It is the world’s most common farm animal as well as humanity’s largest single source of animal protein. Some 24 billion strong, it outnumbers all other birds by an order of magnitude. Yet for 2 centuries, biologists have struggled to explain how the chicken became the chicken.
Now, the first extensive study of the bird’s full genome concludes that people in northern Southeast Asia or southern China domesticated a colorful pheasant sometime after about 7500 B.C.E. Migrants and traders then carried the bird across Asia and on to every continent except Antarctica.
“Our results contradict previous claims that chickens were domesticated in northern China and the Indus Valley,” researchers led by Ming-Shan Wang from the Chinese Academy of Sciences’s Kunming Institute of Zoology write in a paper published today in Cell Research. They also found that the modern chicken’s chief ancestor is a subspecies of red jungle fowl named Gallus gallus spadiceus.
“This is obviously a landmark study,” says Dorian Fuller, an archaeologist at University College London who was not involved in the effort. He adds that the results could shed light on the emergence of agriculture and early trade networks, and what features of the bird made it so attractive to people.
Charles Darwin argued the chicken descended from the red jungle fowl because the birds resemble each other and can make fertile offspring; he speculated that domestication happened in India. But five varieties of the pheasant inhabit a broad arc extending from the jungles of Indonesia to the Himalayan foothills of Pakistan. Which variety led to the chicken, and where, was uncertain. Based on presumed chicken bones, archaeologists claimed, variously, that people domesticated the bird 9000 years ago in northern China and 4000 years ago in Pakistan.
DNA studies promised to resolve the issue, but researchers had few samples from the bird’s wild relatives. So Jianlin Han, a geneticist at the Joint Laboratory on Livestock and Forage Genetic Resources, embarked on a 20-year project to sample indigenous village chickens and wild jungle fowl near more than 120 villages across Asia and Africa.
Wang’s team sequenced the full genomes of 863 birds and compared them. The results suggest modern chickens descend primarily from domesticated and wild varieties in what is now Myanmar, Laos, Thailand, and southern China (see map, right). “This region is a center of domestication,” says co-author and geneticist Olivier Hanotte of the University of Nottingham. The results confirm a hypothesis put forward in 1994 by Japan’s Crown Prince Akishino, an ornithologist, on the basis of mitochondrial DNA data.
Wang’s team did find some evidence for a South Asian contribution: A jungle fowl native to the Indian subcontinent may have interbred with the chicken after its initial domestication in Southeast Asia, the team says.
The new DNA data link domesticated chickens most closely to the Southeast Asian subspecies G. g. spadiceus, however. They suggest the lineage that became the modern chicken branched off from the jungle fowl between 12,800 and 6200 years ago, with domestication occurring sometime after the lineages split.
Fuller doubts the bird was fully domesticated before the arrival of rice and millet farming in northern Southeast Asia about 4500 years ago. Hanotte acknowledges that “we need the help of archaeologists” to understand the human events that triggered domestication.
But Jonathan Kenoyer, an archaeologist and Indus expert at the University of Wisconsin, Madison, remains skeptical that the chicken arose in Southeast Asia. “They need to get ancient DNA” to back up their claims, he says, because genomes of modern birds may provide limited clues to early events in chicken evolution.
Nor does the DNA show what first enticed people to tame the bird. Early varieties were far scrawnier and produced fewer eggs than today’s industrial varieties, and their predators were legion. Some researchers suggest the bird was initially prized for its exotic plumage or for cockfighting. Selling prize fighting cocks remains a lucrative business in Southeast Asia, and the birds’ high value may have spurred traders to carry them long distances.
Smithsonian Institution archaeozoologist Melinda Zeder calls the new paper “fascinating” and says it shows “the domestication and dispersal story is more complicated than we thought.” She urges combining genetic and archaeological data to flesh out the tale. Archaeologists are now gathering chicken bones that suggest farmers in southern China and Southeast Asia first domesticated the bird some 3500 years ago—findings that bolster the genetic work.
Han’s group, meanwhile, is creating a massive data set based on more than 1500 modern chicken genomes from Asia, Europe, and Africa. The researchers plan to analyze chicken dispersal into Europe and Africa, as well as the genetic variations behind traits such as the ability to withstand disease or produce more eggs. “This study opens a whole new page in chicken genomics,” Han says.