A look at the DNA of village dogs points to the region near Mongolia and Tibet.
by Cathleen O’Grady
Sometime before fifteen thousand years ago, humans started domesticating Eurasian gray wolves. Researchers are in agreement that this happened somewhere on the Eurasian continent, but getting any more specific than that has proven difficult. A new paper in PNAS suggests that the region around modern Mongolia or Nepal might be where it all began.
Previous evidence has painted a conflicting picture. The earliest archaeological evidence of dog domestication comes from Europe and Siberia. Meanwhile, genetic evidence from mitochondrial DNA (passed down to individuals by their mothers) and Y chromosomes (which travel down the male line) places the origin of dogs in southern China around 16,000 years ago.
It’s not clear why this conflict exists, but it’s a complex story that will likely be difficult to pin down. Migration, interbreeding, and other processes could make the data difficult to interpret, and there could just be enough gaps in our knowledge that we can’t get a good sense of how the overall picture looks.
The answer’s in the mutts
A team of researchers led by Laura M. Shannon and Adam R. Boyko at Cornell University decided to look in a new place for genetic data: village dogs. These are the free-ranging dogs that live alongside humans in settlements all around the world, with no human-controlled breeding. They usually don’t belong to anyone in particular.
Village dogs are an excellent source of data because of people’s tendency to tinker with dog genomes to create things like shorter noses, fluffier coats, stronger tails, or whatever else takes our fancy. Because of our tinkering, breed dogs have very limited genetic diversity and can only take us so far in answering questions about dog history. The genetic diseases that are widespread in purebred dogs mean we have reams of genetic data, but there are limits on the usefulness of that information.
Village dogs, on the other hand, are genetically diverse, so they can provide meaningful data on where domestic dogs originated and how they spread through the world. To get at this goldmine, the Cornell team gathered genetic data from 549 dogs in 38 countries, as well as 4,676 purebred dogs from 161 breeds.
They followed in the steps of previous studies by using mitochondrial DNA and Y chromosomes but also went a step further by using data from more than 180,000 markers across the full dog genome. Then they combined their own data with previously collected data, creating the largest database of canine diversity to date.
Migration histories
As expected, village dogs had much more genetic diversity than purebred dogs. Even though purebred dogs look very different from one another, they actually have only a very small amount of the overall diversity seen in free-roaming dogs across the world.
Of course, dogs have just as much of an immigration history as humans. European dogs in particular have spread to multiple regions, coming to dominate in many of them. A number of regions have village dogs with clear European ancestry, such as the South Pacific, the Americas, and Africa.
In Eurasia, on the other hand, the only influence from European dogs is what might be expected from normal interbreeding across neighboring regions. In general, the genetic patterns found in dogs matched the regional boundaries known from human history.
To assess where dogs might have been domesticated, the researchers looked at a feature called linkage disequilibrium (LD). This is a measure of how much genes that are normally inherited together on a chunk of the same chromosome start to separate over time. Populations in the place of origin of a species often have low LD, while migrant populations have higher LD as they increase in distance from the origin population.
The research team found the lowest LD in Afghanistan and Central Asia, suggesting domestication in Central Asia, followed by a spread to nearby regions, including Afghanistan, India, and Vietnam. If dogs enjoyed particular success in East Asia, it could explain why previous genetic results pinpointed this as the region of origin.
This study is far from the final word on dog domestication. To begin with, samples of village dog DNA from the regions missed by this team could change the results. There are also other strands of evidence that could change the picture yet again, like analysis of ancient dog and wolf DNA. And of course, there are still questions about exactly how and when domestication happened.
PNAS, 2015. DOI: 10.1073/pnas.1516215112 (About DOIs).