Authors: Brian Fagan
The Intimate Bond (6 page)
The process of wolves becoming dogs was as much social as biological. Part of the transformation involved changes in diet. Both dogs and humans are omnivorous, but young domestic wolves would have had to adapt to a diet of both meat scraps and plant remains. They wouldn't have learned the group hunting skills that were common to wolf packs in the wild. Their diet would have come in part from human hands, from casual scavenging, and from their own hunting of small rodents and other animals. This penchant for solitary hunting might well have become a critical skill for cooperating with humans. Acquiring a varied diet from diverse sources would have required both ingenuity and solicitation. Still, such a diet probably resulted in smaller body size, an adaptation that led to reduced nutritional requirements.
It's all very well for individual wolves to adapt a doglike life, but how would they have perpetuated themselves? Their opportunities to breed with members of wild packs would have been limited at best, especially since most packs discourage outsiders from joining breeding pairs and siblings. Almost certainly, then, breeding occurred between tamed wolves in a domestic association with humans. The first dogs entered a new habitat, where they spread rapidly to fill a new ecological niche, thanks to a premium on early sexual maturity. In strictly ecological terms, one could talk about an act of colonization of human society by wolves that evolved into what we know as dogs. Domestic dogs reach puberty much earlier than wolves in the wild, so the trend toward a
smaller body size and more juvenile appearance than those of wolves accelerated with the expanding population. As anthropologist Darcy Morey eloquently puts it, the best strategy for early dogs would have been “have as many viable pups as you can, as fast as you can, and get them out there.”
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Wolves joined humans, became domesticated dogs, and formed close bonds with them in many places and on many occasions over vast areas of Europe, Eurasia, East Asia, and even the Himalayas (see sidebar “Dogs, Wolves, and DNA”). Their domestication was a diffuse event that was a direct, and inevitable, consequence of people and animals living in close juxtaposition, depending on one another, and developing informal bonds cemented on the human side with rituals and attitudes of profound respect toward other living beings.
Dogs, Wolves, and DNA
Ever since the identification of the ABO blood system in the early twentieth century, genetics has played an important role in studying both animal and human evolution. Modern molecular biological techniques have made it possible to study the genetic information inside the nuclei of each cell in bodies of animals such as dogs, sheep, and cattle. This nuclear DNA is easy to study in living creatures, but degrades quickly after death. In recent years, studies of mitochondrial DNA (mtDNA) outside the cell nuclei in small structures called mitochondria, which passes through the female line, have cast new light on the ancestry of different animals. The mtDNA passes through generations, and changes at a steady and distinctive rate, and only through random mutation. Some of the remarkable findings include the fact that the genetic signature of chickens from the Andes dating to before Columbus is identical to that of Polynesian birds, hinting, perhaps, at contact between the two areas before the European Age of Discovery.
The genetic history of dog origins is still little understood.
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A comparison between DNA from 27 wolf populations in Asia,
Europe, and North America and that of 140 dogs from 67 different breeds leave no doubt that wolves were dogs' ultimate ancestors. However, did domestication occur once or at many locations? One influential study in 2002 produced mitochondrial evidence for an origin in Southeast Asia, but the earliest archaeological finds there date to about seven thousand years ago, much later than those from Europe. A recent paper studied the Y chromosome (inherited through the male line) in 151 dogs from around the world and suggested a place of origin south of the Yangtze River in East Asia. A growing body of evidence points to a Southeast Asian population that spread throughout the world. Yet, the earliest known domesticated canines come from Europe and Southwest Asia. The geneticists point out that European and Eurasian dogs interbred with wolves for a long time, whereas Southeast Asian canines, once domesticated, lived far from them. Thus, they developed their own evolutionary path. By calculating the mutation rate of genetic markers on Y chromosomes from a sample of a hundred Australian dingoes, known to have appeared about forty-two hundred years ago, the researchers calculated that Eurasian and Southeast Asian dogs parted company about seven thousand years ago. Subsequently, Southeast Asian dogs evolved in such a way that their increasing numbers enabled them to replace western forms as they moved east at a time when agriculture was taking hold over wide areas.
The research and debates continue, but it seems certain that wolves and humans got together in many places, in some areas at least fifteen thousand years ago.
Denmark, spring, 8000
BCE
. The hunter crouches among the dense reeds, bow at the ready, and arrow hooked to the string. A pile of arrows lies to his right, ready for use. He watches the geese as they rest on the placid waters of the shallow lake after their arduous journey from the south. Feet in the mud, he remains motionless, waiting for some geese to swim within arrow's range. To his right, his black-and-brown hunting dog lies quietly, and completely still, panting gently, alert. Half a dozen ducks paddle slowly close to shore. Slowly, deliberately, the man draws his bow and takes aim. The dog remains absolutely still, watching intently. Zip, zip . . . the hunter releases once, grabs another arrow and shoots again. The startled birds take to the air, but two struggle in the water, impaled by razor-sharp flint-tipped arrows. A soft command: the dog points, then slips into the water. He swims to the struggling fowl, grabs them one by one, and carries them to shore. The hunter quickly wrings their necks and puts them in a netting bag. His dog wags his tail and looks up expectantly. A pat on the head, and maybe a scrap of food, then back to the hunt as the hunter moves slowly to a new vantage point in quest of more prey. Hours later, dog and master return to camp, the last bird gripped firmly in canine jaws.
As we have seen, wolves and humans got together in many locations, some bitterly cold, others much warmer. Despite the growing, if incomplete, evidence for wolf-dogs, we still do not know precisely when people fully tamed canines. One certainly could not describe wolf-dogs as fully domesticatedâif they existed at all.
What evidence do we have for actual dogs? The earliest unquestionable dog came to light in the grave of a fifty-year-old man and a twenty- to twenty-five-year-old woman unearthed at Bonn-Oberkassel, Germany, by quarrymen's picks in 1914.
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The quarrymen shattered most of the bones before archaeologists investigated the burial with methods that were extremely crude by modern standards. Unfortunately, only a jaw fragment of the fourteen-thousand-year-old beast survives.
When I excavate a burial, I always wonder what events surrounded the interment, whether animal or human. Did the deceased perish from old age, from chronic disease, or a war wound? Was he loved or held in contempt? Did she have children and what rituals surrounded her passing? Many of these questions can be teased from the bones (from telltale signs of injuries caused by hard work, or of serious infections), and from DNA. The Bonn-Oberkassel burial is particularly fascinating, because the earliest-known dog in the world lay with two people, perhaps its master and mistress.
At fourteen thousand years ago, the Bonn-Oberkassel beast is a very ancient dog indeed. But was it a dog or its close wild relative, a wolf? Distinguishing dogs from wolves is notoriously difficult, especially when the surviving bones are fragmentary, as they usually are. Domestic dogs are generally smaller. Their teeth and skulls display minor differences from those of wolves. To tell the two apart is challenging at best, so the experts have turned to a statistical tool known as discriminant function analysis. The researchers have designed a classifier that combines measurements from the bones from known wolves and domesticated dogs to produce spreads of measurements around an average, so that a jaw such as that from Bonn-Oberkassel can be compared with the averages for both wild and domestic canids. Zooarchaeologist Norbert Benecke compared the Oberkassel jaw with other archaeological finds and with bones from Greenland wolves and specimens from zoos, even from Australian dingoes. He found that it belonged firmly in the dog category, making this findâalas, sadly incomplete after the passage of a centuryâthe earliest-known domestic dog yet known.
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Is Oberkassel the earliest dog in the world? Certainly not, for the changeover took hold in many locations at a time when major
environmental changes caused by rapid warming at the end of the Ice Age were under way. At present, the Bonn-Oberkassel beast is the earliest known and, lying alongside a human couple, suggests an intimate human-animal relationship, at minimum one of companionship. By fourteen thousand years ago, dogs begin to turn up in other places, among them, settlements in the Dnieper River Basin on the Central Russian Plain. Two nearly complete skulls are about the same size as those of modern-day Great Danes, large beasts that could possibly have been wolves held in captivity, or even wolf-dogs. There are other dog finds from the Ukraine, but what is striking is that the size of dog bones falls sharply after fifteen thousand years ago, especially among specimens from Southwest Asia dating to around nine thousand to ten thousand years ago, by which time domesticated dogs were commonplace and significantly smaller than wolves.
Eleven Thousand Years of STDs, Canine Style
Dogs suffer from a transmittable genital cancer, a sexually transmitted disease (STD) that causes bleeding genitals or forms grotesque tumors in canines wherever they live. This contagious cancer first appeared in a dog that lived about eleven thousand years ago. Unlike other cancers, which die with the patient, this sexually transmitted cancer passed to other dogs during the victim's mating activities while it was still alive. A team of researchers sequenced this cancer genome, which carries about two million mutations, far more than in most human cancers, which have between a thousand and five thousand.
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They used an infected Australian Aboriginal camp dog and a spaniel from Brazil, two beasts separated by more than sixteen thousand kilometers (ten thousand miles). The genetic makeup of the tumors from both animals was remarkably similar. By using a single mutation, known to accumulate through time, they were able to estimate that the cancer first appeared about eleven thousand years ago, when agriculture was taking hold in Southwest Asia and the Ice Age was in full retreat. They also compared DNA from modern tumor cells to the genotypes of 1,106 coyotes, dogs, and wolves. They believe that the original cancer-carrying beast may have resembled an Alaskan malamute or husky, with a short gray-brown or black coat. The sex of the animal is unknown, but most likely it was a relatively inbred individual.
Transmittable cancer is commonplace among today's dogs, but it existed among a single isolated dog population until about five hundred years ago. Since then, the cancer has spread widely around the world, perhaps carried by dogs that accompanied ships on global voyages during the European Age of Discovery. The tumor mutation rates showed that the Australian and Brazilian dogs' cancer cells separated about 460 years ago.
Cancers, both in animals and in humans, arise when a single cell acquires mutations that cause it to produce more copies of itself. The cancer cells can then metastasize to other parts of the body. But for them to leave the bodies of their original hosts and spread to other individuals is very rare indeed. The only other known transmittable cancer is a fast-moving facial cancer found in a carnivorous marsupial, the Tasmanian devil, spread by biting.
The unknown canine ancestor that hosted transmissible cancer has passed us a genome to help cancer researchers better understand the factors that drive the evolution of many types of cancer. Above all, these researchers have an opportunity to understand what processes cause cancers to become transmissible and that could, one day, arise in either animals or humans.
We'll never know why a man and a woman were buried with their dog fourteen thousand years ago, for the intangibles of the past vanish within generations. Was it a faithful companion, a protector, or a beast valued in the hunt? Was it killed to accompany its owners into the other world or did it die somewhat later? Once again, the archaeological record is silent. That it was cherished is a certainty, for the mourners took the
trouble to inter it with those who were presumably its owners. At the moment, this is the earliest dog burial known, but more important, it is the first in a tradition of dog burials that survived through thousands of years in different societies, whether those of hunters or farmers.
After the Ice Age
It may be no coincidence that the first domesticated dogs appear at a time when the grip of the last Ice Age cold snap was loosening. Within a few thousand years, rapid warming and environmental shifts had wrought profound changes in hunting societies over huge tracts of the world, from western and northern Europe, across Eurasia, and into Southwest Asia. Natural global warming rapidly shrank the great ice sheets that mantled Scandinavia, the Alps, and what is now Canada. Sea levels rose, continental shelves vanished under the ocean, and temperatures climbed. The northern world changed profoundly as ice sheets and open steppe retreated northward and gave way to more closed-in woodland and forest.