Mexico may claim the Chihuahua, and Tibet the shih tzu. But a new genetic study indicates that all small dogs have their origins in the Middle East.
The origin of the domestic dog is a hot topic in evolutionary biology. Scientists agree that today's Fidos came from the domestication of the gray wolf, but they are at odds over where this took place. Previous genetic studies focusing on mitochondrial DNA—inherited only from the mother—have suggested that modern domestic dogs are descended from animals that lived in East Asia between 5000 and 16,000 years ago. But archaeological excavations in Europe and the Middle East have found remains of what appear to be domestic dogs dating back as far as 31,000 years.
Now, a team led by evolutionary geneticist Melissa Gray of the University of California, Los Angeles, has examined nuclear DNA to fill in a crucial piece of the puzzle. The researchers took samples of blood, tissue, or saliva from three populations: large domestic dogs (those weighing more than 30 kilograms), small dogs (weighing less than 9 kilograms), and wild wolves, foxes, and coyotes from around the world. They then looked at a gene called insulin-like growth factor 1 (IGF1). All canines, wild or domestic, have some form of this gene—precisely which form is strongly associated with the size of an animal's skeleton.
The team found that the version of IGF1 carried by all small dogs is found in very few large dogs and no wild canines. But a very similar form of the gene is found in gray wolves from the Middle East. That means that this region is probably the birthplace of the common ancestor of all the world's small dogs. Because they all carry the same variant, it is extremely unlikely that small body size evolved more than once. And for the gene to have had time to spread all over the world, it must have evolved shortly after dogs were first domesticated.
Gray emphasizes that the study, published  today in BMC Biology, doesn’t necessarily mean that dogs were first domesticated in the Middle East. But it's a "strong indication" that that region “has played a significant role in the early history of domestic dogs.” The authors note that archaeologists have found remains of small dogs dating to 12,000 years ago in the area. There are older sites in Europe and Russia, but they contain larger dogs. She says humans living in small agricultural communities may have deliberately bred small dogs because they ate less and could be kept in small spaces.
Adam Boyko, a geneticist at Stanford University in Palo Alto, California, who specializes in the evolution of the domestic dog, is impressed by the study. “This really pokes a hole in the argument of this relatively simple domestication in East Asia, ... which is what people have been arguing based on mitochondrial DNA,” he says.