People have been selectively breeding dogs for thousands of years. A new find on the island of Zhokhov near Siberia suggests that timeline may stretch back further than previously thought.

Researchers from the Russian Academy of Sciences in St. Petersburg have found dog bones in the archaeological remains of old Zhokhovian camps that suggests the people there may have bred dogs for sledding and hunting purposes as early as 15,000 years ago. This would put the earliest domestication and selective breeding of dogs thousands of years earlier than previously thought. The study will be published in the June 2017 issue of the Journal of Archaeological Science: Reports.

Previously, dog bones and the remains of wooden sleds had been found on Zhokhov, but researchers hadn’t been able to confirm whether the dogs were specifically bred for sledding.

In order to test the remains, researchers test skulls to see whether the ratios of the skulls were in line with wolf or dog skulls. These ratios look at height of the skull in the cranial region to total skull length and the ratio of snout height to total skull length.

Using the bones, researchers also estimated the size of the dogs, placing them between 16 and 25 kilograms. The ideal weight for a sled dog is typically between 20 and 25 kilograms.

It is possible that the dogs were not entirely bred by the people but that dogs and wolves were mating naturally and Zhokovians chose the dogs that worked best for their purposes. However, the researchers believe the discovery marks the third stage of domestication. The first two are natural selection based on humans feeding animals and artificial selection by people based on an animal’s ability to be tamed. They concluded that at 9,000 years ago, domesticated dogs were used for hunting and pulling sleds and that domestication process could have started thousands of years before that.

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