Once the field settled down in the years following the initial papers on ancient DNA recovery, a number of labs began to report its successful extraction from fossil wolves and unambiguous dogs, sometimes of great antiquity.
The field advanced in fits and starts, at first with the publication of single cases, then a few related finds and eventually, in 2013, a large series that seems, for now, to have settled the question of the origin of the wolf–dog transition in favour of Europe between 19,000 and 32,000 years ago.4
In the first decade of this century, the protocols for recovering ancient DNA improved a great deal and it became realistic routinely to obtain long sequences from old bone. Once again mitochondrial DNA was the target, for the very good reason that there are far more copies in a cell compared to nuclear DNA. If you are working at the limits, as you always are with ancient DNA, you want to make things as easy for yourself as possible.
DNA sequencing technology had also advanced to a point where it became practicable to sequence all 16,727 bases of the canid mitochondrial genome from fossils. Analysing the complete sequence avoided the potential bias of restricting the analysis to the shorter ‘control region’ used in the earlier papers by Wayne and Vilà and by Savolainen. The large 2013 study used more or less complete mitochondrial sequences of eighteen fossil ‘canids’ along with a large collection of modern dog breeds. Although not every specimen yielded all base pairs of sequences, it was enough to place them accurately on the evolutionary tree. Nuclear DNA, conversely, was too badly preserved to be of much use.
The resulting tree, or phylogram, to use the proper name, again recognised the four main branches (I–IV in the figure here) of modern dog breeds initially published by Wayne and Vilà. The results were fascinating. The fossil dogs on three of the four branches (I, III, IV) of the tree are closely related to modern breeds while the rare fourth, mainly Scandinavian, branch (II) is closest to modern wolves from Sweden and Ukraine. One possible explanation is that dogs on this branch, which include the Norwegian Elkhound and the Jämthund, acquired their mitochondrial DNA from wild wolves in the recent past, after the advent of agriculture.
While all of the ancient dog lineages have survived to the present day, that is not the case for the fossil wolves. Many of these lineages are now extinct or have simply not been picked up in living wolves yet, though the likelihood of that diminishes as more and more modern wolves are sequenced.
There is a wealth of fascinating detail in the 2013 paper by Olaf Thalmann, which I encourage you contemplate at your leisure from the original publication.5 I do, however, want to mention one particularly surprising finding – about dogs in America. Only two fossil dogs were sequenced, one from Argentina and the other from Illinois, USA. From these mitochondrial sequences these dogs were clearly both related to branch I European dogs, though the ages of the fossils (1,000 and 8,500 years BP respectively) mean that they must have arrived well before the first European settlement in the fifteenth century. These dogs accompanied the indigenous Native Americans who had arrived earlier from Asia. None, however, had mitochondrial DNA remotely like that from American wolves. This has to mean that Native American dogs were ultimately descended from European and not American wolves.
There was another surprise in store. Breeds thought to have been descended from indigenous ‘Pre-Columbian’ dogs, like the Chihuahua and Mexican Hairless, also had an exclusively European mitochondrial heritage. Although sample numbers are quite low, it does look as if the indigenous Native American mitochondrial lineages were another casualty of European settlement.
As the dust settles on the controversies still hovering over the timing and location of the transition from wolf to dog, one thing is certain. It all began a very long time ago.
7
Though hardly fixing the dawn of the transition between wolf and dog with any degree of precision, the genetic dates are embedded in the bounds of what we call the Upper Palaeolithic – the last of the three phases of the Old Stone Age.
The origin of this classification, which is still used today, can be traced to John Lubbock, 1st Baron Avebury. A banker by profession, he also had a wide range of other interests including politics, biology and archaeology. His interest in the natural world grew from his friendship with Charles Darwin, who moved to the same village, Downe, in Kent, in 1842 when Lubbock was eight years old. As Lubbock matured, his interest in evolution and archaeology grew. He became an ardent supporter of Darwin’s evolutionary theories and of academic liberalism in general. He bought land in Wiltshire to save the famous prehistoric stone circle at Avebury from destruction and introduced into Parliament a bill that would eventually become the Ancient Monuments Act, the forerunner of all legislation to protect ancient sites.
Lubbock divided the Stone Age into two phases, the Palaeolithic, sometimes known as the Old Stone Age, lasting until roughly 10,000 years ago, and the Neolithic, the New Stone Age which followed it, coinciding with the invention of agriculture. Later an intermediate phase, the Mesolithic or Middle Stone Age, was adopted as the term for the period between the end of the last Ice Age about 17,000 years BP and the dawn of agriculture when the Neolithic began. About 4,000 years ago, the Neolithic gave way to the Bronze and then Iron Ages. The Palaeolithic was further divided into Lower, Middle and Upper phases, with the last of these lasting from about 50,000 years BP until the transition to the Mesolithic. Incidentally, the dates here only apply to the Stone Age in Europe. In other parts of the world the transitions occurred more recently; indeed, in highland New Guinea the Stone Age lasted until well into the twentieth century.
The genetic dating places the wolf–dog transition firmly within the Upper Palaeolithic, a quite extraordinary period in the history of our species, bristling with innovation and new ideas. The hallmark of the Upper Palaeolithic is the appearance of new forms of stone tools, the most durable of evidence. Until then, the only tools were hand axes and spear points. They were carefully made, certainly, but had not changed in basic design for tens of thousands of years. Suddenly, archaeologists were finding delicate arrow points, bone needles, even fish hooks, artefacts never seen in older, deeper layers.
Human fossils were much scarcer than stone tools, but they too showed a change from heavy-boned and robust skeletons whose skulls boasted prominent brow-ridges and receding chins to an altogether lighter and more graceful form. Was this a change brought about by slow adaptation, or was it the sign of the arrival in Europe of a new human species? After years of debate the argument was settled in favour of the wholesale replacement of the indigenous humans – Homo neanderthalensis, the Neanderthals – by a new arrival from Africa. This was Homo sapiens, our own ancestors. Mitochondrial genetics was the deciding factor in settling the argument in favour of replacement.
A few days before Christmas 1994 three speliologists, Eliette Brunel-Deschamps, Christian Heller and Jean-Marie Chauvet, were clambering over the face of the Ardèche gorge in southern France. Here the river cuts through the southern flanks of the limestone Massif Central on its way to join the Rhône near St-Just, well on its way to the Mediterranean. It is in the nature of limestone to form underground cave systems when exposed to the constant attention of slightly acidic groundwater. Over thousands of years the water gradually erodes the rock, hollowing out caverns of sometimes immense proportions.
The entrances to some 4,000 caves, many little more than overhangs, some as vast as medieval cathedrals, punctuate the steep walls of the gorge. Most are clearly visible, while others are hidden by rock-falls and vegetation. It was in order to find these hidden caverns that Jean-Marie Chauvet and his companions were inching their way along the steep sides of the gorge. They were searching for air currents emerging through cracks and crevices that would betray the presence of a cave system deep underground. At one point Chauvet felt a slight breeze coming from the rocks brushing the hairs on the back of his hands. He bent down to sniff the subterranean zephyr, then called his companions over. They agreed that the gentle flow smelled