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
|
|
