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Cows are seemingly simple creatures. Their history is anything but.

An analysis of ancient genomes from domestic cattle and their wild relatives has uncovered the complex family tree of our milk- and steak-producing charges.

The study, published in the journal Science, reveals a history shaped by centuries-long drought and trysts with wild aurochs.

European cattle (Bos taurus) were domesticated around 10,500 years ago in a region that today spans parts of Turkey and the Middle East from wild aurochs (Bos primogenius), large beasts that were eventually snuffed out in the seventeenth century.
Genetic information from modern cattle indicate that a pool of just 80 female aurochs contributed to this initial domestication event. But analysis of modern genomes can only reveal so much about this early history.

One complicating factor is the introduction of genes from zebu (Bos indicus) - the characteristically humped cattle of South Asia that were domesticated around 8000 years ago from Indian aurochs (Bos nomadicus). This occurred further east in the Indus Valley, a region in modern-day Afghanistan, Pakistan and northern India.

To get at some of the early events in cattle history, geneticist Dan Bradley, from Trinity College Dublin, and his colleagues painstakingly extracted DNA from as many old cattle bones as they could get their hands on.

"We tried to do as complete a survey of the ancient Near East as we could," says Bradley.

It was an ambitious project, given the area they were working in. With ancient DNA, "sometimes it's there and sometimes it isn't," says Bradley, "and in the ancient Near East, very often it's not there".

They ended up with data from the genomes of 67 cattle, including six aurochs. The animals spanned a period of history from 8000 years ago through to medieval times.
Early on, matings between domesticated cattle populations and local wild aurochs were common, according to the analysis.

The aurochs breeding with the domesticated cattle were most likely bulls, says Bradley.

"That makes sense," he adds, because the bulls needn't have been captured from the wild. Capturing and keeping a wild female auroch would have proven far more challenging.

Later on, around 4000 years ago, the genetic signature of zebu suddenly makes an appearance.

"There's nothing, and then all of a sudden it's all through the region," says Bradley.

One possible explanation is a centuries-long drought at the time. The so-called 4.2-thousand-year abrupt climate event coincided with the collapse or decline of empires in ancient Mesopotamia, Egypt and the Indus Valley.

"Zebu are better adapted to an arid climate," says Bradley.

The trait may have been deliberately introduced by ancient Near Eastern herders.
It's also possible that herders simply needed to re-stock with zebu cattle after drought wiped out - or dramatically reduced - their taurine herds.

Once again, the input was from the male line. "You can change the genetics of a herd, in terms of years, almost overnight. All you have to do is choose a bull," says Bradley.