Mutated wheat and barley was waiting for humans, ready to be plucked and planted, but animals were different. People could kill and eat them, but they couldn’t take wild beasts back to the village and commence a captive breeding program. Domesticating animals entailed a more complex process.
Throughout history people have tamed all sorts of wild animals—kangaroos, monkeys, cassowaries, cheetahs, moose, giraffes, and brown bears—apparently for the simple pleasure of keeping pets. But when those individual animals die, their tameness dies with them. Domestication, by contrast, is an evolutionary process in which a species adapts to living alongside humans. Whereas taming involves individual animals, domestication involves populations. Pet keeping might be thought of as an initial evaluation to see if the animal might work out as a permanent employee: many animals were interviewed, but few were hired.
Those animals that made the cut—large domestic beasts like cows, pig, sheep, goats, horses, camels, and llamas—have certain things in common. They eat widely available foods such as grasses and leaves (unlike, for instance, koalas, which require specific types of eucalyptus leaves). They reproduce fairly quickly (unlike elephants, which gestate for almost two years). They respond to threats by bunching up rather than by scattering (as white-tailed deer do). They aren’t too aggressive (like African buffalo) or agile (like fence-leaping gazelles). Perhaps most importantly, they live in groups structured by a dominance hierarchy, a trait that allows humans to step in at the top position and assert control of the herd.
This pattern helps explain why certain animals became domestic while others didn’t. But it doesn’t allow us to peer back into history to see precisely how these animals first made the transition from a wild to a domestic state. In order to understand that, we must look at individual cases of domestication.
The domestic goat, which likely emerged from the shifting hunting tactics of humans, offers a relatively clear-cut case. All of the goat breeds that exist today are descended from the wild Bezoar goat, which lives in high mountains stretching from Turkey into Pakistan. Archaeologists in the region have uncovered ancient heaps of goat bones in caves occupied by humans during a period stretching from 45,000 years ago to about 9,000 years ago. For nearly all of that period, most of the bones came from adult animals, both males and females. Then, starting about 9,000 years ago, the bones of subadult males begin to predominate. After that time, very few male goats lived to adulthood. By contrast, almost all of the females did.
In the earlier periods, it seems, the hunters focused on immediate returns, killing adults because those animals had the most meat on their bones. The later pattern—killing males at younger ages while allowing females to grow older—reflected what we might call a sustainable harvesting strategy, intended to ensure that animals would be available for hunting in the long term. The shift most likely reflected dwindling resources. At this time, roughly 12,000 years ago, the people living in the mountains from present-day Turkey to Pakistan underwent the same climate-induced stresses as those at lower altitudes. The weather warmed, and human population boomed, increasing the hunting pressure on local animal populations; this, combined with a cooling climate, caused the herds of wild goats to dwindle. The hunters changed their hunting strategies to meet the new conditions. They killed young males—only a few were needed for breeding—and protected females, who gave birth to kids and thereby sustained the herd’s population.
What started as a hunting strategy became, over hundreds or thousands of years, a new relationship with goats. Hunters followed the herds through the hills, closely observing them, learning their behavior, and selectively culling them to maximize docility and productivity. In time, hunters gave way to shepherds, who controlled their precious livestock with fences and herding dogs.
A similar domestication process is likely for sheep and cows—but not for swine. Omnivorous and intelligent, pigs demand a different theory of domestication, one that accounts for the ways they manipulated people.
Wild boars, Sus scrofa, became domestic pigs, Sus scrofa domesticus, several times, independently. It happened once along the Yellow River in China and perhaps a second time along the Yangtze sometime before 6000 BC. It’s also likely that pigs were domesticated in India and Southeast Asia. The occasion we know most about happened in the Near East, starting at Hallan Cemi, roughly 11,000 years ago.
When something happens a half dozen times or more, it starts to take on an air of inevitability, as if the partnership between pigs and people had been rendered necessary by the nature of each. The habits of wild pigs had not changed much in the few million years before domestication. The same cannot be said of people: after the last great ice age, they had stopped wandering in search of food and settled down into villages. Not long afterward, pigs joined them.
The process of pig domestication didn’t happen as it did with goats because wild boars aren’t much like wild goats. Pigs group themselves not in large herds but in small maternal family groups, known as sounders, averaging a dozen or so individuals. And they live not on open hillsides but in the forest. To hunt them, humans likely sat quietly and waited for one to wander down the path. Boars couldn’t be followed, observed, and selectively killed. Boar hunting could not have turned into boar herding.
A different mechanism was at play, and the story of how wolves turned into dogs might provide the key to understanding it. Dogs were the first domestic animals, emerging 15,000 or 30,000 years ago, long before the invention of farming. According to an earlier theory of dog domestication, our ancestors snatched wolf pups from the den, tamed them, and carefully bred a new species to serve as companions, sentinels, and hunting partners. The first stage, adopting pups, surely happened, given what we know about pet keeping. But the second stage, transforming these captured pups into dogs, poses problems. Modern experiments show that wolves hand-raised from the age of eight days become relatively tame but are most certainly not dogs: they bite their handlers, cannot be trained to sit or stay, and try to run off as soon as they reach sexual maturity.
More likely, wolves entered the school of domestication not as pets but as scavengers. Wolves trailed bands of hunter-gatherers, devouring the remnants of the hunt. When people settled into permanent villages, they produced even more materials for wolves: not only animal scraps but also burnt food, rotten fruits, and spoiled grains and nuts. A genetic mutation that allowed wolves to better digest starch may have played a role in their domestication. Some of the waste would have been cooked grains and meat, which offered further advantages: the process of cooking renders more of the calories in food available, offering animals who could exploit this resource a particularly rich source of energy. Accessing this food posed risks, because humans tend to be hostile to large wild animals wandering among them. Biologists judge an animal’s wariness in terms of “flight distance”—how far an animal will run away if a human approaches. Flight distance is, in part, genetically determined, and variation exists within populations. Those wolves with shorter flight distances—that is, with a greater tolerance for proximity to humans claimed more food and reproduced more successfully.
Boars likely underwent a similar domestication process. Wild boars, unlike wolves, did not need to await a genetic mutation to be able to feed on cereal grains: they had always enjoyed the ability to eat starches, along with just about any other substance humans ate. Like dogs, they would have been attracted to the new source of food found in human garbage heaps. Also like dogs, they would have had to undergo an evolutionary adaptation—becoming less fearful of humans—in order to exploit it.
The selective forces of this new ecological niche favored individuals that were bold (not prone to flee at the sight of humans) but not aggressive (because humans would kill those that posed a threat). The wild animals began to separate into two populations: one tolerated human presence; the other did not. The boars and wolves most adept at exploiting this food source bred themselves into the new, human compatible species that we now know as pigs and dogs.
With dogs, this theory of domestication is largely speculative, based on what we know of dog and wolf behavior. With pigs, however, we can test the theory against archaeological evidence. Ancient dog bones are not uncommon, but their numbers pale in comparison to those of pigs. For every dog Neolithic people kept as a sentry, hunting companion, or pet, they kept dozens of pigs as a food source. People ate pork in quantity and tossed the bones into piles, where they remained buried for millennia, awaiting analysis.
The earliest evidence of pig domestication comes from Hallan Cemi, the site now buried under a lake in Turkey. There, villagers ate mostly young male pigs, and pig bones became more common even as the forest dwindled, suggesting the pigs lived as scavengers within the village rather than as wild beasts roaming the nearby woods. All of the pigs at Hallan Cemi, however, were wild boars: their bones show none of the morphological changes, such as shorter snouts, that indicate domestication. This may have been a simple matter of time: Hallan Cemi was occupied for just four hundred years, and the domestication process may have been only getting started when the village was abandoned.
A longer run of evidence comes from a site known as Cayönü Tepesi, not far from Hallan Cemi along the Boğazçay River, another tributary of the Tigris. Settlement at Cayönü started slightly later than at Hallan Cemi, around 8500 BC, and the site was continuously occupied for nearly 2,000 years. And over that span, the pig bones change from wild to domestic.
Because domestication is an evolutionary process, there is no clear threshold when a group of animals stops being wild. It’s better to think of the human relationship with such animals as a continuum: it starts with hunting, moves on to various forms of human management of wild populations, and continues to full domestication.* Evidence of this process might start with a shift in the demographics of the animals killed—such as the goats in the Zagros or the pigs at Hallan Cemi—and gradually proceed to a change in morphology. This is exactly what we see at Cayönü Tepesi.
As time progressed at Cayönü, the village’s residents killed pigs at progressively younger ages, suggesting human control similar to that at Hallan Cemi. Hundreds of years later, evidence began to appear in the structure of the bones as well. Compared to their wild ancestors, domestic pigs have shorter snouts, smaller brains, and more crowded teeth. These changes are most likely a side effect of domestication. Domestic animals are selected for docility, which is a juvenile trait—young pigs are less aggressive than adults—and the genes for docility come packaged with other juvenile traits, such as a shorter snout and smaller teeth. Just as domestic pigs preserve their juvenile docility into adulthood, they hold on to a more juvenile skull shape too. Scientists know that when they find adult facial bones of a certain shape, they are looking at a domestic pig.
The changing shapes of the bones at Cayönü Tepesi indicate a gradual process of domestication. The archaeological record reveals that, over the 2,000-year period of settlement at the site, wild boars lived among people, gradually evolving into domestic pigs.
The same continuum from wild to domestic characterizes the archaeological record of other domestic animals, but it’s worth noting a crucial difference. Goats and sheep became domestic through their role as prey for human hunters: people first killed wild animals, then managed wild herds, and finally managed domestic herds. Pigs became domestic through their relationship not with humans as hunters but rather with humans as villagers. People tracked down goats, but pigs tracked down people. Once domesticated, goats retained their original habitat, the scrubby hills and grassland outside town, whereas pigs took up residence right alongside humans. From the start, it was a more intimate relationship, involving everyone who lived in town rather than just herders assigned to the task.
Pigs, moreover, had a job to do beyond providing meat. They cleaned up the waste that accumulated in each village they occupied: dead animals, rotten food, and human feces. The villagers could not have understood that this was a useful public health measure, but they would have been happy to be rid of the stink.
Pigs possessed alchemical powers, transforming garbage into food. At first, this must have seemed like a godsend. In time, however, people came to despise the pig for doing precisely the job it had evolved to do.
Excerpted from "Lesser Beasts: A Snout-to-Tail History of the Humble Pig" by Mark Essig. Available from Basic Books, a member of the Perseus Books Group. Copyright © 2015. Reprinted with permission of the publisher. All rights reserved.