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| Wild horses |
By Sarah C. P. Williams, Science Now, May 7, 2012
Shards of pottery with traces of mare's milk, mass
gravesites for horses, and drawings of horses with plows and chariots: These
are some of the signs left by ancient people hinting at the importance of
horses to their lives. But putting a place and date on the domestication of
horses has been a challenge for archaeologists. Now, a team of geneticists
studying modern breeds of the animal has assembled an evolutionary picture of
its storied past. Horses, the scientists conclude, were first domesticated 6000
years ago in the western part of the Eurasian Steppe, modern-day Ukraine and
West Kazakhstan. And as the animals were domesticated, they were regularly
interbred with wild horses, the researchers say.
"This is a very good paper," says biologist
Michael Hofreiter of the University of York in the United Kingdom. "Nobody
has applied this method of population modeling to horses before."
Throughout their history, horses have been interbred,
traded between populations of people, and moved across continents. All of this
makes their genetic history hard to follow. Moreover, the wild ancestor of
horses, Equus ferus, is extinct, complicating researchers' efforts to compare
the genetics of domestic animals with wild ones. Previous research nailed down
a broad area—the Eurasian Steppe, which stretches from Hungary and Romania
through Mongolia—as the region where horses originated and were domesticated.
But earlier genetic studies relied mostly on mitochondrial DNA, which is inherited only from a mother, to try to understand horses'
evolutionary history.
"The problem was that there was a lot of diversity
in the mitochondrial DNA," says biologist Vera Warmuth of the University
of Cambridge in the United Kingdom, the first author of the new study. And the
diversity didn't group the horses into their breed or place of origin.
"Every horse breed has almost all the mitochondrial lineages
represented," she says.
Warmuth instead studied sequences of horse DNA inherited
from both parents and known to be diverse between horse populations. She and
her colleagues collected genetic samples from more than 300 horses at 12
different sites across the steppe. Data were collected for only working animals
bred within a local area, not those bred for show or appearance, to minimize
any human-guided selection that would make some genes more common. Then, the
researchers used computer programs designed to model the spread of a population
to simulate how different locations of horse domestication and spread throughout
the steppe would influence modern genetic diversity. They compared each model
with the real data they had collected to see which fit best.
The best-fit model, the team reports today in the Proceedings
of the National Academy of Sciences, showed the wild ancestor of domestic horses
originating in eastern Eurasia 160,000 years ago and being domesticated in the
western part of the Eurasian Steppe around 6000 years ago. The model also helped explain why
there had been so many female lineages when previous studies had tried to rely
on mitochondrial DNA. "We think that as domestic horses spread out of the
western steppe, local wild females were continuously incorporated into the
spreading herds," says Warmuth. The constant addition of new females made
the genetic patterns—in particular, the female lineages—more complex than if
the domestic population had been totally isolated.
Hofreiter is impressed. "They have still only
narrowed down the domestication region to a fairly big area," he says,
"but they did have enough genetic data to get a signal out of the
noise."
Not all researchers are convinced, however.
Archaeologist Marsha Levine of the University of Cambridge thinks using modern
genetic samples to retrace horses' evolution is a dead end. "There's been
mixing of cultures and mixing of horses in this region for many thousands of
years," she says. "And so when you're looking at any modern horse,
you just don't know where it's from."
Bringing together many kinds of evidence is what will
ultimately answer the whens and wheres of horse domestication, Levine says.
"What we need to be doing is using material from excavations, sequencing
ancient genes, and combining that with what we know from archaeological
evidence about how animals were used in the past."
Ultimately, says Hofreiter, getting to the bottom of horse
domestication will reveal more than just the history of these animals.
"Horse domestication has changed human cultures a lot. It has changed
warfare, it has changed transportation," he says. "Studying the past
of horses can tell us a lot about our own past."
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