In 1999, a study of mitochondrial DNA indicated that the domestic dog may have originated from multiple grey wolf populations, with the dingo and New Guinea singing dog "breeds" having developed at a time when human populations were more isolated from each other. In the third edition of Mammal Species of the World published in 2005, the mammalogist W. Christopher Wozencraft listed under the wolf Canis lupus its wild subspecies, and proposed two additional subspecies: "familiaris Linneaus, 1758 [domestic dog]" and "dingo Meyer, 1793 [domestic dog]". Wozencraft included hallstromi – the New Guinea singing dog – as a taxonomic synonym for the dingo. Wozencraft referred to the mDNA study as one of the guides in forming his decision. The inclusion of familiaris and dingo under a "domestic dog" clade has been noted by other mammalogists. This classification by Wozencraft is debated among zoologists.
Where the domestication of the dog took place remains debated, with the most plausible proposals spanning Western Europe, Central Asia and East Asia. This has been made more complicated by the recent proposal that an initial wolf population split into East and West Eurasian groups. These two groups, before going extinct, were domesticated independently into two distinct dog populations between 14,000 and 6,400 years ago. The Western Eurasian dog population was gradually and partially replaced by East Asian dogs introduced by humans at least 6,400 years ago. This proposal is also debated.
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