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Domestication's family tree
DNA is revealing that taming animals was not a simple process

FEW subjects, these days, can escape the embrace of genetics. That is especially true of archaeology. The study of genes has already illuminated humanity's history, showing how and when the species spread from its African roots to the farthest corners of the world. Now it is uncovering details of the most significant period of that history, the beginning of agriculture.

The latest piece of the jigsaw was published in this week's Nature by Christopher Troy of Trinity College, Dublin, and David MacHugh of University College, Dublin. They and their colleagues have been trying to work out whether modern European cattle were domesticated from the now extinct auroch (Bos primigenius) that once roamed the continent, or are the descendants of cattle that were brought from the Middle East by the settlers who are believed to have introduced agriculture to Europe around 7,000 years ago.

To answer this question, Dr Troy and Dr MacHugh turned to mitochondrial DNA. This particular form of the genetic material is more abundant in a cell than is the familiar DNA of the cell's nucleus. That is because each cell has many mitochondria. They are the cellular components that release energy from glucose, and they have their own DNA because they were, in the distant evolutionary past, free-living bacteria. By contrast, a cell has but a single nucleus. Extracting mitochondrial DNA from old bones is therefore easier than extracting nuclear DNA.

That is what Dr Troy and Dr MacHugh did. They took mitochondrial DNA samples from four fossil aurochs found in Britain and compared them with the mitochondrial DNA of modern cattle from Europe, Africa and the Middle East. As time passes, mutations accumulate in the DNA, as one "letter" of the genetic code is replaced by another. By looking at the number of differences between the DNA-sequences of two creatures it is possible to see how closely related they are. It is also possible to estimate how much time has passed since they shared a common ancestor, since the rate at which letters are substituted usually remains constant for particular types of creature.

As the diagram shows, the aurochs, though related to Middle Eastern and European cattle, are on a branch by themselves. The aboriginal Europeans did not, it seems, have the wit to domesticate cattle. It took a bunch of immigrants to show them how.

That, until recently, would have been regarded as a textbook example of the way that agriculture developed. Species, it was theorised, were domesticated only once and the result "diffused" to the rest of the world. Over the past few years, though, other genetic studies have revealed a more interesting pattern.

The diagram of the cattle family tree published by Dr Troy and Dr MacHugh incorporates another branch, discovered a little while ago by Daniel Bradley, one of their collaborators who also works at Trinity College. Dr Bradley was responsible for testing the theory that modern cattle are the result of not one but two separate domestications. This theory, which predates even Charles Darwin, is based on the very different anatomies of cattle found in Europe and the Middle East, compared with those from India. In particular, the westerly cattle lack the shoulder humps of zebu, the Indian breed.

Those who support the idea of a single domestication suggest that the distinctions could be the result of subsequent selective breeding. Dr Bradley, though, used mitochondrial DNA to show that the most recent common ancestor of Bos taurus (the western cow) and Bos indicus (the zebu) may have lived as much as 1m years ago‹well before Homo sapiens existed.

'Til the cows come home

In Africa, the story is more confusing. African cattle have features of both Bos taurus and Bos indicus, but their mitochondrial DNA suggests that, despite this apparently intermediate nature, they all belong to Bos taurus. Mitochondrial DNA, however, is unusual in being passed down only from mothers to offspring (sperm leave their mitochondria behind when they fertilise an egg). When Dr Bradley examined the nuclear DNA of cattle, and in particular that of their Y chromosomes, which confer maleness, he found a different picture.

Zebu-like cattle in Africa did, indeed, turn out to have Indian genes in them. But those genes have come, overwhelmingly, from male Indian cattle. That suggests cattle originally came to Africa from the Middle East, as geography might predict. But it also suggests that, when trade eventually brought Indian cattle to Africa, the zebu took the fancy of African stock-breeders, who deliberately studded their females with Indian males. That explains the mixture of characteristics, and also why the female-linked DNA looks Middle Eastern and the male-linked DNA looks Indian.

Cattle are not the only animals to have been domesticated on more than one occasion. Mitochondrial DNA suggests that goats, sheep, pigs, yaks and buffalo were each domesticated at least twice. Dogs were domesticated at least four times. And the mitochondrial tree for horses is so tangled that it is impossible to say exactly how many times people first slung themselves into the saddle.

That is intriguing for two reasons. First, it suggests that lots of people had the idea of domesticating animals independently, rather than the process being tried out only once for each species. Second, it adds weight to the idea that the reason such a limited number of animals has been domesticated is not because people stopped when they felt they had enough species for their needs, but rather because they tried many times and frequently failed. It was only with the ancestors of the species that now grace farmyards that they got results.

Melinda Zeder of the Smithsonian Institution in Washington, DC, who studies the process by which goats were domesticated, observes that the wild forms of those species that have been domesticated tend to live in groups and have fairly clear dominance hierarchies. This makes it easy for them to fit in with humanity. Animals such as gazelles, which would, on the face of things, be good candidates for domestication, do not have such hierarchies, so they would not easily submit to the discipline that the farmyard requires. Whether the free-spirited gazelle is better off than the cosseted goat, cow or sheep is an open question.

Apr 26th 2001
From The Economist print edition