Modern chickens were domesticated 6,000 years ago from Asian junglefowl. From their home continent, they spread to Europe, arriving in Greece by 500 BC. In 2015, the United Nations said there are roughly 19 billion chickens alive on Earth at any one time, the Economist reports. Yet how they came to be one of the world's most populous birds is something of a mystery.
In a study published in the journal Molecular Biology and Evolution, scientists have now tracked changes in the DNA of ancient chickens to find out when and where specific traits that were valuable to humans emerged—and what social changes were happening at this time.
Previously, scientists had identified two genes linked to the loss of seasonal reproduction, which allows for faster egg-laying, and reduced aggression and fear of humans.
The team, from the U.K., Germany and Canada, analyzed 100 archaeological chicken bones looking for specific genetic changes linked to domestication. They used samples spanning 2,200 years, taken from sites across Europe.
Findings showed these genes started being favored in 920 AD in Northern Europe. Study author Mark Thomas said in a statement: "Several independent archaeological studies have documented substantial increases in the frequency of chicken remains between the 9th and 12th centuries AD, as well as a shift towards the management of adult hens, presumably to increase egg production. Intriguingly, this is the period when selection on the TSHR variant [one of the genes involved] most likely kicked off."
Researchers found these evolutionary changes coincided with increased urbanization and the widespread adoption of Christianity. At this time, traditional monastic practice among the Benedictine Order appear to have favored chicken consumption.
"The significant intensification of chicken and egg production, evident in the Medieval European archaeological record, has been linked to Christian fasting practices which forbade the consumption of meat from four-legged animals during fasting periods, but allowed for the consumption of birds, eggs and fish," the study says.
These factors appear to have worked together to place selection pressures on chickens, causing them to produce more eggs and become less aggressive towards humans. "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 Northern Europe," study author Anders Eriksson said in a statement.
"Intriguingly, they also coincide with several socio-cultural changes, including a general increase in the popularity of Christian beliefs, new religious dietary rules and increase in urbanization (favoring traits that mean that animals could be kept in small spaces). We cannot say which one of these was most important but most likely a combination of all these factors affected selective pressures on European chickens and consequently their evolution," said author Anders Eriksson.
Greger Larson, another author, added: "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."
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