Eight billion chickens are consumed annually by salivating Americans. This makes the plucked dinosaur relative the most popular meat in the United States and the backbone of billion-dollar franchises like Kentucky Fried Chicken. According to new research, we can credit part of our culture’s chicken frenzy to a surprising source: zealous medieval Catholics.

In the journal Molecular Biology and Evolution, a team of University of Oxford researchers explains that the modern domestic chicken is a result of the decisions made by Europeans during the High Middle Ages. While scientists already knew that chickens were domesticated from Asian jungle fowl 6,000 years ago, these researchers used a new form of statistical analysis to determine that it was around 1000 A.D. when the chicken traits we love today began to emerge — namely, reduced aggression, fast egg-laying, and yellow skin pigmentation among the well-fed birds.

These traits emerged, the researchers believe, because of gene selection that resulted, perhaps unintentionally, from societal pressures at the time. Increased urbanization and Christian edicts that enforced fasting and prohibited eating four-legged animals pushed chickens onto Medieval dinner plates — and ultimately affect what we eat today.

“With our new method we see that the time of selection coincides with an increase in the amount of chicken bones in the archaeological records across North America,” said study co-author and University of Cambridge zoologist Anders Eriksson, Ph.D., in a statement. “Intriguingly, they also coincide with several socio-cultural changes … We cannot say which one of these was the most important but most likely a combination of all these factors affected selective pressures on European chickens and consequently their evolution.”

TELFORD, ENGLAND - NOVEMBER 20: A Brahma is seen at the National Poultry Show on November 20, 2016 in Telford, England. The annual event continues to grow with around 7000 entries this year from all around the world. (Photo by Leon Neal/Getty Images)
Religious laws altered chicken evolution.

Previously, scientists discovered that domestic chickens have genetic variants in two genes, which strongly indicate that gene selection occurred. These genes, thyroid-stimulating hormone receptor (TSHR) and beta-carotene dioxygenase 2 (BCD02), affect how aggressive a chicken is, how fast it can lay eggs and its skin pigmentation.

In this study, the researchers looked for these genes in 100 archaeological samples of chicken bones collected in 2014. Using a mathematical model to analyze the chicken DNA, they were able to pinpoint the moment these traits increased in frequency among chickens. Selection for the TSHR gene, in particular, began in 920 A.D. — the same time when people across Northern Europe increasingly ate chicken and were influenced by a Benedictine Monastic Order that decreed people couldn’t eat four-legged animals during fasting periods. And during the medieval period, people fasted a lot.

The researchers behind this discovery hope to use the same method to explore the links between other domesticated wildlife and the cultural changes that were happening in human populations at the same time.

“We tend to think that there were wild animals, and then there were domesticated animals,” said University of Oxford archaeologist and co-author Gregor Larson in a statement. “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.”

Photos via Getty Images / Leon Neal, Wikimedia Commons/Flickr