Dog Sense (8 page)

Since 1997, there has been a steady flow of more detailed studies of dog and wolf DNA, and, as a result of these, our conclusions about the exact moment of the dog's domestication have changed and are still changing today. DNA technology is relatively new, and while it may give unequivocal answers when used for “fingerprinting” (e.g., confirming the parentage of a particular puppy in a dispute over pedigrees), its use in reconstructing events long since passed is much more open to interpretation. Different types of DNA can give different answers; for example, the story told by the type contained in the nucleus of most mammals' cells (the subcellular organelle where paternal and maternal DNA mix) is often different from that told by the type associated with other parts of the cell, such as the mitochondria (which contains only maternal DNA). As new analyses have appeared and been integrated into the picture, the estimate that dogs might have been domesticated more than a hundred thousand years ago has since been revised down considerably—to between fifteen thousand and twenty-five thousand years ago.

One reason for this drastic downward revision is that problems have been found in the method used to calculate how much time has elapsed
since two animals had a common ancestor. The DNA most commonly used for this purpose comes not from the nucleus but from the mitochondria (whose genetic content is abbreviated as mtDNA). Very occasionally, only once every few thousand years, mitochondrial DNA mutates, such that mother and daughter, who would otherwise have identical mtDNA, exhibit sequences that differ at just one location (this applies only to mothers—fathers do not pass on any mtDNA to their offspring, male or female). Unlike other kinds of mutation, these changes have no effect on the health or fecundity of the animal, and so are passed on “silently” down the generations, spreading throughout all the daughter's descendants. By counting the number of differences between two individual animals' mtDNA, scientists can estimate the amount of time the two individuals' lineages have been diverging—and can thus form an idea of how long ago their most recent shared female ancestor lived. The bigger the number of distinct mutations, the older the two animals' joint lineage must be.

Errors slip into this sort of mtDNA dating when scientists, having determined how many unshared genetic mutations exist between two individuals, attempt to figure out just how often these mutations may have occurred in both animals. The regularity of these mutations varies from one kind of animal to another. However, scientists know from the fossil record and from carbon dating that the dog's ancestor, the wolf, diverged from the coyote about 1 million years ago. A simple comparison between the number of differences between dog and wolf, and the number between wolf and coyote, suggests that the dog and the wolf had been separated for about one-tenth of that time—in other words, for about a hundred thousand years. This calculation, however, relies on the mutations in mtDNA occurring at the same rate in domestic and wild animals. Since the 1997 study, it has become apparent that mtDNA mutations occur more frequently in domesticated animals than in wild ones. The same comparative method used in the 1997 study has consistently overestimated the time since domestication for virtually every animal to which it has been applied: For example, the DNA of the pig, probably first domesticated nine thousand years ago, suggests a domestication of sixty thousand to five hundred thousand years ago; and the horse, more than three hundred thousand years ago
instead of about six thousand. The mutation rate must therefore be faster during domestication than in the wild, speeding up the rate at which mtDNA changes from once every few thousand years to once every few hundred. Studies of other species suggest that this accelerated rate is a side effect of chronically high levels of stress hormones, caused by living in crowded conditions and in close proximity with man. Thus the estimate of a hundred thousand–plus years is highly likely to be an overestimate, perhaps by a factor of five or more, bringing the interval since the dog's domestication down to a much more realistic twenty thousand years or so.

In addition to comparing the dog's DNA with that of the wolf, scientists can examine how much variation there is between different types of dog, as a way of determining how long they have been around. However, this procedure, too, superficially seems to suggest that dogs were domesticated much earlier than twenty thousand years ago. A recent analysis of the DNA that codes for the dog's immune system has produced an estimate of several hundred thousand years since domestication—a figure even more unlikely than the hundred thousand years indicated by the mtDNA, since it predates the evolution of our own species. On the other hand, such an estimate assumes that mutation is the only source of variation and that all dogs are descended from a single pair of wolves. A similar degree of diversity could occur if, say, several wolves had been domesticated, each of which had distinctive DNA. But this is likely to be the case only if each of those wolves had lived in a different part of the world—a supposition that, in turn, implies several domestication events.

The apparent contradictions between the archaeological evidence and the DNA evidence can be reconciled if we posit not just one domestication event but several, in different parts of the world. It is now becoming possible to examine the DNA of fossilized dog teeth taken from Neolithic burial sites. While only a few dozen individuals have been sequenced so far, the results tend to confirm that wolves were indeed domesticated at several, possibly many, different locations.

Scientists have also begun to find proof for multiple domestications by looking at a different type of DNA, extracted from living dogs. The DNA that codes for the immune system is inherited from both parents, not just the mother, as mtDNA is. The much greater diversity in the
DNA for the immune system suggests that dogs have far more forefathers than foremothers; in other words, dogs overall seem to have many male wolf ancestors between them, but only a few female wolf ancestors. Thus the genetic material from the “extra” males must have been introduced after domestication had started. The early domestic dog bitches would presumably have been attractive to, and so occasionally mated by, wild male wolves. Moreover, their puppies would have been born in close proximity to humans. And provided that the genetic contribution of their wolf father did not make them too intractable, they could have survived to contribute to the dog genome. There is no reason why a mating between a male dog and a female wolf should not also produce puppies, but they would be born in the wild and, hence, would be more likely to contribute to the wolf's genome than to the dog's.

Thanks to recent scientific developments, we now know that the diversity of the modern dog's genome is not hopelessly incompatible with the archaeological evidence surrounding the dog's domestication. Nevertheless, there is still a discrepancy—possibly as large as five thousand to ten thousand years—between the most likely date suggested by the DNA (twenty thousand or more years ago) and the oldest date that most archaeologists will agree to (fourteen thousand years). The reason for this discrepancy probably lies in the type of evidence that archaeologists will accept as evidence for domestication. Human remains and the bones of wolves have been found together at sites going back a half-million years, long before modern humans evolved, but archaeologists do not accept these joint burials alone as signs of domestication. Rather, they look for evidence of domestication either in the remains of animals that are clearly distinguishable from wolves (e.g., those with a wider skull, a shorter muzzle, or smaller teeth) or in signs that the animals, even if otherwise indistinguishable from wolves, had a special place in human society—preferably both.

Probably the earliest well-established archaeological example of a dog that is both biologically distinct from wolves and specially connected to humans is the burial, about twelve thousand years ago in what is now northern Israel, of a human with one hand resting on the body of a puppy. Not only does the position of the puppy show that it had a close relationship with that person, but its teeth are also significantly
smaller than those of any wolf that lived nearby at that time, indicating that it must have come from domestic stock.

Neither the physical signs of domestication in this puppy, so distinct from its wild counterpart, nor the evident bond between the animal and its owner, can have arisen overnight. Rather, the puppy must have been preceded by many generations of dogs who made up the transition from wild wolf to domesticated pet. Such transitions may be virtually invisible to archaeology, but the subsequent rapid emergence of dogs all over the Old World is compatible with the idea that there was not one domestication but several. In the next two millennia after this twelve-thousand-year-old burial, other similar burials—either of humans and dogs together or of dogs on their own—occurred in various parts of Europe. These sites have been found in, among other places, the United Kingdom, suggesting that dogs had also spread quickly from their points of origin, which are thought to be far to the east. Scientists also believe that, at roughly the same time, humans were taking other domestic dogs, probably from another focus of domestication in East Asia, out of Siberia and into what is now Alaska. (At the time, both were part of a single landmass known as Beringia, which, depending on the period, stretched as far as 600 miles from north to south.) These dogs moved with one of the early waves of colonists down the west coast of North America and then into the interior: The earliest confirmed dog remains in the United States, in Danger Cave, Utah, are perhaps ten thousand years old. Meanwhile halfway across the world, humans took dogs with them as they moved into the farthest reaches of southeast Asia; the DNA of the eight hundred thousand street dogs found in Bali today shows that they are the descendants of dogs who arrived there overland, before Bali became an island twelve thousand years ago.

This rather rapid appearance, in the archaeological record, of dogs all over the globe can potentially be explained by many independent domestications taking place almost simultaneously—but it is also plausible that domestication of the dog did actually start much earlier than the archaeology indicates. The point at which archaeologists can be sure that the dog had already become a domesticated animal may actually reflect not the beginning of the transition from the wolf but, rather, the culmination of a fundamental change in the relationship between
man and dog, one that had already taken thousands of years to develop. This process could not be complete until the dog had become an integral part of human culture, and also until it no longer needed to maintain the physiognomy of the wolf, because many of its essential needs were being taken care of by its owner. Thus the five-thousand-year discrepancy between the date of domestication as shown by the archaeological record and that indicated by the dog's DNA can be explained by positing an extended period over which domestication took place gradually. These earliest dogs, or proto-dogs as they are sometimes called, would have been indistinguishable from wolves in terms of physical appearance, and they were probably treated in a strictly utilitarian way. For example, they may have been communal “property,” as today's village dogs are, rather than having a single “owner.”

To be sure, any pre-domestication theory that posits several thousand years of coexistence between wolves and people before the transformation to domestic dogs must account for the lack of archaeological evidence over this period—say, from twenty thousand to fifteen thousand years ago. If dogs existed during this period, possibly even earlier, why are they absent from human burials for the whole of this period but then suddenly start appearing in burials, across the globe, over the course of “only” a couple of millennia? The archaeological record itself may hold the answer.

The earliest known dog burial is more than fourteen thousand years old. Located in Bonn-Oberkassel in Germany, it was discovered in a quarry in 1924 and seems to have consisted of a partial skeleton of a dog buried alongside two humans. Unfortunately the outbreak of World War I led to the loss of much of this material; yet a single piece of the dog's jaw remains, the arrangement of its teeth showing that it was clearly not from a wolf. Archaeological evidence indicates that, from then on, dog burials became almost commonplace. (Other kinds of animals were buried as well—but not nearly as often as dogs.)
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Some dogs were buried alongside people; others had their own dedicated graveyards. In what is now the southeastern United States, dog burials were so common during the period between nine thousand and three thousand years ago that it is their relative infrequency from
later
burial
grounds that archaeologists feel they need to explain, rather than the other way around.

Mankind had been burying its dead for tens of thousands of years before dog burials began. Many ancient human graves contain animal remains; some may have come to be there accidentally, but many were obviously included deliberately, indicating a powerful emotional link between early humans and the animals they found around them. Consider this description of the contents of a grave, dug twenty-eight thousand years ago in Russia, that contained the remains of a boy, a girl, and a sixty-year-old man. Buried with them were thousands of pieces of deer's antlers, polar foxes' teeth, and mammoth ivory, which had probably been incorporated into necklaces or as decorations on their long-disintegrated clothes. Beside the boy was a sculpture of a mammoth, itself carved from mammoth ivory. In another grave nearby there was a small ivory sculpture of a horse (a hunted animal at this point, not domesticated). These people clearly had an important relationship with their local animals, one that included representing them in their art and possibly featuring them in religious rites. This relationship, however, seems to have been exclusively that of hunter and quarry.