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Canine conundrum: did dog days dawn in Europe?

Is it a wolf? Is it a dog? No, it’s a Czechoslovakian Wolfdog. dutch-tiger

Dogs have a special connection with humans, more so than any other animal. But until recently little was known about how we formed this bond. Charles Darwin once speculated: “I do not believe … that all our dogs have descended from any one wild species.” Modern genetic tools now show that Darwin’s guess was wrong. All dogs seem to have descended from grey wolves. But if that is the case, when and where did the process of domestication begin?

There have been conflicting ideas about dog’s origins. Genetic studies have suggested the Middle East and East Asia. Now, if a new study published in Science by Olaf Thalmann at the University of Turku and colleagues is right, we may have a different answer. Thalmann’s claim, based on a comprehensive genetic analysis of fossils and modern day species, suggests that dogs were first domesticated in Europe at time when humans were still hunter-gatherers.

The analysis is based on mitochondrial DNA (mtDNA), which is passed down from mothers to their children with no input from fathers. Each cell has multiple copies of the mtDNA, making it the easiest bit of DNA to extract from ancient bones. Differences between individuals’ mtDNA allow the construction of a branching tree of maternal inheritance, leading back to the last common female-line ancestor. Thalmann used the mtDNA of many modern and ancient wolves and dogs to create just such a tree.

The dog samples formed a few clusters of close relatives within the tree, with the wolves much more distantly related to each other, between them. Each cluster must represent the domestication of one female (or a number of closely related ones), or the crossing of a male dog and female wolf at a later point. According to Thalmann’s analysis, each cluster was more closely related to European wolves than it was to those from other parts of the world. This suggests that domestication of dogs happened in Europe.

The age of each cluster was also estimated. The results were consistent with the fossil record, which show that dogs existed possibly as early as 36,000 years, long before the advent of farming (which happened 13,000 years ago).

While these are exciting findings, it is important to remember if an analysis included more samples, the results may change. Most importantly, additional wolves could be closer related to the dog clusters than are the current closest European wolves. The authors argue that it is unlikely that distant wolves would be so closely related to European ones, but I am unsure about this. Wolves are an exceptionally mobile species, and even within the published tree, there are relatively close relatives that live on different continents.

It is also likely that mtDNA from many domestication and hybridisation events has been lost over the millennia, and there is no guarantee that these were all European. For instance, within the study’s results, there were intermediate wolf-dog fossils that did not cluster with modern dogs. These could be the events where the mtDNA was lost instead of persisting into modern populations. Whether they became extinct or joined the ancestral dog population but without passing down their mtDNA to the modern day is anybody’s guess.

There is also no guarantee that even if the surviving mtDNA is from Europe that this is true for the rest of the genome. Since the mitochondrial genome is inherited separately from the DNA in the cell’s nucleus, it is possible that additional contributions to the nucleus were made by wolves outside of Europe. These could have been from male wolves, or cases where a mitochondrial lineage went extinct. If an ancestral dog population contained DNA from wolves from several places, there could still be regions of the modern dog genome that descend from other parts of the world.

As humans spread around the world, they rapidly became the new top predator. Life for wolves would get tougher, while the new opportunity as part of the efficient human-dog hunting team opened up. And as the canine family are thought to be particularly quick to evolve (giving them a propensity to domestication) it would be surprising if domestication was limited to one region.

The study does not address how domestication was initiated, but it is easy to assume that hunters taking and training wolf cubs was the first step. However, it is also worth considering that the least scared or least hostile wolves followed humans to feed off their leftovers, and dogs could have started to evolve before any human thought about it. Such a model is sometimes thought of as needing human settlements, which would be ruled out if as Thalmann suggests the domestication date is before humans took up agriculture (that then led permanent settlements). However, the large carcasses of the hunted animals may have done just as well.

Whether or not it turns out that our best friend originated in Europe, it has been exciting to watch the story of such a familiar animal slowly come together with each new study.

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