There’s a legend that when Jesus was born, the roast chicken on King Herod’s plate rose up and shouted, “Christus natus est!” or "Christ is born!"
That may be the most famous chicken in the history of Christianity. But it turns out that the fowl and the religion could be even more intertwined: As the Press Association reports, Christian dietary rules during the Middle Ages could have radically altered the characteristics of modern chickens.
The first chickens, Asian jungle fowl, were domesticated some 6,000 years ago. But since that time, the animals have attained a range of different traits. To examine when these changes took place, an international team of scientists examined chicken DNA gathered from 100 chicken bones found in European archeological sites spanning roughly 2,200 years. They analyzed the chicken DNA using a statistical framework that allowed them to pinpoint when natural selection for certain traits began and how strong that selection pressure was. The researchers published their results in the journal Molecular Biology and Evolution.
As Ricki Lewis writes for the PLOS DNA Science Blog, variations in two genes in particular made for better chickens: the thyroid-stimulating hormone receptor (TSHR) and the beta-carotene influencing gene BCDO2. Having two copies of a variant of TSHR likely allows the chickens to lay eggs faster, reduced aggression and increased their tolerance to humans. The other variant, BCDO2, is involved in beta-carotene processing, which causes the chicken skin to be yellow instead of white or grey. It’s believed that chicken lovers saw yellow skin as a sign of a healthy chicken and could have selected for this trait.
Based on the statistical model, the researchers suggest that selection of these two traits kicked into high gear in the chicken population around 920 A.D. This was a time in which populations in northern Europe were eating a lot of chicken, archaeological records show.
"This significant intensification of chicken and egg production has been linked to Christian fasting practices, originating with the Benedictine Monastic Order," Anders Eriksson, an author of the study, explains in the release. These rules forbade consumption of meat from four-legged animals during fasts—but chickens and eggs were okay.
By 1,000 A.D. those rules had spread to the whole population, according to the release. It was also a time of increased urbanization, meaning chickens may have been bred closer together on small plots land, which would have placed increased pressure on selection for chickens that produced eggs faster and could live closer together without fighting.
According to the Press Association, only 40 percent of chickens studied that were older than 1,000 years had the THSR variant. Now, all modern chickens have the trait.
Greger Larson, an author on the article, says in the press release:
“We tend to think that there were wild animals, and then there were domestic animals. We tend to discount how selection pressures on domestic plants and animals varied through time in response to different preferences or ecological factors. This study demonstrates just how easy it is to drive a trait to a high frequency in an evolutionary blink of an eye, and suggests that simply because a domestic trait is ubiquitous, it may not have been a target for selection at the very beginning of the domestication process.”
As for all the crazy-looking varieties of long-tailed, fuzzy-headed and brightly colored chickens, most of those traits had nothing to do with selective pressure. Instead, many of them were bred for chicken shows during the Victorian Era when "Hen Fever" took over the United States and Great Britain.
These days, however, whether you are religious or not, that roast chicken on your table was likely shaped—at least in part—by Christianity in the Middle Ages.