From the Chihuahua to the Saint-Bernard, passing through the Borzois greyhounds with their incredibly elongated skulls, today’s dogs present an exceptional variety of shapes, while all are descended from the same ancestor, the gray wolf. This high variability is only very recent, since it is linked to the intensive selections carried out over the last 200 years for the creation of the 355 breeds now recognized by the Fédération Cynologique Internationale. But what do we know about the appearance of the first dogs, in prehistoric times? This is the question we addressed in our article published today May 18 in the scientific journal Proceedings of the Royal Society B.
Our research has shown, for the first time, that at this very early period dogs already had a wide variety of head sizes and shapes.
All current dogs come from the same ancestor
All dogs come from the same ancestor: the gray wolf. At least 15,000 years ago in the Upper Paleolithic (the exact date and location of domestication remains a matter of debate), fearless and aggressive wolves belonging to a now-extinct lineage would have been attracted to human encampments, probably to take advantage of leftover food. The prehistoric men would then have approached these wolves, these bringing them a help to hunt or to protect their camps against the attacks of other predators. We would have tamed the less wild of them, making them reproduce and thus domesticating them over time.
This domestication has been accompanied by many genetic, physiological, behavioral and even physical modifications, most of them unintentional. Among the morphological changes, archaeozoologists (the experts on human-animal relations in the past) and paleogeneticists have noted variations in coat color, a reduction in size, less marked differences between males and females and the preservation of traits rather juvenile, which results in changes in the dimensions of the skull with a strongly marked and shortened muzzle and more frequent dental anomalies (absence or rotation of certain teeth) due to lack of space.
Moreover, a study conducted since the 1960s in Siberia has shown that by selecting the most curious and least aggressive foxes over the generations (thereby recreating the hypothetical conditions of the first encounters between men and wolves), the animals became more and more docile, their level of stress (appreciated by the secretion of cortisol) decreasing, and that they presented the same morphological differences as those observed by archaeozoologists during the transition from wolves to dogs. Domestication would also have modified the anatomy of the muscles of the face, so as to allow the raising of the eyebrows.
A diversification of dogs from the Neolithic?
Later during the Neolithic in Western Eurasia, humans gradually opted for a sedentary and agriculturally oriented life. These changes in our way of life most likely affected our canine sidekicks, making them even more different from their wild ancestor. In particular, prehistoric men were able to select morphologies adapted to the performance of certain tasks, such as hunting big game or defending camps and villages.
However, only a few studies have attempted to describe the morphology of dogs from bone remains. For example, a Scottish study attempted a facial reconstruction from the skull of a dog dated to around 4,500 years ago and found in a necropolis in the Cuween Hill region of Scotland’s Orkney archipelago. On the reconstructed bones, whose size evokes our modern border collie, silicone and clay were used to reconstruct the volume of the muscles. A skin was then added, the fur having been chosen so as to recall the European gray wolf. A similar reconstruction was done recently for an even older dog, dated to around 7,600 years ago.
Other studies, unfortunately scattered, have relied on measurements made on the bones to describe the shape of these prehistoric dogs. This research comes up against the problem of the conservation of bone remains (cranial remains are rare and often very fragmented), refers to small samples and is limited to the study of certain regions or periods, without seeking to have a more global variability of dogs in Europe on the scale of prehistory. In addition, the method used is generally very rudimentary and does not make it possible to precisely describe the shape of the bones (at best we have estimates of robustness or height at the withers from measurements made on the long bones, and size indications from measurements made on the elements of the skull). Thus, to date, no study has precisely and reliably documented the morphological variability of dogs on the scale of Prehistory and Europe.
In our study, we studied a sample of more than 500 lower jaws (mandibles) of European dogs dated from 11,100 to 5,000 years before our days, i.e. from the Mesolithic to the very beginning of the Bronze Age, when dogs were already well differentiated from wolves. We based ourselves on the mandible because it is the most frequent and best preserved bone in an archaeological context. Moreover, the mandible remains a good indicator of the general shape of the head and it can be used to give a functional meaning to the variations of shapes observed. We can therefore estimate whether the masticatory muscles were more or less developed, and which ones acted the most during the bite.
We used 3D methods to precisely describe the shape of these mandibles, i.e. the size and proportions within the bone. To quantify this variability and compare it to that of our current dogs, we used a repository consisting of around a hundred modern dogs of various breeds or returned to the wild state (Australian dingoes), as well as a few wolves (modern and old).
The results of our study
Our study has shown, for the first time, that at this very early period dogs already had a wide variety of head sizes and shapes. European prehistoric dogs either had mandibles of the same size as some current medium-sized dogs like the husky or the golden retriever, or of the same size as our current beagles, or even small dogs like the Pomeranian (also called dwarf spitz ) or the dachshund. In any case, they all had significantly smaller jaws than the smallest modern or archaeological wolves in our sample. We did not find extremely large (like modern Rottweilers or Borzois Greyhounds for example) or extremely small (like Yorkies or Chihuahuas) sizes.
In terms of shape either, we have not identified a very extreme shape, so no equivalent to highly modified breeds such as the Rottweiler, Borzoi Greyhound, French Bulldog, Dachshund or Chihuahua. Most of the dogs had an average conformation, similar to today’s beagles or other breeds such as the husky, but there was some variability with more elongated heads (mandibles resembling those of Sloughi or whippet sighthounds, or Pomerania).
If we expected this result and this lower variability of prehistoric dogs compared to modern dogs, we did not expect what we demonstrated next. We highlighted that part of the variability of prehistoric dogs did not seem to have an equivalent among our current dogs or among wolves. Which is surprising, given that we made sure to include all possible types of morphology by integrating the extremes (small or large dogs with short or long muzzles, dogs with a slightly modified cranial morphology such as beagles or dingoes). One might therefore have expected prehistoric dogs to position themselves somewhere in this variability.
It is true that our modern sample was not exhaustive at the time of the study, but we have since carried out additional analyzes by adding stray dogs (without particularly selected morphology), and it turns out that they are not enough. to explain these unique forms observed in European prehistoric dogs. It is more than likely that by adding dogs to the modern corpus, we always make this observation. This leads us to wonder if certain forms might not have disappeared.
In addition, we have identified anatomical peculiarities in prehistoric dogs compared to modern dogs, which makes it possible to recognize them for sure. These discriminating traits can, among other things, illustrate the adaptation of dogs to selection pressures related to their environment and their way of life. Indeed, prehistoric European dogs have strong, arched jaws, suggesting that they used their temporal muscle more. One possible explanation is that they ate harder, harder-to-chew foods than our kibble-fed dogs. Another hypothesis is that it would have been useful for them to defend camps and villages or to help catch big game when hunting.
Finally, we showed greater flexibility within the mandible of archaeological dogs: in modern dogs, the shape of the front of the jaw is strongly linked to that of the back of the jaw, due to developmental constraints , whereas this is less the case in prehistoric dogs. This greater flexibility could have allowed dogs to adapt more easily to sudden changes in diet, for example.
In this study, we aimed to describe very globally the morphological variability of European dogs in prehistory, by comparing them to modern dogs, without trying to explain this variability or to follow the morphological evolution of dogs during prehistory. . Future work will be necessary to decipher, with rigor, how geographical and cultural differences (affecting the place granted to the dog in societies or their diet) may have impacted the morphology of our canine allies during this period.