Where and when dogs were initially domesticated by our ancestors is one of the most puzzling unresolved questions of human prehistory.
We know that modern dog breeds originated from the grey wolf (Canis lupus) and were domesticated some time during the last Ice Age – at least 15,000 years ago.
But exactly where it happened, and if it were in one single location or multiple places, is still undetermined.
In a new study published in Nature, researchers used the DNA of ancient wolves to further delve into the evolution of dogs and found that their ancestry could be traced back to two different populations of wolves.
“Through this project we have greatly increased the number of sequenced ancient wolf genomes, allowing us to create a detailed picture of wolf ancestry over time, including around the time of dog origins,” says co-first author Dr Anders Bergström, a post-doctoral researcher in the Ancient Genomics lab at the Francis Crick Institute, England.
“By trying to place the dog piece into this picture, we found that dogs derive ancestry from at least two separate wolf populations – an eastern source that contributed to all dogs and a separate more westerly source, that contributed to some dogs.”
Two distinct populations of ancient wolves
The grey wolf has been present across most of the northern hemisphere for the past few hundred thousand years. An international group of geneticists and archaeologists has sequenced the genomes of 72 ancient wolves excavated from Europe, Siberia and North America.
They also used data from the genomes of 68 modern wolves, and 169 modern and 33 ancient dogs, so that the total dataset spanned the past 100,000 years.
By analysing these genomes, the team found that early dogs in Siberia, the Americas, East Asia and Europe appear to have a single, shared origin from an eastern Eurasian species of wolf.
Whereas early dogs from the Middle East, Africa, and southern Europe appear to have developed (in addition to the eastern Eurasian species) up to half of their ancestry from a distinct population related to modern southwest Eurasian wolves.
So, either wolves underwent domestication more than once and the different populations subsequently mixed together, or domestication occurred only once (in the eastern Eurasian species) and these early dogs then mixed with wild wolves.
Tracing natural selection in action
Because the 72 ancient wolf genomes studied spanned about 30,000 generations, it was also possible to look back and build a timeline of how wolf DNA has changed over time.
“This is the first time scientists have directly tracked natural selection in a large animal over a timescale of 100,000 years, seeing evolution play out in real time rather than trying to reconstruct it from DNA today,” explains senior author Dr Pontus Skoglund, group leader of the Ancient Genomics lab at the Francis Crick Institute.
“We found several cases where mutations spread to the whole wolf species, which was possible because the species was highly connected over large distances.
“This connectivity is perhaps a reason why wolves managed to survive the Ice Age while many other large carnivores vanished.”
Mutations in one gene in particular went from being very rare to present in every wolf over a period of about 10,000 years (30,000 to 40,000 years ago) and is still present in all wolves and dogs today.
The variants affect a gene called IFT88 on chromosome 25, which is involved in the development of bones in the skill and jaw.
The rapid spread of these mutations in the population may have been driven by a change in the types of prey available during the Ice Age, giving an advantage to wolves with a certain head shape. But the gene could also have other unknown functions in wolves.
The team is continuing to hunt for a close ancient wolf ancestor of dogs, to hopefully reveal more precisely where domestication most likely took place. It is now focusing on genomes from other locations not included in this study, including more southerly regions.