first domesticated in East Asia.^[14].

Due to the difficulty in assessing the structural differences in bones, the identification of a domestic dog based on cultural evidence is of special value. Perhaps the earliest clear evidence for this domestication is the first dog found buried together with human from 12,000 years ago in Israel.^[15][16]

Domestication of the wolf over time has produced a number of physical changes typical of all domesticated mammals. These include: a reduction in overall size; changes in coat coloration and markings; a shorter jaw initially with crowding of the teeth and, later, with the shrinking in size of the teeth; a reduction in brain size and thus in cranial capacity (particularly those areas relating to alertness and sensory processing, necessary in the wild); and the development of a pronounced “stop”, or vertical drop in front of the forehead (brachycephaly). Behaviorally, the wagging of tails and barking are behaviours only found in wolf puppies, retained via neoteny throughout the dog's life. Certain wolf-like behaviors, such as the regurgitation of partially digested food for the young, have also disappeared.

DNA evidence

Specialization–Breeds such as the Newfoundland and the Labrador were bred from large black mixed-breed dogs, such as this one from Atlantic Canada

Prior to the use of DNA, researchers were divided into two schools of thought:

• Most supposed that these early dogs were descendants of tamed wolves, which interbred and evolved into a domesticated species.
• Other scientists, while believing wolves were the chief contributor, suspected that jackals or coyotes contributed to the dog's ancestry.

Carles Vila, who has conducted the most extensive study to date, has shown that DNA evidence has ruled out any ancestor canine species except the wolf. Vila's team analyzed 162 different examples of wolf DNA from 27 populations in Europe, Asia, and North America. These results were compared with DNA from 140 individual dogs from 67 breeds gathered from around the world. Using blood or hair samples, DNA was extracted and genetic distance for mitochondrial DNA was estimated between individuals.^[17]

Based on this DNA evidence, most of the domesticated dogs were found to be members of one of four groups. The largest and most diverse group contains sequences found in the most ancient dog breeds, including the dingo of Australia, the New Guinea Singing Dog, and many modern breeds, like the collie and retriever. Other groups such as the German shepherd showed a closer relation to wolf sequences than to those of the main dog group, suggesting that such breeds had been produced by crossing dogs with wild wolves. It is also possible that this is evidence that dogs may have been domesticated from wolves on different occasions and at different places. Vilà is still uncertain whether domestication happened once–after which domesticated dogs bred with wolves from time to time–or whether it happened more than once.

A later study by Peter Savolainen et al. identified mitochondrial DNA evidence suggesting a common origin from a single East Asian gene pool for all dog populations.^[14] However, a more recent study by Bridgett vonHoldt et al. using a much larger data set of nuclear markers points to the Middle East as the source of most of the genetic diversity in the domestic dog and a more likely origin of domestication events.^[18]

The most puzzling fact of the DNA evidence is that the variability in molecular distance between dogs and wolves seems greater than the 10,000–20,000 years assigned to domestication. Yet the process and economics of domestication by humans only emerged later in this period in any case. Based upon the molecular clock studies conducted, it would seem that dogs separated from the wolf lineage approximately 100,000 years ago. Although clear evidence for fossil dogs becomes obscure beyond about 14,000 years ago, there are fossils of wolf bones in association with early humans from well beyond 100,000 years ago.^[19] Tamed wolves might have taken up with hunter-gatherers without changing in ways that the fossil record could clearly capture. The influx of new genes from those crossings could very well explain the extraordinarily high number of dog breeds that exist today, the researchers suggest. Dogs have much greater genetic variability than other domesticated animals, such as cats, asserts Vilà. Once agriculture took hold, dogs would have been selected for different tasks, their wolf-like natures becoming a handicap as they became herders and guards. Molecular biologist Elaine Ostrander is of the view that "When we became an agricultural society, what we needed dogs for changed enormously, and a further and irrevocable division occurred at that point."^[20] This may be the point that stands out in the fossil record, when dogs and wolves began to develop noticeably different morphologies. A recent study of African dogs found a high level of mtDNA diversity. The authors suggest that a new view of the domestication of the dog may be needed.^[21] A study by the Kunming Institute of Zoology found that the domestic dog is descended from wolves tamed less than 16,300 years ago south of the Yangtse river in China.^[22] An older report said that all dog mitochondrial DNA came from three wild Asian female wolves.^[23]

Experimental evidence

As an experiment in the domestication of wolves, the "farm fox" experiment of Russian scientist Dmitry Belyaev attempted to reenact of how domestication may have occurred.^[24] Researchers working with selectively breeding wild silver foxes over 35 generations and 40 years for the sole trait of friendliness to humans, created more dog-like animals. The "domestic elite" foxes are much more friendly to humans and actually seek human attention, but they also show new physical traits that parallel the selection for tameness, even though the physical traits were not originally selected for. They include spotted or black-and-white coats, floppy ears, tails that curl over their backs, the barking vocalization, and earlier sexual maturity. It was reported "On average, the domestic foxes respond to sounds two days earlier and open their eyes one day earlier than their non-domesticated cousins. More striking is that their socialization period has greatly increased. Instead of developing a fear response at 6 weeks of age, the domesticated foxes don't show it until 9 weeks of age or later. The whimpering and tail wagging is a holdover from puppyhood, as are the foreshortened face and muzzle. Even the new coat colours can be explained by the altered timing of development. One researcher found that the migration of certain melanocytes (which determine colour) was delayed, resulting in a black and white 'star' pattern."

Before the wolf

There were significantly sized canid carnivores such as Hesperocyon gregarius. The Hesperocyon was a slender, long tailed animal that inhabited the tropical rainforests of North America (about 37 million years ago). The Hesperocyon hunted birds and their eggs, eating seeds and fruit as well. Another canid, Borophaginae, hunted larger animals such as horses and camels. However, the Borophagines met an untimely end about 5 million years ago when the climate cooled and meat supplies began to dwindle. About 2.5 million years ago, the borophagines became extinct. With the Borophagines gone, the other branch from Hesperocyon, known as Leptocyon, began to thrive. The Leptocyon was a slender, narrow-snouted carnivore whose teeth (unlike the Borophagines) had no special bone crushing ability. Leptocyons hunted smaller prey and used the back lower carnassial teeth to crush plant matter. It was for this reason the Laptocyon surivived and the Borophagine did not. In a few million years, the Laptocyon split into a number of different species, leading to the canid family tree: South American Zorros (foxes), wolf-like carnivores (coyotes, jackals, wolves), and fox-like carnivores (red fox and its relatives).^[25][26]

Specialization

As humans migrated around the planet, a variety of dog forms migrated with them. The agricultural revolution and subsequent urban revolution led to an increase in the dog population and a demand for specialization. These circumstances would provide the opportunity for selective breeding to create specialized types of working dogs and pets.

Neoteny in the rapid evolution of diverse dog breeds

The Bulldog is well known for its short muzzle and saggy skin on its face

This rapid evolution of dogs from wolves is an example of neoteny or paedomorphism. As with many species, the young wolves are more social and less dominant than adults; therefore, the selection for these characteristics, whether deliberate or inadvertent, is more likely to result in a simple retention of juvenile characteristics into adulthood than to generate a complex of independent new changes in behavior. (This is true of many domesticated animals, including humans themselves, who have many characteristics similar to young bonobos.) This paedomorphic selection naturally results in a retention of juvenile physical characteristics as well. Compared to wolves, many adult dog breeds retain such juvenile characteristics as soft fuzzy fur, round torsos, large heads and eyes, ears that hang down rather than stand erect, etc.; characteristics which are shared by most juvenile mammals, and therefore generally elicit some degree of protective and nurturing behavior cross-species from most adult mammals, including humans, who term such characteristics "cute" or "appealing".

A hunter with a large pack of beagles, a breed of hunting dogs, 1885

The example of canine neoteny goes even further, in that the various breeds are differently neotenized according to the type of behavior that was selected.^[27]

• Herding dogs exhibit the controlled characteristics of hunting dogs. Members of this group, such as Border Collies, Belgian Malinois and German Shepherds use tactics of hunter and prey to intimidate and keep control of herds and flocks. Their natural instinct to bring down an animal under their charge is muted by training. Other members of the group, including Welsh Corgis, Canaan dogs, and Cattle dogs herd with a more aggressive demeanor (such as biting and nipping at the heels of the animals) and make use of body design to elude the defences of their charges.
• Gun dog breeds used in hunting—that is, pointers, setters, spaniels, and retrievers—have an intermediate degree of paedomorphism; they are at the point where they share in the pack's hunting behavior, but are still in a junior role, not participating in the actual attack. They identify potential prey and freeze into immobility, for instance, but refrain from then stalking the prey as an adult predator would do next; this results in the "pointing" behavior for which such dogs are bred. Similarly, they seize dead or wounded prey and bring it back to the "pack", even though they did not attack it themselves, that is, "retrieving" behavior. Their physical characteristics are closer to that of the mature wild canine than the sheepdog breeds, but they typically do not have erect ears, etc.

The fad for intentionally cross-bred dogs, such as this poodle hybrid, have resulted in the label "designer dogs"

• Scenthounds maintain an intermediate body type and behavior pattern that causes them to actually pursue prey by tracking their scent, but tend to refrain from actual individual attacks in favor of vocally summoning the pack leaders (in this case, humans) to do the job. They often have a characteristic vocalization called a bay. Some examples are the Beagle, Bloodhound, Basset Hound, Coonhound, Dachshund, Fox Hound, Otter Hound, and Harrier.
• Sighthounds, who pursue and attack perceived prey on sight, maintain the mature canine size and some features, such as narrow chest and lean bodies, but have largely lost the erect ears of the wolf and thick double layered coats. Some examples are the Afghan Hound, Borzoi, Saluki, Sloughi, Pharaoh Hound, Azawakh, Whippet, and Greyhound.
• Mastiff-types are large dogs, both tall and massive with barrel-like chests, large bones, and thick skulls. They have traditionally been bred for war, protection, and guardian work.
• Bulldog-types are medium sized dogs bred for combat against both wild and domesticated animals. These dogs have a massive, square skull and large bones with an extremely muscular build and broad shoulders.
• Terriers similarly have adult aggressive behavior, famously coupled with a lack of juvenile submission, and display correspondingly adult physical features such as erect ears, although many breeds have also been selected for size and sometimes dwarfed legs to enable them to pursue prey in their burrows.

The least paedomorphic behavior pattern may be that of the basenji, bred in Africa to hunt alongside humans almost on a peer basis; this breed is often described as highly independent, neither needing nor appreciating a great deal of human attention or nurturing, often described as "catlike" in its behavior. It too has the body plan of an adult canine predator. Of course, dogs in general possess a significant ability to modify their behavior according to experience, including adapting to the behavior of their "pack leaders"—again, humans. This allows them to be trained to behave in a way that is not specifically the most natural to their breed; nevertheless, the accumulated experience of thousands of years shows that some combinations of nature and nurture are quite daunting, for instance, training whippets to guard flocks of sheep.

Notes

References

Further reading

• Klütsch CFC, Seppälä EH, Lohi H, Fall T, Hedhammar Å, Uhlén M, Savolainen P (2010) Regional occurrence, high frequency but low diversity of mitochondrial DNA haplogroup d1 suggests a recent dog-wolf hybridization in Scandinavia. Animal Genetics, online early.
• Morey, Darcy. Dogs: Domestication and the Development of a Social Bond (Cambridge University Press; 2010) 384 pages; uses zooarchaeology to explores ties between humans and canines over the past 15,000 years with a focus on the New World and Arctic regions.

External links

• Wayne, Robert K.. "Molecular evolution of the dog family". http://www.idir.net/~wolf2dog/wayne2.htm. Retrieved 2007-04-11. 
• Brisbin Jr., I.L.; T.S. Risch (15 April). "Primitive dogs, their ecology and behavior: Unique opportunities to study the early development of the human-canine bond.". Journal of the American Veterinary Medical Association 210 (8): 1122. PMID 9108912. 
• Trut, L.N. (1999). "Early Canid Domestication: The Farm-Fox Experiment". American Scientist, 87: 160-169 87: 160–169. doi:10.1511/1999.2.160. 
• Mestel, Rosie (October 1, 1994). "Living World: Ascent of the Dog". http://discovermagazine.com/1994/oct/ascentofthedog434.