Religious dogma in the Middle Ages helped create the modern domestic chicken, research suggests.
Scientists found traits such as reduced aggression, faster egg-laying and an ability to live in close proximity to other birds emerged in chickens in about AD 1000.
Chicken evolution might have been strongly influenced by the impact of Christian beliefs on what people ate.
During the Middle Ages, religious edicts enforced fasting and the exclusion of four-legged animals from menus.
However, the consumption of chickens and eggs was permitted during fasts.
Increasing urbanisation might have helped drive the evolution of modern domesticated chickens, the study, published in the journal Molecular Biology and Evolution, said.
"Ancient DNA allows us to observe how genes have changed in the past, but the problem has always been to get high enough time resolution to link genetic evolution to potential causes," Oxford University lead researcher Dr Liisa Loog said.
"But with enough data and a novel statistical framework, we now have timings that are precise enough to correlate them with ecological and cultural shifts."
Chickens were domesticated from Asian jungle fowl around 6000 years ago.
But the new study, which combined DNA data from archaeological chicken bones with statistical modelling, showed some of the most important features of the present-day chicken arose in the high Middle Ages during a time of soaring demand for poultry.
They traced the evolutionary history of more than 70 chickens, looking for changes in the THSR gene that determines levels of aggression.
Natural selection favoured chickens with THSR variants that helped them cope with living close to one another, the study found.
THSR variants also led to faster egg laying and a reduced fear of humans.
A thousand years ago, just 40 per cent of the chickens studied had this gene, which is present in all modern domesticated chickens.
"We tend to think that there were wild animals and then there were domestic animals rather than thinking about the selection pressures on domestic plants and animals that varied through time," Dr Loog said.
"This study shows how easy it is to turn a trait into something that becomes fixed in an animal in an evolutionary blink of an eye."
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