February 2003

Genetic Hoofprints

The DNA trail leading back to the origins of today’s
cattle has taken some surprising turns along the way.

By Daniel G. Bradley

The genes present in the 1.3 billion cattle living on the Earth today represent a stream of inheritance that stretches back 10,000 years. The founding event in the legacy of the domesticated farm animal was the capture of the formidable wild ox, or aurochs. Taming a long-horned beast six feet tall at the shoulder must have been a daunting task, but it was just one of a series of plant and animal domestications that forever changed the way most people live.

But just what is the genetic heritage of domestic cattle? Was more than one kind of aurochs brought under human control, and if so, how many ancestral species does that heritage encompass? How much variation is present in the genome? What experiences does its DNA encode to help the animal deal with heat, cold, hunger, thirst, disease, and all the other stresses of life? Answers to such questions do more than satisfy curiosity. Humanity’s dependence on cattle runs deep, and our own well-being is therefore bound up with that of the animal. For example, if the selective breeding that has created a super milker should also inadvertently lead to vulnerability to a particular disease, how deep is the genetic reservoir that could still be called on to fight that vulnerability?

If the selective breeding that has created a super milker should also inadvertently lead to vulnerability to a particular disease, how deep is the genetic reservoir that could still be called on to fight that vulnerability?

For the past dozen years my colleagues and I at the Smurfit Institute of Trinity College in Dublin have been tracing the genetic origins of modern breeds of cattle. The work has taken us from Great Britain to South Asia to the Sahara, and from modern factory farms to pastoralist societies. Guided by the signposts of DNA, we have virtually traveled back in time along the genetic stream, from the present to the ancient past, to the era when some determined bands of people first tamed an ox.

Before modern techniques of molecular genetics became available, the only meaningful information about how and when domestications took place came from archaeology. The study of those world-changing events, embedded as they are in prehistory, is a tricky business. Yet archaeozoologists have devised a number of ingenious ways of determining, for instance, whether dusty, 8,000-year-old collections of bones are the remains of hunted wild beasts or the former members of a domestic herd. Domestication is likely, for instance, if bone deposits show the animals died at roughly the same age. The conclusion also holds if the bones represent more males than females (herders often slaughter males but keep females to produce offspring) or if the bones indicate changes in structure and a slight decrease in size (features that begin to show up in animals after generations of domesticated life). In the case of cattle, yet another feature of domestic service is diagnostic: the presence of certain kinds of wear and tear in joints or vertebrae, which indicates that the animals were once beasts of burden.

East Anatolian Red is a Near Eastern breed. Photo by Ronan Loftus

An archaeological site can be designated a domestication center only when excavations indicate that hunting gradually gave way to herding. Archaeologists have documented such a transition in the Fertile Crescent of the Near East. Extending outward from the land bounded by the Tigris and Euphrates Rivers, this region was the center of domestication for an unparalleled number of plant and animal species, including barley, oats, and wheat among the cereal grains, and the “big four” domesticated animals: cattle, goats, pigs, and sheep. But there is evidence that cattle were also brought under domestication farther east, in what are now Pakistan and India.

By tracing the ancestry of living animals, geneticists can verify and amplify archaeological findings, thereby giving a more detailed view of domestication and its history. Blood samples and hair follicles from individual cattle are used to provide samples of certain genes, such as the ones in the mitochondrion, the powerhouse of the cell. The DNA sequences of genes can then be compared. Mutations in DNA sequences accumulate at a pace that remains approximately steady over time. The known mutation rates of common sequences can serve as “molecular clocks,” which enable molecular biologists to estimate when ancestral lines branched away from each other. The sequences my colleagues and I have studied indicate that hundreds of thousands of years ago—that is, long before any domestication took place—two distinct kinds of wild cattle emerged, both of which are represented in modern populations.

Patterns of genetic variation are like ripples in a pond, which persist even after the stone that caused them has sunk from view into the depths. The geneticist can judge from the size and direction of the ripples where and sometimes when a stone was dropped, as well as how big it was. Within the geographical distribution patterns of cattle genes, we have determined that not one but two big stones—corresponding to separate domestications of the two divergent kinds of wild ox—were thrown into the pond 10,000 years ago. The resulting ripples continue to expand and overlap.

Some scholars argued that all domestic cattle had a common origin in a single domestication center in the Fertile Crescent. Others believed, on the basis of archaeological evidence, that the cattle of the Indian subcontinent were separately tamed.

Ever since Darwin, opinions had cycled between the one- and the two-stone scenario. Some scholars argued that all domestic cattle had a common origin in a single domestication center in the Fertile Crescent. Others believed, on the basis of archaeological evidence, that the cattle of the Indian subcontinent were separately tamed. Our work has shown that the cattle of Europe, northern Asia, and Africa all have closely related DNA sequences and that they all belong to a group that corresponds most closely to the humpless cattle known as Bos taurus. But the genes of the humped, zebu cattle native to India, known as Bos indicus, tell a different story. On the bovine family tree, zebu are ten times further removed from the three members of the B. taurus group than those three are from one another. The Indian humped cattle belong to a genetically distinct group of their own. So the genetic evidence firmly sides with the archaeological findings: early farmers, in what are now Pakistan and India, did indeed capture and tame their own zebu-like version of the wild ox.

Gene sequences can also be read as a record of how herding may have spread outward from those two domestication centers. Our genetic work had indicated that European cows were related to the ones that were domesticated in the Fertile Crescent. But we wanted to find out whether they also carried a genetic inheritance from the aurochs that still inhabited Europe when cattle were being herded there. (The last remnant population of European aurochs was hunted to extinction in a Polish forest in 1627.)

Alentejana is a breed from southern Portugal. Photo by Daniel G. Bradley

In 2001 we managed to extract DNA gene sequences from six large aurochs bones discovered in Britain. The bones were between roughly 3,000 and 8,000 years old. The ones of more recent date were from wild oxen that had lived as neighbors of domestic herds then kept in Britain. Our analyses showed that the British aurochs sequences were closer to sequences present in B. taurus than to those of B. indicus, but they were quite different from any encountered in modern cattle. These wild oxen appear to have made no detectable contribution to the domestic gene pool; they did not interbreed with their domestic contemporaries. The forebears of European cattle, then, were wholesale importations from the Near East. Today a British cow’s mitochondrial genes are much more similar to the genes of a cow—ancient or modern—from Syria or Turkey, than to the genes of the wild ox that used to roam the island.

Although we detected no other sources of genetic input in the DNA of modern European cattle, the animals themselves are outwardly much changed since their ancestors migrated from the Fertile Crescent. By selectively breeding for traits such as milk production, physical conformation, and even coat coloration, people have altered the appearance, size, and utility of livestock. After decades of scientific animal breeding, certain traits have been enhanced to an extraordinary degree. Milk production per animal has doubled in the developed world in the past twenty years, largely because of genetic manipulation. Top Holstein-Friesian cattle favored by large-scale dairy farmers can easily produce forty liters of milk a day; in contrast, a West African N’Dama cow may give only four.

Cattle raised on most European and North American farms today have a pedigree going back only to nineteenth-century founder animals of common breeds, such as Hereford, Holstein-Friesian, and Aberdeen Angus.

Yet selective breeding, by definition, also narrows the genetic base of herds, and it may have side effects as well. Breeding for milk production, for instance, could lead to reproductive problems and increased susceptibility to disease. The predicament is not as extreme as it is in crop species, where the genetic base can be sharply narrowed. But the widespread use of artificial insemination in cattle inevitably implies that the male ancestors of most of the world’s elite milking herds are all close relatives. Cattle raised on most European and North American farms today have a pedigree going back only to nineteenth-century founder animals of common breeds, such as Hereford, Holstein-Friesian, and Aberdeen Angus. And though the individual animals from which modern breeds were formed would have varied in appearance, most of the animals that belong to a particular breed today look remarkably uniform. Fortunately, though, there is still some diversity among European breeds, and valuable genetic resources may be tied up in less common breeds, such as the distinctive Scottish Highland and the Portuguese Alentejana cattle.

To find an exuberant variety of breeds, however, one has to look beyond the industrialized West, to regions where cattle have a place not only as commodities in the economy, but in the culture as well. The primacy of the cow is still intact not just in India (where cattle are considered sacred and are more numerous than in any other nation) but also in Africa. In African pastoral societies, milk, meat, and sometimes blood from cattle are major sources of protein; cattle dung provides both fuel and building material; and steers, or castrated bulls, are used as draft animals. Herd ownership can also symbolize status, and individual cattle can serve as large, mobile units of wealth. For example, bridegrooms often present cattle as gifts to the bride’s family. Cows also serve religious or ritual functions. The Kapsiki people of northern Cameroon, for instance, keep a cattle breed (also known as Kapsiki) specially for the skins, which are made into burial shrouds.

The Kuri breed of Chad is a type of humpless cattle (Bos taurus), but molecular data shows it has received a substantial influx of genes from neighboring humped cattle (Bos indicus). Photo by Daniel G. Bradley

Such cultural intimacy between the people of Africa and their cattle does not mean that African herders refrain from any selective cattle breeding. But Africans breed their herds without the obsession for uniformity that has emerged in the West. One of the most evident traits that African herders select for is extreme horn size. The enigmatic Kuri cattle, herded near Lake Chad in north Africa, bear horns of enormous length and girth. When these cattle move side by side in the herd, their hollow horns knock together, producing a characteristic resonant sound. The horns are selected for their appearance rather than any utility. Ankole cattle, from the great lakes region of East Africa, are also bred for horn shape and size. Not surprisingly, the animals are prized status symbols.

According to our genetic analyses, African cattle originated neither from Indian humped cattle nor from Near Eastern cattle. Those findings support the separate-origins theory of cattle domestication favored by archaeologists, who had maintained that in Africa, too, cattle domestication was local. Our results confirm that African cattle stem from the domestication of a B. taurus type of wild ox that inhabited northern Africa when the Sahara region was much less arid than it is today. It may even be the case that the distinctive pastoral lifestyle of African tribes such as the Masai is of tremendous antiquity, and could pre-date the capture of cattle and development of milking in the Fertile Crescent.

Although the earliest African cattle were of the B. taurus type, modern African breeds show multiple influences, traceable through their genetic history. I recently joined the geneticist Olivier Hanotte and his coworkers at the International Livestock Research Institute in Nairobi, Kenya, in a study that was part of a worldwide effort to chart genetic variation in cattle breeds, in the hope of conserving diversity.

Hanotte sampled the genes of fifty indigenous breeds of cattle in twenty-three countries across the length and breadth of Africa. Then, with a technique called principal-component analysis, we were able to peel away the various overlays of genetic variation caused by interbreeding, as they appeared across the continent—rather like peeling the layers of an onion. As each overlay was removed, we exposed a new, previously unseen trend, or pattern of variation. One major trend was the dispersal over millennia of the original B. taurus-type cattle from the Sahara region to the forests of West Africa and down to the southern cape. A second, minor genetic trend was a trickle of Near Eastern and European cattle into the continent, where they mixed with native breeds.

The third and most pronounced trend in our genetic data, however, pointed to a great influx into Africa of zebu-type, B. indicus cattle from South Asia. A herd in Sudan, for instance, can now carry mitochondrial DNA from the domestication of a northern African wild ox, an event that may have taken place only a hundred miles from Sudan, but more than 7,000 years in the past. The same herd may also include some genes that have leaked southward from the Fertile Crescent in the same period. But strikingly, most of the genes in the Sudanese herd are likely to be of the B. indicus type. And the cattle are likely to be humped, like Indian zebu.

Butana is an East African breed of humped cattle. Photo by Ronan Loftus

When we overlaid the gene types on a map of Africa, we discovered that the numbers of B. indicus genes peaked on the east coast. The clusters suggest that zebu were first imported into Africa in large numbers by sea, rather than via an overland route such as one crossing the isthmus of Suez. Perhaps the humped cattle were brought first to Arabia, then on to East Africa. We also determined that the most purely B. indicus of African breeds live on the island of Madagascar, a finding that also supports the idea that the Asian cattle came to Africa by sea. The trade between Africa and India seems to have been ancient and reciprocal. African cereals such as sorghum and finger millet appear in India as early as the second millennium B.C., about the same time B. indicus may have first appeared in Africa.

Since their arrival in Africa, Indian cattle genes have thrived and through interbreeding, have spread throughout the continent. Zebu are generally well adapted to hot and dry environments, a boon in African regions that are becoming increasingly arid. And in the late nineteenth century, when the cattle disease rinderpest became epidemic and decimated B. taurus herds, zebu genes conferred some resistance.

In an age when most cattle in the developed world have a slim family tree, humanity should treasure, and perhaps will come to be thankful for, the rich weave of ancestry that persists on the plains of Africa. Pastoral societies also preserve the cultural importance of this largest of domesticated species. In Western societies, this cultural element has mostly disappeared from people’s everyday lives. Cattle retain their significance only behind the fenced-in properties of agribusinesses and the well-guarded entrances to commodity-trading floors.

Here in my home, Ireland, the economy was dominated for millennia by cattle farming. The system stretched back to the time of the herders who built the stone-walled fields of Céide, in the west of the island, fields that have lain buried under peat bogs for 5,000 years. The Irish word for a road, bothar, means a path wide enough to accommodate a cow. And in the wider cultural setting cattle have literally been the alpha, if not the omega, of the Western world. After all, the first letter of the alphabet you are now reading had its genesis as a symbolic representation of an ox.

“Cattle is in the blood,” is the way Daniel G. Bradley describes himself. A lecturer in genetics and a fellow at Trinity College, Dublin, Bradley grew up on a small farm in northern Ireland and recalls tending cattle on spring mornings before going to school. Even today not all his work is done in the lab. To help trace the origins of African cattle, he has traveled several times to western Africa and, with help from the U.N. Food and Agriculture Organization, he has visited remote pastoral villages in Guinea and Guinea-Bissau. He once drove through a desert strewn with spent rocket shells to reach an area of Chad that is home to huge-horned African Kuri cattle.

Copyright © Natural History Magazine, Inc., 2003

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