The Origins of the Domestic Dog

The archaeological record indicates that man’s best friend – the domestic dog Canis familaris – was likely also its first domestic animal. What though are its origins? Was it domesticated once or did several events take place independent of one another?

Circumstantial evidence suggests that dogs have diverse origins. During most of the late Pleistocene, humans and wolves co-existed over a wide geographical area, providing ample opportunity for independent domestication events and continued genetic exchange between wolves and dogs. Indeed, the extreme phenotypic diversity of dogs, even during the early stages of domestication, is highly suggestive of a varied genetic heritage. Whilst this question cannot be resolved through archaeological evidence, it is possible that genetic evidence may, therefore, be able to help.

When portions of mitochondrial DNA from wolves and domestic dogs was sequenced, the control region of wolves and dogs was demonstrated to be highly polymorphic. The distribution of wolf haplotypes displayed geographic specificity, with most localities containing haplotypes unique to a particular region. Sequence diversity amongst dogs was similar. However, mitochondrial haplotype diversity in dogs could not be partitioned according to breed. Many breeds shared sequences with other breeds. No dog sequence differed from any wolf sequence by more than 12 substitutions, whereas dog differed from coyotes and jackals by at least 20 substitutions and 2 insertions. This supported a wolf ancestry for dogs.

However, because mitochondrial DNA is maternally inherited, interbreeding between female dogs and male coyotes or jackals would not be detected. Therefore, a more limited study of nuclear DNA was also carried out. This also supported the conclusion that the wolf was the ancestor of the domestic dog.

Various methods of phylogenetic analysis distinguised four distinct clades. Clade 1 included 19 of the dog haplotypes. This group included representatives of many common breeds, as well as ancient breeds such as the dingo, New Guinea singing dog, and African basenji. Clade II included dog haplotype D8, found in two Scandinavian breeds: the elkhound and jamthund. This was also closely related to two wolf haplotypes found in Italy, France, Romania and Greece. Clade III contained 3 dog haplotypes, which were found in a variety of breeds such as the German shepherd, Siberian husky, and Mexican hairless. Clade IV also contained 3 haplotypes that were identical or very similar to wolf haplotype W6 found in Romania and western Russia, which suggests recented hybridisation between dogs and wolves.

Dog haplotype clades II and IV are most closely related to wolf sequences from eastern Europe. The coyote and wolf sequences were estimated as diverging one million years ago based upon the fossil record. Consequently, this implies that the sequence divergence between the most different genotypes in clade I (the most diverse group) could have originated as much as 135,000 years ago. The sequence divergence within clade I implies an origin of greater than 14,000 years before present, which is what is indicated by the archaeological record. Nevertheless, bones of wolves have been found in association with those of hominids from as early as the middle Pleistocene, up to 400,000 years ago.

It is suggested that early domestic dogs may not have been morphologically distinct from their wild relatives. Conceivably, the change from nomadic hunter-gatherer societies to more sedentary agricultural population centres around 10,000 to 15,000 years ago may have imposed new selective regimes on dogs that resulted in marked phenotypic divergence from wild wolves. Although breeds show uniformity with respect to behaviour and morphology, most breeds show evidence of a genetically diverse heritage because they contain different haplotypes. Moreover, dog sequences cluster with different groups of wolf haplotypes. Therefore, after the origin of dogs from a wild ancestor, dogs and wolves have continued to exchange genes.

References:

Morey, D.F. 1994. The Early Evolution of the Domestic Dog. American Scientist 82: 336-347

Vila, C.V., Savolainen, P., Maldonado, J.E., Amorim, I.R., Rice, J.E., Honeycutt, R.L., Crandall, K.A., Lundeberg., Wayne, R.K. 1997. Multiple and Ancient Origins of the Domestic Dog. Science 276: 1687-1689