OXFORD, U.K.—Chicken is the most ubiquitous meat on menus around the world, from chicken Kiev to chicken McNuggets. But the bird wasn’t a common food in Europe until about 1000 years ago. That’s when the Catholic Church got tough and banned meat from four-legged animals on fasts—which numbered 130 days out of the year. Suddenly, demand for meat from two-legged chickens surged, according to a talk here yesterday at the seventh International Symposium on Biomolecular Archaeology. The edict also appears to have influenced the evolution of a gene that made the birds lay eggs year-round and set in motion changes that helped make them plumper.
Chickens trace their ancestry back thousands of years to the red jungle fowl of Southeast Asia—and perhaps some other birds that got into the mix. Yet, little is known about when they became meaty morsels. The birds may have been domesticated more than once in Asia, and the first ones that show up reliably in the archaeological record 7000 to 3000 years ago in China, India, Egypt, and Greece, for example, were colorful but scrappy fowl. Chickens were a delicacy for Romans, medieval Europeans favored hardier birds, such as geese and pheasants, that they didn’t have to feed or protect from predators—and the size of chickens shrunk. This suggests they were bred primarily for cockfighting, egg laying, and as exotic garden ornaments.
A breakthrough on how the birds were domesticated—and a gene that played a key role—came in 2010 with a study of the genomes of eight different populations of present-day chickens from around the world. Researchers found that they all carried two copies of one version of a gene, called the thyroid stimulating hormone receptor (TSHR), which apparently set in motion changes that plumped up the birds. This dominant version of the gene, or allele, had swept through all domesticated chickens, regardless of whether they were broilers bred for size or strains bred for laying many eggs. Although the precise function of the gene is not known, it regulates metabolism and reproduction, so probably stimulated chickens to lay more eggs year-round. Once hens began laying eggs all year, they probably had to be kept indoors in more crowded conditions. This—and the new yen for chicken meat—may have indirectly set in motion selection for fatter chickens. The authors proposed that this gene variant was critical for domestication because it was in all domesticated chickens.
If this version of the TSHR gene was a trigger for the initial domestication of chickens, as that study proposed, then it should also be found in ancient domesticated fowl. But when University of Oxford evolutionary biologist Greger Larson and his colleagues gathered DNA from 80 domesticated chickens from 12 European archaeological sites dating from 280 B.C.E. to the 18th century in Greece, central Europe, and the United Kingdom, they found that few carried this now dominant variant of the TSHR gene, as he reported at the meeting.
Larson’s team reconstructed the number of chickens that had the dominant version of the TSHR gene variant over time, and found that it suddenly swept through chickens at multiple archaeological sites in the United Kingdom about 1000 years ago, turning up rapidly in 40% of the chickens they sequenced.
What happened? Larson posed the question to zooarchaeologist Naomi Sykes of the University of Nottingham in the United Kingdom, who had counted chicken bones at archaeological sites in Europe as part of a chicken project. She found that the number of chicken bones doubled between the mid–10th century to the year 1000, jumping from 5% to 6% of the meat bones to 12% to 14%. This change in diet included a boost in eggs and fish consumption, which also was reflected in historical records for food accounts from great houses and monasteries.
These changes came right after the Benedictine Reform in the United Kingdom in the mid–10th century, which followed earlier monastic reforms in Europe that required that fasting became a religious and legal requirement. These new laws had a big impact on diet for all people living in Christian nations, because fasting took place for about 130 days a year, Sykes says. Europeans were to abandon eating meat from “quadrupeds”—sheep, goats, and other four-legged animals on these days. But chickens, with two legs, were acceptable. And the more people ate chicken, the more they bred them, selecting birds that laid eggs year-round and were meatier—and, presumably, carried the TSHR variant.
Other researchers who heard the talk said that it demonstrated the power of ancient DNA studies to show the evolution of a trait through time. Domestication isn’t one event—it is a continual process, as humans tweak the makeup of the animals they live with. “It’s cool because it shows we’re moving beyond thinking of domestication as a single event … you can see the psychology of early farmers over time who go from just wanting to make a wild variant [of plant or animal] grow to making the damn thing tasty,” says Tom Gilbert, a geneticist at the University of Copenhagen.
It also shows the dramatic impact that humans have on the evolution of the animals and plants around them. “It speaks a lot about the effects humans’ decisions have on the environment—even a political or religious decision really can impact the biology of animals,” says Ludovic Orlando, a geneticist at the University of Copenhagen.
Others noted that they hope Larson’s team will get ancient DNA from chickens outside of Europe, as well. “There are a lot of people who paid no attention to Benedictine monks,” says archaeologist David Meltzer of Southern Methodist University in Dallas, Texas. Larson says his team already is gathering ancient fowl from the Middle East and beyond.