The time of origin for each dog mtDNA clade can be estimated from the mean genetic distance of the sequences in each clade to the original wolf mtDNA type,
and the mutation rate of the analysed region. Under the assumption that the clade originates from a single wolf mtDNA type, the age of the clade will ap-proximately correspond to the origin of the dogs originating from that female wolf. However, there are two problems with the available data that render a precise dating of the origin of the dog based on the available mtDNA data impossible.
The first problem concerns the dating of the palaeontological fix-point needed to calibrate the mutation rate. The wild canid most closely related to the wolf is the coyote, and the time for the split into two species from their common ancestor is the best available calibrating point for the mutation rate of dog mtDNA. Counting the number of sequence differences between wolf and coyote in the analysed mtDNA region, and dividing by the divergence time, gives the mutation rate. However, while a first appearance of wolves ~700,000 years ago and of coyotes ~1 million years ago (Kurtén, 1968; Kurtén and Anderson, 1980) indicates a date for the divergence between the two species of approximately 1 million years ago, the time of divergence has not been definitely established and an earlier date of up to approximately 2 million years cannot be ruled out.
Therefore, without more exact palaeontological evidence, a precise dating of the
Clade AClade BClade C
East Asia Europe SW Asia
Fig. 2.3. Minimum-spanning networks showing genetic relationships among mtDNA types of phylogenetic clades A, B and C (Savolainen et al., 2002). mtDNA types (circles) are separated by one mutational step, ignoring indels. mtDNA types found in East Asia, Europe and Southwest Asia are indicated in separate networks.
The sizes of grey circles are proportional to mtDNA-type frequency in the respective populations. Small uncoloured circles denote mtDNA types not found in the regional population. Black dots are hypothetical intermediates. Uncoloured squares are wolf mtDNA types. Subclusters of clade A discussed in the main text, three in the East Asian and one in the European network, are marked by black lines.
origin of the dog will not be possible, but it can nevertheless give an indication of the probable range of time, and this information can be used together with other data from the archaeological record on the first appearance of dogs. To simplify the following argument, the 1-million-year date for the wolf–coyote divergence will be used for the calculations based on mtDNA, but bearing in mind the pos-sibility that the split occurred up to 2 million years ago, in which case the age of the datings should be doubled. The second problem is that the mutation rate of the analysed 582 base pairs region does not give resolution between mtDNA types in the time scale needed to monitor the last 20,000 or 30,000 years. According to the calculations above, the rate of mutation is approximately one mutation per 24,000 years in a lineage (Savolainen et al., 2002). This implies that if two wolves, having mtDNA types differing by a single mutation, were domesticated say 15,000 years ago, the dogs of today originating from those two lineages would have mtDNA types differing from each other only by the single original mutation or by just one or two additional mutations. If there were several wolves having mtDNA types differing by just a few mutations, it would not be possible to fully resolve the mtDNA types of the dogs originating from them. Thus, the period around 15,000 years ago, which is suggested to be the time for the origin of the domestic dog according to the archaeological record, cannot be fully studied using the 582 base pairs mtDNA region available at present.
In a domestication event with a subsequent population expansion, a star-like phylogeny, with the founder mtDNA type in the centre and new mtDNA types distributed radially, would be expected. The networks of clades B and C are star-like, indicating an origin from a single wolf mtDNA type (Figs 2.2 and 2.3). In contrast, clade A has a complicated pattern without an easily identifiable central node. A distance of up to 11 substitutional steps between mtDNA types would indicate that clade A is much older than clades B and C, and derives from an initial domestication of wolves. However, as discussed above, the dog mtDNA types in clade A do not necessarily originate from a single wolf mtDNA type even though clade A is an almost completely continuous group of mtDNA types sepa-rated from each other by single mutational steps. It is possible that clade A origi-nally was a clade of several closely related wolf mtDNA types, and that several wolves having a number of these mtDNA types belonging to clade A were domes-ticated. Looking more closely at clade A it has, instead of a single central node, several subclusters with star-like shape, suggesting that clade A may have origi-nated from several wolf mtDNA types (Figs 2.2 and 2.3). The approximate age of clade A, assuming a single origin from the wolf and a subsequent population expansion, is estimated from the mean pairwise distance between East Asian dog-mtDNA-sequences (3.39 substitutions, SD=0.13) and the mutation rate to 41,000
± 4000 years. This calculation may be biased by the population history among the dogs and wolves. Alternatively, the maximum age of clade A can be estimated from the number of steps between the most distantly related mtDNA types, 11 substitutions apart which corresponds to ~120,000 years. According to these cal-culations clade A would have originated 40,000–120,000 years ago, and if it is
supposed that it was formed in a domestication event from a single wolf mtDNA type, the domestic dog would have originated 40,000–120,000 years ago. If instead an origin of clade A from several different wolf mtDNA types is assumed, several reasonably defined subclusters can be found. To give an alternative dating of the domestication of the dog, three subclusters of clade A possibly representing three origins of dog from wolf, marked by lines in Fig. 2.3, can be studied. The mean genetic distances of the sequences belonging to these subclusters to their respective nodes (0.45, 0.65 and 1.07 substitutions with SD=0.13, 0.09, 0.27, respectively) give estimates of 11,000 ± 4000, 16,000 ± 3000 and 26,000 ± 8000 years for their ages, respectively. Thus, clade A has a substructure, suggesting that it could have been formed by several wolf mtDNA types, possibly ~15,000 years ago. Assuming single wolf mtDNA types as founders of clades B and C, the mean distances among East Asian sequences to the nodes (0.54 and 0.71 substitutions,
SD=0.08 and 0.10) give estimated ages of 13,000 ± 3000 and 17,000 ± 3000 years for clades B and C, respectively. Thus, clade A was formed ~40,000–120,000 years ago but has a substructure indicating later population events, and clades B and C were formed ~15,000 years ago. Depending on how these data are inter-preted, it can suggest a first origin of domestic dogs either ~40,000–120,000 years ago forming only clade A, after which clades B and C were introduced into the dog gene-pool through crossbreeding between dogs and wolves ~15,000 years ago, but it can also suggest a single origin ~15,000 years ago involving all the three clades A, B and C.
The key question for the dating of the origin of the dog is the number of wolf mtDNA types that formed dog-clade A, but this question cannot be answered based on the present mtDNA data. However, indirect evidence in the form of the universal presence of clades A, B and C at similar proportions talks in favour of a simultaneous origin of the three clades. Assuming that clade A would have origi-nated from an initial domestication approximately 40,000 years ago, the dog pop-ulation originating from that domestication event would be expected to have spread to other parts of the world. An introduction of clades B and C, 15,000 years ago, into the already existing domestic dog gene pool, by regional cross-breedings with wolves, would require a very thorough mixing to have occurred, through migration to all parts of Eurasia, to level the frequencies of the three clades to the very similar proportions now found throughout the world.
Alternatively, if clade A originated from a domestication 40,000 years ago, these dogs could have remained isolated in the original geographical region, not spreading to other parts of the world. Clades B and C would then, 15,000 years ago, have been introduced into the domestic dog gene pool by regional cross-breedings with wolves, after which the dogs would have spread to other parts of the world. While the amount of migration and trade 15,000 years ago is not known, this scenario seems unlikely. It is also contradicted by the age of the oldest subcluster of clade A in Europe (as determined from the mean genetic distance from mtDNA types unique to the western part of the world to the nodal mtDNA type shared with East Asia; 0.39 substitutions, SD=0.09), which is estimated to be only 9000 ± 3000 years old (Fig. 2.3).
Thus, a first origin of dog mtDNA clade A, a long time before the origin of dog clades B and C, does not seem probable considering the very similar proportions of clades A, B and C around the world. The total sum of circum-stantial mtDNA evidence therefore indicates a single origin, in both place and time, for the three clades, approximately 15,000 years ago. Furthermore, in the absence of a clear result from the mtDNA data, the best evidence for the time of the first origin of the domestic dog remains the archaeological record, which indi-cates an origin approximately 15,000 years ago. A synthesis of mtDNA data: the similar proportions of clades A, B and C, and the larger genetic variation for clades A and B; and archaeological data: the oldest evidence of domestic dog dated at approximately 15,000 years ago, therefore point to an origin of the domestic dog in East Asia ~15,000 BP. In this event, clade A would have had several origins from wolf mtDNA types, at least around ten, and the first domes-tication of wolves would not have been an isolated event, but rather a common practice in the human population in question.