One of the questions we frequently get is why we are taking blood from mostly unwanted, "village" or "pariah" dogs. These questions come from those who have a real interest in the science and those that just dislike dogs and can't imagine anyone would be interested in them, let alone in unbred street dogs. For the purposes of this post, I'll assume readers fall in the former group. First, let's start with the basic theoretical justification.
For the moment, ignore the 15,000 years ago bit. What we know is that dogs are most related to Eurasian grey wolves and therefore were domesticated from them, presumably somewhere in Eurasia. Sometime in the past, when dogs were intially domesticated, a population of wolves became dogs through either selective breeding by humans or "self-domestication", where some wolves started to hang around human settlements, presumably surviving on their trash. Over time some of these wolves became less afraid of humans and became so specialized in their habits that they essentially stopped interbreeding with wild wolves (though dogs, wolves and coyotes can all still interbreed). Now, we have no idea how large this population of proto-dogs was, but we do know that it was probably much larger than the number of dogs used to start most modern breeds ~200 years ago (most breeds were founded by about a half dozen sires and maybe a dozen females).
After initial domestication, dogs probably lived "breed-less" lives as human commensals (hanging around humans, not really helping or harming them but living off their trash) for many thousands of years. During this time, dog populations quickly expanded and spread across the globe. In the last few hundreds of years, several hundred dog breeds were formed from local dogs in many parts of the world; these breed dogs have entirely replaced the non-breed "indigenous" dogs in some parts of the world, notably in Western Europe and the USA. However, most dogs throughout the world still live their lives as non-breed, indigenous, commensal dogs. We refer to these dogs as "pariah" or "village" dogs. They tend to be smallish (25-40 pounds), often tan, short-haired dogs, though the type varies a bit according to the region you're in. The important point is that these dogs have not undergone the intense genetic bottleneck associated with breed formation. Thus, while breed dogs have only a small subset of the total genetic diversity of all dogs, it is likely that village dogs have a much greater range of the total diversity. Thus, they are very useful for looking at the original domestication event. They are informative of the original genetic bottleneck that led to the formation of domestic dogs many thousands of years ago.
By sampling village dogs from across the world, we get an idea of how diverse these dogs are in different regions of the globe. Then, we can look for patterns in the amount of diversity. Specifically, we look for a region with very high genetic diversity and then look for the surrounding regions to have gradually less and less diversity as it gets further away. This is what is seen in humans, with the highest diversity seen around Ethiopia and diversity decreasing gradually as one moves further and further away from Ethiopia. That's how we know modern humans evolved there. The idea is that the region where a species formed will start out with all the diversity of that species. As some animals move away from that site and colonize other areas, they bring with them only a subset of the total diversity. Then as animals move even further away from the site of origin, they take with them only a subset of a subset of the diversity, and so on. This is why breed dogs are not useful for uncovering the site of origin of domestic dogs: all modern breeds across the world have only a tiny subset of the total diversity seen in all dogs, so it's hard to compare them and know that you're seeing an effect of the origin domestication and colonization events and not some artifact of recent breeding.
In future posts I'll explain more about the controversy over how long ago dogs were domesticated and cover other aspects of the science behind our research, but I have to leave now to collect more samples.