THE ORIGIN OF THE DOMESTIC DOG. A FRIEND STILL ENIGMATIC?

Alberto Bertelli

The dog, by the light of genetic studies and archaeozoological evidences, appears to be without doubt the first animal domesticated by the man. In spite of this from the evolutionary point of view, the origin of our best friend is still under investigation and far from being clarified definitely.
It is common to read that the dog would be nothing more than a direct descendant of the wolf and its systematic position is tantamount to a subspecies of the same wolf: Canis lupus familaris. Everything seems steadfast and clear but in Nature, it is always good to remember this, there are never clear-cut boundaries.
The studies and discoveries that have occurred over the years about the history of the dog, helped to clarify some points but, at the same time, to raise new questions.
It is advisable that the reader remembers an important detail: the domestic dog can only produce fertile hybrids with wolves, coyotes (Canis latrans) and golden jackal (Canis aureus) - Eurasian jackal -, the process is facilitated by the fact that these species are characterized by the same karyotype (chromosomes number, in this case 48), this particular might have played a key role in the evolution of the dog, as evidenced by the genetic traces found by the analysis, but it is equally important to remember that the crosses between dogs, wolves, coyotes and jackals are not frequent or widespread; Konrad Lorenz was the first to consider the genetic influence of the Golden Jackal in the evolution of the dog.
One of the earliest and most complete researches about this argument was developed in 1997 by Robert Wayne, Carles Vilà and their group of the University of California, Los Angeles (UCLA ): the researchers proceeded comparing the mitochondrial DNA (i. e. DNA that each organism inherits from the mother) of 140 dogs from 67 breeds, 162 wolves from 27 different locations distributed between Europe, Asia and North America; 5 coyotes; 2 golden jackals, 2 black-backed jackals (Canis mesomelas) and 8 jackals of Siemen (Canis siemensis).
All dogs, independently of the breed, were found genetically kindred to the wolves, while it was evidenced a greater distance from coyotes and jackals; because the examination was conducted according to the matrilineal level, it could not be shown any genetic contribution of male jackals or coyotes, also the domestic dog doesn't descend from a single lineage of wolves but from different populations. This last result seemed to confirm what was already known: since the late Pleistocene (from 2.58 million years to 11.700 years) men and wolves shared vast and several geographical areas and the morphological diversity that characterizes the dogs would suggest a multiple origin.

The percentage of genotypical differences induced the researchers to speculate that the separation of the dog from the wolf would have occurred about 135,000 years ago, but this date would seem to contradict the archaeological data by which the first evidence of domestication of the dog goes back to around 14,000 years ago: this is about burials where the dog was buried and carefully adorned; the oldest one was found in Germany in Bonn-Oberkassel, in fact dating back to 14,000 years. Sometimes these sepulchral monuments, widespread across all continents except Antarctica, were double (man and dog). These vestiges, moreover, hold importance as evidence of the emotional relationship that linked men and dogs. The chronological discrepancy between laboratory results and archaeological surveys was explained: the oldest dogs probably were not well differentiated morphologically by wolves, and then the canine bone finds, without the confirmation of the genetic analysis, can be confused with the remains of wolf, a part of the mandible as we will see later, also the absence of older burials and marked morphological differences are not sufficient to demonstrate the absence of a complete domestication .

The study of Wayne and colleagues seemed to prove definitively that the wolf was the only - or the main - ancestor of the dog.
The place - or the places - and the era, in which the passage from wolf to dog would take place, still remained as subject of speculation .
In 2002, Peter Savolainen, Department of Biotechnology at the Royal Institute of Technology in Stockholm, analysed the mitochondrial DNA of 654 dogs, representing the majority of dog breeds worldwide, comparing his data with 100 samples analysed and provided by Ya -Ping Zhang and Jing Luo of Academy of Sciences of Kunming and with the samples of 38 European and Asian wolves.
The results defined the existence of four groups of variable affinity between wolves and dogs . The three larger groups, comprising a greater number of dogs and wolves mutually related, resulted distributed across Eurasia .
The type of geographic distribution and the similarity of DNA samples seemed to indicate an original unique area, identifiable with Central Asia .
There were not detailed enough to be able to identify with precision the nation but, Savolainen ended, the actual China may correspond to the area where it happened the first domestication of the dog. The era of the origin was then deducted from the scale of genetic variation among clades (groups) resulted after the analysis, compared to the average time of change from one species to another, in this case on the basis of palaeontological data, which corresponds to a million years about; this calculation would indicate an origin of dog between 15,000 and 40,000 years ago, and the event could have been the result of a practice repeated by human populations in a wide span of time .

In spite of this, the questions were not yet exhausted. The bone finds discovered in the Americas - for example, in the state of Utah, in Danger Cave - demonstrate the existence of the dog there since ancient times: between 9000 and more than 10,000 years before the modern age, without excluding ages more distant.
This could raise the suspicion of a further process of domestication occurred independently in the New World.
A subsequent study ( Leonard J. A. et al. 2002) confronted :

- The remains of 37 dogs found in various pre-Columbian archaeological sites of Mexico, Peru and Bolivia ;
- 11 samples of DNA from remains found in Alaska, dating back to a time before the first European exploration of Vitus Bering and Aleksey Chirikov during 1741 ;
- The DNA of 140 dogs and 259 wolves living.

The results demonstrate that the New World dogs and the Europeans dogs share a common origin from the same wolves (or primitive dogs) coming from Eurasia .
This implies that the ancient human colonists who crossed the Eurasian Bering isthmus in several waves, within the span of 14,000 and 10,000 years ago, and brought their dogs with them, also the wide variety of mitochondrial DNA which characterizes the dogs in the New World shows that those ancient dog populations were already differentiated and numerous. It should be added, by way of completeness, that we should not forget the contribution of the dogs arrived in the American Continent after 1492 - discovery of America - with the colonists and, before that date, the probable genetic interference of the Coyotes, at least as regards the dogs of the Great Northern Plains.

In summary, it seemed that demonstrated the first real step towards the domestication would have occurred in Central Asia/western China, during a period corresponding to 15,000 years ago, from more than one wolves populations, confirming some seemingly purely palaeontological studies conducted in during the 70s.
Recent paleontological discoveries have brought new findings: the find of a complete skull and perfectly preserved 33,000-year-old, with the morphological characteristics of dog (not of wolf!) In the cave of Razboinichya - Altai Mountains, Siberia. The excellent preservation has allowed the dating by radiocarbon analysis conducted by three independent laboratories. This extraordinary result was not considered a true demonstration of domestication, however, it confirms the existence of a canid differentiated from the wolf, before the last ice age, during a period away than the dating taken into account until now.

After 2002, also studies and comparisons of character took turns, especially of ethological chracter, and the results highlighted by genetics were not enough to fill doubts and incongruities that seem to undermine the theory of the origin of the dog from the wolf .
It is considered as proposition that the domestic species, if they are placed in a position to return to the wild, take the morphology and behaviour of the original wild forms; apparently the dog is an exception: the dogs become wild, even after several generations, don't become wolves again, nor in morphological or behavioral terms, but overall reach a status that makes them similar to the wild dogs of South Asia and Oceania known as "Pariah Dogs" .
This can be explained in two ways: either artificial selection has so changed the dog as to make it definitely a new species, genetically allied to the wolf but definitely separate from it, or it is likely that something of the past of the dog, is escaping us.
Studies, developed in Italy in 2006 by Verardi, Lucchini and Randi, have confirmed that the dog and the wolf, however genetically related and being in a position to interbreed giving fertile offspring, tend to remain separate: the hybridization between the two remains a exceptional event, which is contained by severe behavioural limits, aside from the size and the spread of the populations of wolves and dogs and their eventual sympatria.
The beginning of the domestication process is usually explained by utilitarian purposes and mutual benefit between wolf and man: the wolves become "hunt fellows" of the man, or the wolves become "guardians/commensals" of the camps, but there is no demonstration that a wolf has become or may become really an aid to human hunting activity, nor that it can become a guardian or a commensal of the man: the ethological characteristics that have allowed the dog to become the domestic animal by antonomasia are unique and totally absent in the wolf.

Even some morphological differences generate perplexity: the dogs in comparison with the wolves - on the same size and debarring a few exceptions - show skulls lighter, smaller teeth, palate wider, broader braincase.
Some features are explainable by the artificial selection, but not all of them, such as, for example, the shape of the coronoid process of the mandible (on which the temporal and masseter muscles are inserted) that in the dog is typically curved backwards along the ascending branch - frontal margin - (Olsen J. S. & Olsen J. W. 1977) (see the schematic drawing).
The genetic affinity between wolf and dog may not be sufficient to prove the direct descent from the wolf to the dog and a new hypothesis could be advanced: the dog would be derived from a species similar to the wolf but already differentiated before domestication.

Indeed a species existed that could prove the existence of a true "proto-dog"; in China, in the strata dating back 200,000-500,000 years ago, were found in 1934 the remains of a canid, denominated Canis lupus variabilis. In spite of having been described and classified as a small subspecies of the wolf, it shows physical characteristics remarkably similar to those of the domestic dog, especially at the level of the mandible and the skull, so it may be a separate species than the wolf.
It is also shown that the genetic affinity, in particular based on mitochondrial DNA , can be justified from a common origin as well as by a direct lineage .
The dog may not be directly descended from the wolf, but may share a common origin through Canis lupus variabilis already endowed "in nuce", presumably, with those ethological characteristics that we recognize today in our best friend; if demonstrated and accepted this theory definitively, some doubts would be clarified and the systematic position of the dog would be defined as a separate species (Canis familiaris) rather than as a subspecies of the wolf.

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Verardi, A., Lucchini, V., Randi, E. 2006. Detecting introgressive hybridization between free-ranging domestic dogs and wild wolves (Canis lupus) by admixture linkage disequilibrium analysis. Molecular Ecology 15: 2845-2855.

Web references:

http://archive.archaeology.org/online/interviews/morey.html

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022821