THE STATUS OF DOMESTIC DOGS as “man’s best friend” indicates that they evoke more emotional responses than do other animals. Indeed, people often lose their objectivity over questions about dogs. Terms such as beauty, intelligence, and loyalty are frequently ascribed to dogs as though they are qualities inherent to dogs. To understand the relationship between humans and dogs requires, however, that we know more about the natural history of domestic dogs as they relate to their wild ancestors and about the history and process of their domestication.
Why Is Domestication Important?
Although most people have no difficulty relating to dogs as something very special in their lives, probably few of them have contemplated the evolutionary significance of domestic dogs in the history of human civilizations. First of all, dogs were the first domestic animal, a fact that has had profound consequences in human history. Domestic animals are so ubiquitous now that we often take for granted both them and all the conveniences they offer us in our daily lives. As Jared Diamond (1997) has pointed out, however, large domestic animals can be one of the deciding factors in the rise and fall of civilizations. Domestic animals are vital sources of food, labor, transportation, and clothing—all essential items for societies. When civilizations clash, those with the right domestic animals (such as horses in a war) can have an overwhelming advantage that tilts the eventual outcome in their favor.
The very idea of a certain invention is often more important than the way that invention is achieved. Once an idea is born, people find different ways to realize it. In the case of domestication, the very idea that animals can be harnessed to human betterment is much more important than which specific animal to domesticate and in what sequence. Consequently, if dogs are truly the first animal to be domesticated, then they can rightfully claim to have instilled in humans the notion that animals can be raised in human dwellings, a radical idea that has had profound effects on the earth’s biota (as in the case of dingoes in Australia), on people’s relationships with animals, and on the way people live. Once such a notion became established, domesticating other animals would be a matter of course by choosing the right species and by experimenting with the right techniques for domestication through trial and error.
It is perhaps no coincidence that the first domestication happened to the wolf, a carnivoran with high social intelligence and well adapted to living with fellow beings. Diamond (1997) notes that most large mammals are highly resistant to human domestication. There are only about 14 successful and economically important large domestic mammals, most of them originally from Eurasia and all of them herbivores. None of the 14 large herbivores possesses the kind of social complexity that dogs have and presumably were initially far less domesticable than dogs. Intriguingly, all of the top five domestic mammals in terms of economic importance—sheep, goats, pigs, cows, and horses—were first domesticated in and around the Fertile Crescent in western Asia (a possible exception is the pig, which may have been first domesticated in China). It is perhaps no coincidence that these five domestic mammals are also among the first to show up in archaeological records in western Asia (between 8,000 and 4,000 years ago). One of the earliest records of domestic dogs is from Israel about 12,000 years ago, well before the records of the big five. Could it be that people in the Middle East and western Asia first learned the ideas of domestication through their association with dogs and quickly applied the same idea to other large herbivores? If so, such an idea was nothing short of revolutionary, leading people to work with large, wary herbivores, to harness greater power, to travel longer distances, and to carry more goods than they otherwise would have been able to by their own means.
History of Dog Domestication
In 1977, a father-and-son team, Stanley J. Olsen and John W. Olsen, advanced the thesis that the likely area of origin for domestic dogs was China. They subsequently attempted to tackle systematically the fossil records of domestic dogs at a time when issues about animal domestication were increasingly attracting archaeologists’ attention (Olsen 1985). They started by assuming that a small subspecies of wolf would be the most likely candidate, and the modern Chinese and Mongolian subspecies Canis lupus chanco seemed to fit the bill. The Olsens then studied the Canis materials from the Zhoukoudian sites, about 50 km southwest of the capital city, Beijing, famous for the discovery of Peking Man (“Peking” is the Wade-Giles spelling of “Beijing”), a middle Pleistocene record of Homo erectus (500,000 years ago). The Canis in question had been named Canis lupus variabilis by Pei Wenzhong in 1934, and Stanley Olsen pointed out that in size and morphology it was intermediate between C. lupus chanco and a species represented by a domestic dog skull from a Chinese Neolithic site in Hemudu.
As the first synthesis of then known fossil records, Olsen’s effort to make the connection between Chinese fossils and domestic dogs was certainly a worthy one. Olsen, however, realized that the co-occurrence of hominid and canid in the middle Pleistocene (around 500,000 years ago) was far too early to be associated with domestication events and thus cautioned that “although this association of hominids and wolves at this early period does not imply in any way either taming or early domestication, it does place both genera of animals in contemporary association that apparently continued until such time that these events did occur” (1985:42). His assumption of the continuity of hominids and canids in China is probably incorrect. Current knowledge suggests that the archaic Homo erectus likely had no direct relationship with Homo sapiens, who arrived much later in Eurasia directly from Africa (although an alternative multiregional hypothesis does exist that claims links between H. erectus and H. sapiens). On the canid side, our own research also indicates that the middle Pleistocene Zhoukoudian canid actually belongs to a species of its own, Canis variabilis, which is only distantly related to the modern wolves. Therefore, the Zhoukoudian materials probably have little relevance to dog domestication. This is not to say that China could not have been the center of origin for domestic dogs, but that the comparison of C. variabilis materials and domestic dogs is probably not appropriate.
At about the same time the Olsens (1977) proposed that domestic dogs had originated in China, two Israeli zooarchaeologists, Simon J. M. Davis and François R. Valla (1978), announced the discovery of a canid puppy buried with a human in Mallaha, an archaeological site near the old Huleh Lake in the upper Jordan Valley of Israel. The Mallaha site belonged to the Natufian culture, which is believed to have been one of the last hunter-gatherer groups, dating to approximately 12,000 to 10,000 years ago. The Natufian people lived in circular dwellings in what were perhaps the earliest permanently settled villages, foreshadowing later agricultural societies. However, it is the way a human was buried with a dog that really captured the archaeologists’ imagination.
In the entrance to the dwelling, 25 cm below a large slab of limestone, which is often indicative of the presence of a burial, a human skeleton lies curled in a fetal position on its right side. The skeleton belongs to an individual of old age, judging by the wear on the teeth, but its sex is uncertain due to damage to the pelvic area. The left hand of this skeleton reaches across the head and is partly buried below the head. This hand is wrapped around the chest of a puppy, and the person’s head is resting on top of the puppy. The intimacy of these two individuals is truly remarkable and argues strongly for a relationship that is more than a mere casual association.
Analysis of the teeth and of the fusion sequence of the long bones in the puppy skeleton indicates that it is of an individual around four to five months old. Adult canid materials were also discovered, including a lower jaw from the same dwelling and an isolated lower carnassial tooth (m1) from the terrace of Hayonim in western Galilee (the latter specimen is associated with a Natufian assemblage that has yielded a 14C date of 11,920 B.P.). The size of these three specimens falls in the lower extremes of modern wolves in the Middle East, but is far smaller than late Pleistocene wolves (around 45,000 to 14,000 years ago), which tended to be much larger due to the colder climate at that time. However, the Natufian canids fall comfortably in the size range of living dogs and of fossil dogs found at archaeological sites, although they tend to be on the upper end of the range for domestic dogs. Analyses of other morphological features, such as the degree of crowding of the premolars, a character of domestic dogs due to the shortening of their rostrum, also place the Natufian canid in the transitional position between wolves and dogs. It thus seems reasonable to postulate that the Natufian canid belonged to an early dog species that has subsequently gotten smaller. Davis and Valla speculated that “the puppy … offers proof that an affectionate rather than gastronomic relationship existed between it and the buried person” and that it was “man-the-hunter” who had domesticated the wolf (1978:609).
This example brings us to another distinction of dogs: they and cats (which were domesticated later and also in the Near and Middle East) are the only carnivores among early domesticates. Such a distinction is important because carnivorans share the same food resources as humans. Large herbivores convert vegetation not usable by humans into consumable products such as meat and milk. It is difficult to imagine that dogs were first raised for food because they consume more humanly edible food than they produce. Therefore, for dogs to have been maintained with resources precious for humans themselves, the dogs must have served a vital function that was otherwise unobtainable. Davis and Valla (1978) were probably right that dogs did not serve as human food until much later, when agricultural surpluses could sustain more dogs (this is not to say that early humans did not eat dogs at all; they probably did opportunistically). Was it collaborative hunting or companionship that provided the compelling reason for a human to keep a dog?
Other discoveries of early domestic dogs have occasionally been reported in scholarly journals, but they are often difficult to verify due to the poor quality of the records. For example, German scientist Günter Nobis reported in 1979 a partial right jaw of a domestic dog from a Cro-Magnon site in Oberkassel, near Bonn. This jaw is dated to around 14,000 B.P. Only four teeth are left in the jaw (canine, fourth premolar, and first and second molars), and it appears that the second and third premolars were lost during life (the alveoli for the roots of these two teeth are no longer visible). If the remains’ identity as a dog can be confirmed, this find would constitute one of the earliest records of domestic dogs, and Nobis speculated that the German dog may represent a domestication independent from those at the other sites in Eurasia. However, it is difficult to ascertain a dog identity for the Oberkassel specimen, given its lack of skeletal regions that allow diagnosis, such as the forehead area and coronoid process, parts of the cranial anatomy that are important in the identification of dogs. Consequently, specialists have not widely accepted this German record as evidence of early dog domestication.
Mikhail V. Sablin and Gennady A. Khlopachev (2002) have reported two nearly complete dog skulls from the upper Paleolithic site of Eliseevichi 1 in the Bryansk Region of the central Russian Plain. Radiocarbon dates on associated fauna (mostly mammoth, arctic fox, and reindeer) have yielded a range of 17,000 to 13,000 years B.P.—the upper range of the date indicating that this find would be the earliest record for dogs. The two skulls are of large, northern wolf size. The main dog features on the Eliseevichi skulls are a short rostrum and a broad palate, characters that supposedly resemble those of the Siberian husky. Furthermore, on one of the skulls (MAE 447/5298), there is a large hole on the left side of the braincase, which Sablin and Khlopachev interpret to be a man-made break for access to the brain; that is, they postulate that its brain was consumed by humans. Intriguing as the new Russian materials may be, evidence for the argument that the Eliseevichi skulls belonged to domestic dogs appears to be ambiguous. The P4 on ZIN 23781(24) (the other skull from Eliseevichi) has a length of 27.3 mm, compared with the P3 length of 17.4 mm. The size proportion between P3 and P4 is that of a gray wolf rather than that of a dog, which would have a relatively reduced P3 to accommodate a shortened rostrum. However, one cannot rule out that the Eliseevichi canid was a very primitive, wolflike dog that was in a beginning phase of domestication—morphology at this stage of divergence is usually incapable of unambiguously resolving the difference. DNA studies may help, but until some DNA can be extracted from the specimens, the identity of the Russian materials will likely remain uncertain.
Morphology of Domestic Dogs
Although most authors agree that some kind of a southern (Chinese or Arabian) wolf gave rise to domestic dogs because of the overwhelming morphological and genetic similarities between the wolf and the dog, these similarities become a confounding problem in distinguishing the remains of dogs from those of wolves at archaeological sites. As a result, the discoveries of supposed domestic dogs at the archaeological sites in China, Israel, and Europe have been disputed due to the absence of a clear criterion to distinguish dogs from wolves in fossil materials. For example, Stanley Olsen (1985) dismissed the Natufian materials as possibly an aberrant specimen of wolf, whereas Tamar Dayan (1994), an Israeli evolutionary biologist, marshaled further evidence that the Israeli burial materials were more likely be domestic dogs. Morphological characteristics, or the lack thereof, lie at the center of the issue.
We begin by looking at modern dogs in order to establish a firm basis of comparison. With a few exceptions, modern dogs are generally smaller than wolves. Along with smaller size, adult dogs also exhibit juvenile characters as though they have never grown up, including such features as short snouts, floppy ears, and large eyes. Biologists call these characters paedomorphic features, referring to the phenomenon that animals that have reached sexual maturity still retain juvenile features. Some evolutionary biologists have speculated that dogs with paedomorphic characters appeal to the human sense of affection. In other words, a “cuteness factor” seems to be at work in bridging the gap between dogs and humans. A paedomorphic dog may also be more submissive, a vital characteristic for a successful domestic relationship.
Skulls and jaws are the most commonly preserved elements at archaeological sites. Dogs tend to have a more domed forehead (easily seen in the profile view, which shows an obvious bulge above the orbit, or bony socket, of the eye) as well as a shortened rostrum and jaws. Associated with the short rostrum are more crowded premolars because there is less room in the jaws. To alleviate this crowding, the premolars are also reduced in size. In addition, some authors claim to have observed in dogs a narrower and more concave posterior border in profile view of the coronoid process (the upwardly protruding bony plate above the mandibular joint) of the lower jaw. The temporalis muscle inserts on this process. This muscle closes the jaw and is much more robustly developed in wolves. Again, its form in dogs is part of the paedomorphic form of the skull.
These morphological generalities work in identifying most modern domestic dogs. When it comes to distinguishing dogs from wolves in the remains found at archaeological sites, however, many of these characters break down as dogs become more wolflike as the archaeological records move closer to the origin of domestic dogs. At a certain point, it becomes almost impossible to ascertain the true identity of a doglike specimen at an archaeological site. At that point, a definitive human–dog association, as seen in the Natufian burial site, becomes more valuable.
A Russian Experiment on the Fox
Morphological characters of dogs are certainly important, particularly in archaeological context, but how dogs were modified behaviorally is equally insightful in gaining a comprehensive understanding of dog domestication. Behavioral considerations are especially relevant given the fact that one of the most critical factors in domestication is the successful modification of animal behavior toward humans. An elegant Russian experiment helped to make this point clear.
Dmitry K. Belyaev believed that the key factor that initiated all the genetic and morphologic changes of wolves into domestic dogs was behavior modification. In particular, the amenability to domestication, or tameability, was the most important factor in determining the success or failure of domestication. Belyaev and his comrades began working with foxes in a long-term experiment of more than 40 years (described in Trut 1999). The Russian geneticists selectively bred foxes for a single trait: tameness. After about 30 to 35 generations, a large percentage of the foxes showed the unmistaken signs of domestic animals: they were docile and eager to please. Remarkably, these behavioral selections also brought about several physical changes. The foxes began to have floppy ears, short and rolled tails, short legs, lighter coat colors, and malocclusion of the teeth (the upper jaw shorter than the lower jaw, as seen in some bulldogs)—characters common in domestic dogs, as Charles Darwin noted long ago.
Belyaev explained his experiment in terms of selection for tameability and its consequence for developmental processes. As noted, many of the physical and behavioral characters in dogs may be seen in light of the paedomorphic process—that is, the retention of juvenile features in adults. After all, to be tame is to shut down the fear response, a behavior that serves wild species well. In a sense, domestic dogs never grow up and are stuck in juvenile stages both behaviorally and morphologically.
Genetic History of Domestic Dogs
Whereas archaeologists are rarely able to find more than a few fragmentary fossils to hint at the records of early dogs, molecular biologists have increasingly turned to DNA for information about the history of dogs. Changes in DNA over the generations are often preserved in the vast and rich sources of the dog genome. In December 2005, a complete map of the DNA code of a female boxer was published by a group led by Kerstin Lindblad-Toh. The dog genome, as it is commonly known for this kind of project, was the first to be sequenced among the 271 species of carnivorans. In fact, the complete genome of only a few organisms has been sequenced; among mammals, the dog is only the fifth species to be sequenced (the first four are the human, the chimpanzee, the mouse, and the rat). The selection of the dog to reveal its complete genetic code thus highlights the dog’s special status in human society and its importance in medical and scientific research.
Among the results of the dog genome study is the ability to identify a small collection of rapidly evolving sequences in various parts of the dog genome that can be used to trace the phylogenetic relationships among living canids. A selection of about 11,100 base pairs of DNA for 30 of the 34 living canid species has permitted the reconstruction of the phylogenetic tree of the family Canidae. Not surprisingly, domestic dogs and wolves are closely related, as morphologists and molecular biologists have long suspected.
Molecular biology has also contributed to a recent controversy over the time and place of origin of the first domestic dogs. Carles Vilà and his colleagues proposed in 1997 that multiple domestication events happened as early as 100,000 years ago. Such an estimate is based on the concept of the molecular clock, which assumes a steady rate of change in molecular composition in a certain region of the DNA. By using one or more existing fossil records as a minimum starting point for the origin of a particular species, a molecular biologist can estimate the rate of base-pair changes in a given amount of time. For example, Vilà and his colleagues used 1 million years as the minimum time of divergence between wolf and coyote. Thus a small amount of molecular difference in the mitochondrial DNA sequence between wolf and dog can be translated to a time period of 100,000 years since the divergence of these canines.
This estimate places dog domestication far earlier than any of the existing fossil evidence indicates. Vilà and his group suggested that early domestic dogs were far too wolflike to be recognizable in the fossil record and that it was only after about 15,000 years ago that artificial selection achieved enough morphological difference for us to be able to recognize dogs in the archaeological records.
In contrast to the study by Vilà and his colleagues (1997), Peter Savolainen and his co-workers (2002) contend that the first domestication of dogs was done in East Asia about 15,000 years ago. Savolainen analyzed a large sample of 654 dog breeds, each with 582 base pairs of mitochondrial DNA, and found a larger genetic variation in East Asia than in other regions, which suggests an East Asian origin for the domestic dog. Because mitochondrial DNA is inherited from the maternal side, Savolainen’s group could trace modern dogs to five female wolf lines. Of these lines, a lineage (“Clade A”) that includes three closely linked positions in Chinese and Mongolian wolf genes has the highest genetic diversity and thus was postulated to contain the ancestral population of domestic dogs.
Dogs of the New World
Despite these controversies, a general consensus exists that dogs were domesticated prior to 15,000 B.P. and that domestic dogs originated in Eurasia. But what happened to dogs elsewhere? In particular, what is the relationship between New World dogs and their Old World counterparts? The dog was the only domesticated species that was present across Eurasia and the Americas before the arrival of Columbus in the fifteenth century. Wolves, in contrast, had a Holarctic (the northern continents of Europe, Asia, and North America) distribution and thus had every opportunity to be associated with humans when they crossed the Beringian land bridge. As noted, fossil evidence indicates that dogs were domesticated around 14,000 to 12,000 years ago, a time period that either closely coincides with the first arrival of humans in the New World, about 15,000 years ago, or even slightly lags behind the human crossing of Beringia. If this timing is taken literally, then dogs could not have been with the first humans to come to the Americas. The questions we have to ask therefore are: Was there any connection between the New World dogs and the Old World dogs, or were the New World dogs independently domesticated by the Native Americans?
To answer these questions, Jennifer A. Leonard and her colleagues (2002) devised another genetic study of dogs. Modern New World dogs cannot be used in such a study because their ancestors were likely to have been interbred with dogs brought by European colonists, and their genetic makeup would not reveal their ancestral conditions. Leonard and her colleagues, therefore, extracted DNA from the bones of 37 dog specimens recovered from pre-Columbian archaeological sites in Mexico, Peru, and Bolivia, which ensured that no interbreeding with European dogs had occurred. Analysis of this ancient DNA revealed that native American dogs are closely related to Eurasian ancestral lineages, suggesting that dogs accompanied late Pleistocene humans when they crossed the Beringian landmass. If so, dogs must have played a remarkable role in the early occupation of the Americas, a case in which a domestic animal assisted humans to settle in a new world.
There is also circumstantial fossil evidence that humans brought their dogs along when they first crossed Beringia to arrive in North America around 15,000 years ago. Ted Galusha, the late collector and curator of the Frick Laboratory at the American Museum of Natural History in New York, noted that several skulls from the late Pleistocene deposits near Fairbanks, Alaska, were extremely short-faced for wild wolves and approached the facial proportions of modern Eskimo dogs. Galusha did not publish his results, which he turned over to Stanley Olsen for further study. Olsen was convinced that these fossil skulls belonged to the forerunners of the Eskimo dogs. However, true wolves were also undoubtedly present in the same deposits, making it difficult to resolve the question of whether the short-faced individuals were part of the range of variation within the wolf population. The genetic study of 11 samples from Alaska by Leonard and her colleagues (2002) confirms the individuals’ identity as dogs.
Australian Dingo
The Australian dingo (Canis lupus dingo) is another interesting example of humans’ first domestic animal braving a new world together with its masters. When the first Europeans arrived in Australia in the eighteenth century, the dingo was the only large placental mammal there, besides the Australian Aborigines, among a diverse group of marsupials. The name “dingo” comes from the Eora Aboriginal tribe of southeastern Australia. In external morphology, dingoes, with their tan coat color, closely resemble domestic dogs, particularly those from southern Asia. In their behavior, however, they tend to be more independent than dogs, and they roam wild over Australia. Although occasionally used for pets or hunting, dingoes generally resist full domestication by modern humans.
Almost from the beginning of the Europeans’ arrival, there was debate about the dingo’s possible origin as a feral dog and its possible relationship with southern Asian dogs. Did it arrive in Australia as a semidomestic animal and then secondarily become feral, or did it come on its own as a wild canid? Because there was substantial open water of at least 50 km between islands along the Southeast Asian archipelago, even during maximum glaciation when the sea level was at its lowest, could a wild canid have swum across the sea without human aid? Or were dingoes carried on boats by Southeast Asian fisherman and sea cucumber harvesters? Rodents and bats are the only placental mammals to have made it to Australia on their own, presumably by island hopping, rafting, and flying from Southeast Asian islands. No large mammal has made the journey, and traveling by boat with humans seems to be the most likely scenario for the dingoes.
Dingoes closely resemble southern Asian dogs and wolves morphologically. In archaeological records, the earliest definite dingo remains date to approximately 3,500 years ago. Peter Savolainen and his colleagues (2004) again applied molecular techniques to answer questions about the connection between New World and Old World dogs. Their analysis of mitochondrial DNA from 211 Australian dingoes, wolves, and domestic dogs suggests that dingoes largely have the genetic makeup of domestic dogs. They share a particular gene type called A29 with dogs from East Asia and arctic America. By calculating the amount of genetic divergence, Savolainen’s group concluded that dingoes arrived in Australia around 5,000 years ago in a single migration event, a date roughly consistent with the archaeological dates. If these dates from the archaeological and the molecular evidence are anywhere close to the true time of the dingoes’ arrival, then the dingoes must have come by boat in a much later wave of human dispersal than that of the first settlers of Australia, about 50,000 years ago.
Despite these studies, questions remain about how dingoes arrived in Australia. Did they become feral because they were not fully domesticated in the first place? Or did they find easy prey in the form of marsupials and thus have little use for humans after landing in Australia? On an island continent without other placental predators, dingoes probably faced relatively light competition and found it easy to stay wild. Whatever the actual scenario, dingoes are a unique example of a carnivoran that invaded a new continent by an unconventional means (boats), through either natural association with or domestication by humans, and ultimately became the top dog in the community.
Why Were Dogs Domesticated First?
As noted, dogs were the first animal to be fully domesticated by human hunter-gatherers, and, as such, they played a unique role in human history, instilling the idea that animals could be harnessed for human purposes. As the first and presumably the only domestic animal at the time, dogs accompanied humans to the Americas and, in a sense, used human technology (the boat) to expand into Australia. They are thus pivotal in the evolution of human societies, and, to a certain extent, their importance is on a par with that of such key inventions as stone tools, bronze, and agriculture. One is thus tempted to ask the question: Why were dogs domesticated first? Why did it take a highly social carnivore to give humans the idea of domestication? Did the mammal have to be intelligent to be able to coexist with us? Or had dogs reached a level of sociability similar to that of humans? The beginning of sedentary human communities is associated with the origin of agriculture. Perhaps dogs were the only domestic animal that fit the hunter-gatherer lifestyle. Perhaps humans observed and learned from wolves’ pack-hunting techniques? Could it be, as Stanley Olsen (1985) speculated, that humans as pack hunters themselves were brought close to the wolves because of their shared tactics in obtaining prey?
Or perhaps hunting had nothing to do with the early contact of humans and wolves. Olsen was probably the first to put this idea on the table: “It is possible that hungry wolves were enticed (not necessarily by human design) to come closer to a campfire, where meat was being cooked and the refuse discarded in the immediate vicinity of the camp. Perhaps wolves that had attached themselves loosely to human habitation areas would consider such camps as their home territory, and the warning growls toward intruders would also warn the human inhabitants of the approach of such outsiders” (1985:18).
The conventional notion of domestication often contains an element of human design. The idea that early humans intentionally selected certain desirable traits or eliminated undesirable traits, thereby creating a breed that suited certain human needs, is rooted in the modern practice of artificial selection in agriculture and animal husbandry. Although such societal intentions are part of decision making when it comes to modern practices of domestication, human motivations in historical context are difficult, if not impossible, to fathom and cannot be demonstrated empirically by the archaeological records. To circumvent this difficulty, Darcy F. Morey (1994) proposed a different approach to the question of the initial domestication of dogs. He suggested that it is not necessary to presume human intentions to make sense of early domestication.
Morey indicated that an evolutionary perspective would do away with anthropocentric approaches. He argued that focusing on the human role in domestication ignores the evolutionary stakes for the participating animals. The success of the dog–human association in almost every conceivable environment on every continent suggests that dogs have profited well from this arrangement, particularly in contrast to the dwindling wolf populations worldwide. Perhaps the ancestral dogs (or wolves) found such an association advantageous and thus willingly participated in an experiment that eventually benefited both parties. Such a view does not have to invoke the conscious act of humans to tame a wild carnivore and thus is free from the intractable and messy arguments about human intentions.
Morey (1994) suggested that late Pleistocene hunter-gatherers (from around 12,000 years ago) were in regular contact with wolves because both hunted many of the same prey. Some wolf pups were then probably incorporated into a human social setting, and some adopted pups survived to adulthood. These wolves must have learned that subordinate status to dominant humans was the condition to be tolerated in human society and that being able to solicit food from people was also a valuable skill. The diet of wolves that lived around human settlements must have changed as well, from predominantly meat to a variety of meat and plants, some of it garnered or scavenged from humans’ scraps. In such a domestic setting, smaller body size is also a favorable trait. Small size and submissive behavior can easily be achieved by paedomorphosis during individual development.
Behavioral biologists Raymond Coppinger and Lorna Coppinger (2001) have taken Morey’s (1994) model of the dog’s self-domestication a step further. They envision the following scenario for dog domestication. First, agriculture created human settlements, a way of living that contrasted with the nomadic hunter-gatherer lifestyle. In every human village, there were discarded products such as bones, carcasses, grains, fruits, as well as human waste. The Coppingers argue that this human dump site became the first niche for some wolves. These wolves would frequent the garbage dump to gain access to the new food source. Those wolves that were less frightened by humans tended to be more successful in making a living this way because they would waste less energy evading humans when they saw them approach. Such wolves by definition were more tame, thereby leading to the early association of wolves and humans, which ultimately led to the domestication of dogs.
One of the important distinctions of the Coppingers’ (2001) model is that it does away with wolves’ sociality in the equation. They argue that sociality has nothing to do with domestication. In fact, they suggest that the incipient dogs subsisted on low-quality food and must have had smaller brains (along with a smaller body size, smaller heads, and smaller teeth) than wild wolves, which had to coordinate the pack (chapter 5). Contrary to the popular belief that dogs possess a high intelligence, the Coppingers point out that dogs not only have a smaller brain than wolves, but also are less intelligent.
Why Were Canids Domesticated?
Although the garbage-dump model is an attractive alternative, perhaps the biggest difficulty with Raymond and Lorna Coppinger’s (2001) hypothesis is that once the sociality of wolves is deemed irrelevant, the domestication of dogs is no longer a unique association of two social species: Homo sapiens and Canis lupus. Other hypocarnivorous carnivorans are also adept at feeding on human waste dumps. Raccoons and bears are two well-known examples, although the former were not in contact with humans until the Paleo-Indians arrived in North America, at which time humans already had dogs as their companions. Bears, though, were around Mesolithic hunter-gatherers, but were never domesticated. But bears are not social predators and may not be easily domesticable.
Before we address the question of why canids first, let us broaden the perspective somewhat to consider the spectrum of early domestic animals. With the exception of cats, which had a relatively minor economic role (catching mice) in early human societies, dogs stood alone as the only carnivore among 14 successfully domesticated large herbivores. If we get past the anthropocentric notion of domestication as a human invention, which Darcy Morey (1994) and the Coppingers (2001) suggest we do, then humans might have been educated about the possibility of living with other animals by association with certain species. However, early humans were hunter-gatherers, and most of the large herbivores were probably their prey. It is therefore difficult to envision that any of the prey species were not especially guarded when encountering humans. Furthermore, there is little that would entice these herbivores to come close to human settlements; the same human waste dumps that were attractive to the wolves as a supplement to their diets were hardly worth visiting from the herbivores’ point of view.
In contrast, carnivorans as predators are less afraid of approaching other species; they have to do it routinely in order to catch their prey. This is not to say that early humans and wolves did not have an antagonistic relationship; all top predators occasionally confront other predators either to loot the other’s spoils or to attack the other. But it is perhaps far easier for a carnivoran to approach another predator, such as a human, than it is for an ungulate herbivore to approach a predator. Therefore, it may have been inevitable that the first domestic animal came from a member of the Carnivora.
Various families of carnivorans were part of the mammalian fauna surrounding early Homo sapiens. We can probably quickly eliminate Mustelidae (weasels, badgers, and otters), Viverridae (civets), Herpestidae (mongooses), and Procyonidae (raccoons and ringtail “cats”) as families of small carnivorans that could not play the same role as dogs in human society (the New World Procyonidae were not even in contact with humans until the Paleo-Indians expanded into the Americas). That elimination leaves four families of large carnivorans to consider: Felidae (cats, lions, and tigers), Ursidae (bears and giant panda), Hyaenidae (hyenas and aardwolves), and Canidae. Although some felids can be highly social predators, they are hypercarnivores (chapter 4) that have essentially a pure meat diet. In other words, human dump sites were probably not appealing to them and, in any case, probably could not have provided them with enough food for subsistence (cats obviously were an exception because they are small enough not to pose a burden on humans). Ursids, as a generalized omnivore, would also have found garbage dumps attractive, but, as noted, bears do not have the right temperament to be readily tamed. Even if they could be tamed, they lack the wolves’ running abilities and hunting skills and thus would not have contributed enough for a long-term relationship with humans. Finally, hyaenids are close enough to canids in their dietary requirements and social behavior to have frequented human dump sites. Hyaenids, however, appear to lack the requisite temperament to be domesticated.
Thus Canidae is the only group of carnivorans that possesses the right qualities to be domesticated. Canids are not too large and thus can be dominated by humans, and they are mesocarnivorous enough to be able to handle a variety of foods. Such a dietary preference probably predisposed canids to scavenge human garbage dumps. A final critical factor is dogs’ temperament. Among all domestic animals, dogs are among the most submissive to humans. Even if we discount the longer history of dog domestication, dogs seem to be more predisposed toward humans than other domestic animals. Cats, for example, have almost an equally long history of domestication (approximately 9,000 years), but as all cat owners know, cats are far more independent and less solicitous toward humans—millennia of artificial selection has not brought about a cat that is as submissive to humans as dogs are. This submissive attitude toward humans probably also facilitated the training for other functions once dogs fully integrated themselves into human societies: as companions, hunting partners, sentinels, guards, and so on.
In a study of social cognition in dogs, Brian Hare and his colleagues (2002) suggest that dogs possess a high level of social-communicative skills with humans. Their study measured the ability of dogs to follow visual, gestural, and other cues to accomplish tasks. Dogs in such experiments outperformed chimpanzees and wolves in comprehending human communicative signals. Hare and his colleagues concluded that dogs’ ability to use human social cues was developed in the process of domestication. We caution, however, that many studies demonstrating special abilities by domestic animals are probably testing mental capacities acquired during long processes of domestication and that these skills are not likely to have been present during initial domestication.
Regardless of dogs’ initial capability, a combination of morphological and behavioral characteristics seems to have made them uniquely qualified to be incorporated into human society. In a sense, it was almost inevitable that dogs were the first animals to have a mutually beneficial relationship with humans.
The Dizzying Array of Dog Breeds
With the possible exception of the goldfish, with its often bizarre morphology artificially bred from a species of carp, domestic dogs are perhaps the most differentiated from their wild ancestors. The wonderful variety of sizes, shapes, and coat colors they have would make an unsuspecting taxonomist place them in separate families if he or she did not know that these traits are artificially selected. By contrast, domestic cats are far more uniform in their morphology, and coat color is the main source of variation. Such a tremendous amount of variation within a single subspecies of wolves is almost inconceivable in nature. (These various traits have no survival value and would have been quickly eliminated if they were to occur naturally.) Nonetheless, these variations seem to indicate a vast reservoir of genetic variability not seen in other mammal species.
Why are dogs so variable? One clue may lie in why cats are so invariable. Jill A. Holliday and Scott J. Steppan (2004) attempted to address this question by comparing the morphological diversity of fossil cats and dogs. Cats are extreme hypercarnivores, and their entire muscle-skeletal-dental system seems to be perfected for a single purpose—to catch prey. With the exception of the saber-toothed cats, much of the history of cat evolution consists of species that resemble one another closely—so closely, in fact, that the fossils of different extinct cats are often difficult to distinguish from one another. It appears that cats’ cranial and dental morphology has been fine-tuned to the point that changes to such a body plan would perhaps be detrimental to their survival, so natural selection has tended to eliminate these changes. Holliday and Steppan thus ask the question: Does increasing hypercarnivory limit morphological variation? Framed in scientific terms, the question becomes: Are increasing degrees of specialization correlated with decreasing phenotypic variation?
The answer to this question is “yes”: specialization to hypercarnivory has a strong effect on subsequent morphological diversity, particularly regarding the jaws and dentition. In layman’s terms, this means that cats tend to be locked in their morphological system or, alternatively, that there has been a strong selective pressure to eliminate variations from a nearly perfect system. Canids, however, are more flexible in their dental and cranial morphology because primitively they are mesocarnivores. Therefore, evolutionarily speaking, canids appear to have more built-in flexibility than felids have.
However, we need to be cautious against too literal an application of this discovery. For one thing, Holliday and Steppan’s (2004) measurements are of skeletal and dental areas that are often evolutionarily the most variable. These areas do not necessarily change that much among dogs. For example, the relative proportion of the shearing and grinding part of the dentition, an indication of the degree of hypercarnivory (chapter 4), is not nearly as variable as the proportions of other parts of the dog skeleton. Instead, much of the skeletal variation in dogs is related to the timing of development. In other words, paedomorphosis probably played a larger role in the great morphological variations in the body size and cranial proportions of dogs.