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Friday, March 29, 2013

Closing in on the origin of dog domestication

In 1975, the skull of an ancient canid (a member of the dog family) was discovered in Razboinichya cave within the Altai Republic, a mountainous region in southern Siberia. The skull was suspected to be that of a dog rather than a wolf or other canid. However, radiocarbon dating showed the skull to be about 33,000 years old, which is more than double the age of the oldest confirmed dog specimens. If the Altai skull was in fact a dog, that would significantly set back the date of dog domestication. The problem is that it's difficult to discriminate between early dog and wolf skulls. Researchers led by Anna Druzhkova of the Siberian Branch of the Russian Academy of Sciences solved that problem by doing mitochondrial DNA (mtDNA) analysis on the skull. They showed that it is indeed from a dog. This means that dogs have been around for a very long time, long before the invention of agriculture or the wheel.


The Razboinichya canid.
A)   aerial view, B) profile, C) palate, D) left mandible, E) left lower tooth row
(scale on ruler in cm). Sub-triangular hole in the skull is the place of initial sampling for 14C dating in 2007.

There is little doubt that dogs are the domesticated descendents of grey wolves. There is, however, a fair bit of controversy as to when those domestication events first occurred. We have archeological evidence of the existence of dogs (as a distinct species from wolves) at least 14,000 years ago, but they undoubtedly emerged much earlier. Without DNA analysis, it’s difficult to prove whether ancient specimens are really dogs rather than wolves. MtDNA, which is inherited only from the mother and hence has a less complicated provenance, has been particularly helpful in creating phylogenetic trees.

The scientists extracted mtDNA from the Altai skull and compared it to the mtDNA of 72 different dogs, 30 wolves, 4 coyotes and 35 prehistoric canids. Most importantly, three of the wolf specimens were from fragments of contemporaneous remains found within the same cave as the dog skull. If the Altai skull had belonged to a wolf, it should have most closely matched the other wolves living in the same area at the same time. This was not what the researchers found. Instead, the Altai skull mtDNA most closely matched the DNA of dogs and was not closely related to the wolves that coexisted with it. 

We still can’t say for sure when dogs first split off genetically from wolves. However, these data suggest that it happened more than 33,000 years ago, by which point they were already separate species. 

For some fascinating interviews about the origin of dog domestication, listen to this episode of Skeptically Speaking.

Druzhkova, A., Thalmann, O., Trifonov, V., Leonard, J., Vorobieva, N., Ovodov, N., Graphodatsky, A., & Wayne, R. (2013). Ancient DNA Analysis Affirms the Canid from Altai as a Primitive Dog PLoS ONE, 8 (3) DOI: 10.1371/journal.pone.0057754 

Ovodov, N., Crockford, S., Kuzmin, Y., Higham, T., Hodgins, G., & van der Plicht, J. (2011). A 33,000-Year-Old Incipient Dog from the Altai Mountains of Siberia: Evidence of the Earliest Domestication Disrupted by the Last Glacial Maximum PLoS ONE, 6 (7) DOI: 10.1371/journal.pone.0022821.

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