Why do we eat so much chicken?

This is down to how we domesticated these birds between the 9th and 12th centuries when chicken and eggs became acceptable foods during religious fasts.

It all hinged on the fact that chickens have genes that make them able to live in confined spaces, crucial to its use today as a cheap form of meat.

One explanation of this is medieval urbanisation, which favoured animals that could live in enclosed environments. Another, scientists said, was the fasting rule.

As two-legged poultry was preferred, selection favoured birds that could be cooped up and reared in large numbers.

Liisa Loog, of Oxford University, said that the work, published in the journal Molecular Biology and Evolution, showed domestication was not a single event.

“We tend to think 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,” she 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. Just because a domestic trait is everywhere in animals today does not mean it was there at the start of the domestication process.”