Origins and genetic legacy of prehistoric dogs: the evolution of prehistoric dogs
Summarized by Jon Belcher, a fourth-year geology major at the University of South Florida. He plans to become a hydrogeologist after graduation or become a sailor in the US Navy, whichever happens first. He is an avid backpacker and enjoys cooking.
What data were used?: Bergström and others compiled and analyzed genetic data to better understand the population history of dogs. To do so, Bergström and others sequenced 27 ancient dog genomes (i.e., the complete set of genes in an organism), and compared them against both ancient human genomes and modern-day wolf genomes. The data was obtained from using other studies, and the sequencing was performed by the researchers themselves.
Methods: The ancient dog genomes were matched to the ancient human genomes in both time and space, providing a map for how and when dogs and humans spread across the globe. To compare dogs and wolves, the aforementioned prehistoric dog genomes and modern-day wolf genomes were both sequenced, and the similarities analyzed.
Results: It was found that the population history of dogs mirrored human lineages, suggesting that when humans expanded their ranges and moved into new areas, they took their dogs with them. Dogs largely moved with humans and evolved alongside them. However, there are some instances where the pattern of humans and dogs moving together doesn’t appear to be the case; it is believed that dogs were occasionally traded or otherwise moved between groups of humans, or that humans moved without dogs. One example given in the article is that there were clear genetic similarities between human and dog populations in East Asia and Europe. Humans and dogs in East Asia and Europe were both more closely related to each other than other populations groups that were geographically closer, such as those in the Near East (which is synonymous with the Middle East).
The researchers found that the genomic data, both ancient and modern, were consistent with the idea that there was a single point in the evolutionary transition of wolves from dogs. Furthermore, gene flow between these two species has been mainly unidirectional since that point. The concept of gene flow can best be summarized as the mixing and mingling of genes between two populations through individuals from both populations interbreeding. It was found that the wolves studied were equally related to all dog breeds analyzed, as shown in Figure 2 in the paper. The researchers used this information to further support the idea that the gene flow was unidirectional.
Modern breeds, as seen in the figure below, are mainly composites of the ancient groups studied. There are not many modern breeds that are descended only from one dog lineage. Dogs such as the Alaskan Malamute descend from the Baikal, America, and Modern European lineages. This information is useful in trying to decipher the origin of dog breeds and tracking how different lineages have interacted over time.
Why is this study important?: This information helped to reveal how dogs have changed over time, including how they have spread over the globe and how modern breeds formed. This relationship between the movement of humans and dogs is a complex one and can have varying outcomes on the actual genetic makeup of populations.
The geographic origin of dogs remains unclear, as the researchers could not narrow down a singular location from the data. To further their research, the authors of this paper suggest collecting data from even older populations of wolves and dogs, and utilizing other disciplines such as archaeology, anthropology and ethology (i.e., the science of animal behavior) to help pinpoint that precise moment where the first modern dogs originated.
The big picture: While earlier studies have suggested bidirectional flow of genes between dogs and wolves, this study found that gene flow was mainly from dogs to wolves. Thus, the study helped to overturn incorrect thinking.
The study also raised multiple new questions. One from the article is “how did dogs spread across Eurasia and the Americas by the Holocene”? This question is raised because there is no currently known major human movements that are concordant with this proliferation of dogs. Another question is why, if gene flow can be bidirectional between dogs and wolves, is it only observed as unidirectional. Do dogs that have a higher amount of wolf genes tend to become wilder and thus are either killed or set free and do not survive? Or do wolves generally not have the chance to spread genes to dogs? The data presented in the article mostly talks about no gene flow for “some wolves”, so does this mean that there are other wolf populations that do receive dog gene flow? Further studies may shed light on these questions.
Citation: Bergström, A., Frantz, L., Schmidt, R., Ersmark, E., Lebrasseur, O., Girdland-Flink, L., Lin, A.T., Storå, J., Sjögren, K.G., Anthony, D. and Antipina, E., 2020. Origins and genetic legacy of prehistoric dogs. Science, 370(6516), pp.557-564.