Having waited long for his owner’s return, Argos has been neglected for years yet sees Odysseus at once for who he is. The dog’s last acts are to drop his ears and wag his tail in a gesture of greeting. Promptly afterward, “Argos passed into the darkness of death, now that he had fulfilled his destiny of faith and seen his master once more after 20 years.”
For centuries, human fortunes have been tied to those of our proverbial best friend, and vice versa. Yet little is known with certainty about the biological origins of the animal species closest to our hearts.
But an international study published in Cell Research journal Tuesday sheds some light on precisely this question in one of the most extensive investigations into dog DNA. The research — the combined efforts of China, Canada, Finland, Singapore, Sweden and the United States, itself a nod to dogs’ near-universal appeal — offers a genetic road map of how the diverse species emerged across continents and millennia.
The first dogs lived about 33,000 years ago in Southeast Asia, according to the study.
Using whole genome sequences from 58 canids (wolf and primitive dog species), researchers determined that dogs from Southeast Asia have the greatest genetic diversity compared with populations from other parts of the world. They also exhibit the most genetic similarity to gray wolves, long believed to be dogs’ closest wild ancestor.
“As a single species,” the study notes, “the domestic dog embodies one of the largest collections of phenotypic diversity for any species living on earth. Our study, for the first time, reveals the extraordinary journey that the domestic dog has traveled on this planet during the past 33,000 years.”
Because each genome is related to the next, a broad view of genome variations through dog generations, as the researchers conducted, provides notable clues about where it all began.
Following the hypothesis that the greatest genetic diversity would be found in the geographical origin of a species, scientists homed in on Southeast Asia for dogs. Then, after calculating how long it takes for a certain number of mutations to appear in the genome, they determined that the split between gray wolves and Southeast Asian dogs occurred 33,000 years in the past.
(In 2013, a DNA analysis of an ancient dog skull dated the specimen to the same general time, as scientists concluded the fossil was 33,000 years old.)
This inaugural “founder” population of dogs was about 4,600 creatures strong.
From there, Fido’s ancestors traveled far and wide. Or, as the researchers put it, “The domestic dog, one of our closest companions in the animal kingdom, has followed us to every continent of the world.”
About 15,000 years ago, a subgroup of the original dogs migrated from Southeast Asia to the Middle East, Africa and Europe — what researchers termed a “global dispersal of dogs.” One of these groups returned to northern China about 10,500 years ago, before relocating again to the Americas.
These paths were charted by looking at the networks of admixture — the genetic mingling that occurs when animals from previously separated populations interbreed — and the layered degrees of genetic diversity among dog types.
The researchers noted “dramatic” levels of genetic diversity among dog breeds. Whereas the Tibetan Mastiff, an East Asian breed, shows levels of diversity comparable with that of Chinese indigenous dogs, many European dog breeds have “considerably reduced genetic diversity.”
Despite these new conclusions, there is still much about the dog’s history that remains mysterious and unknown. For instance, what might have caused East Asian dog populations to genetically diverge from wolves in the first place?
One possible explanation, according to the study’s scientists, is that the initial divergence that eventually led to domestic dogs was a split in the wolf population, a genetic differentiation between South Chinese wolves (dogs’ ancestors) and northern wolves. This might suggest that domestication did not occur until the global dispersal.
On the other hand, it is also possible that the “ancient dog-wolf split” was actually the first step in the domestication of wolves and their evolution into domestic dogs.
“The mild population bottleneck in dogs suggests that dog domestication may have been a long process that started from a group of wolves that became loosely associated and scavenged with humans,” the study proposes, “before experiencing waves of selection for phenotypes that gradually favored stronger bonding with humans (a process called self-domestication).”
As no archaeological evidence has yet been discovered to support these threads in genetic data, dog ancestry retains an enigmatic aura. In the meantime, we may revel in the fact that domestic dogs are not only “man’s best friend,” but also, as the researchers declare, “one of the most beautiful genetic sculptures shaped by nature and man.”
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