The Wolf Within. Professor Bryan Sykes
important strand was Darwin’s observations of the creation of new forms by deliberate breeding, which he referred to as artificial selection. His favourite examples were the extravagant strains of domestic pigeon created by fanciers, the main reason being that he was pretty certain that they all descended from just one wild species, the rock dove Columba livia. As in all his work, Darwin was thorough and meticulous. He kept the main varieties of pigeon himself at home, and through his network of contacts collected as many skins as he was able from far and wide. He spent days in the collections at the British Museum and even enrolled in two London pigeon-fanciers’ clubs.
As well as pigeons, Darwin studied pigs, cattle, sheep, goats, horses and asses, domestic rabbits, chickens, turkeys and ducks, even goldfish, not to mention plants of many kinds. And, importantly for us, dogs. The first chapter of his accumulated thoughts on evolution through artificial selection, published in 1868 as The Variation of Animals and Plants under Domestication, is devoted entirely to dogs.
Right at the start Darwin sets out the principal question surrounding the origin of dogs.
The first and chief point of interest in this chapter is whether the numerous domesticated varieties of the dog have descended from a single wild species or from several. Some authors believe that all have descended from the wolf, or from the jackal or from an unknown and extinct species. Others again believe, and this of late has been the favourite tenet, that they have descended from several species extinct and recent, more or less commingled together.
Then he adds: ‘We shall probably never be able to ascertain their origin with certainty.’
Darwin’s questions on the origin of dogs remained unanswered for over 120 years until the new science of molecular genetics began to take an interest. In the chapters that follow we will explore what this new science has to say about the evolution of dogs and how, for once, Darwin has been proved wrong. We have been able to ascertain the origin of dogs with certainty.
3
I Met a Traveller from an Antique Land
This is not the first time that I have hijacked this line from Shelley’s ‘Ozymandias’. It conveys perfectly the sense of antiquity and timeless continuity I still feel when I gaze at my favourite guide to the past – mitochondrial DNA.
To explain, we need to go back thirty years to a key paper published in the leading scientific journal Nature by the New Zealand-born evolutionary biologist Allan Wilson from the University of California, Berkeley.1 Wilson and his team had taken placenta or cell lines from 147 women from all over the world and isolated DNA from the mitochondria. Mitochondria are components of our cells that reside in the cytoplasm, that part of the cell that surrounds the cell nucleus but is still contained within the cell membrane. They are integral components of the cell, but they have their own separate origin. Back in the distant past they were free-living algae that became engulfed by a primitive cell and have remained there ever since. Being originally separate organisms, mitochondria still retain their own DNA. Their special property is that they enable the cell to use oxygen to burn food. Until then, cells only had the apparatus for anaerobic metabolism and could not cope with atmospheric oxygen. With the help of their newly acquired mitochondria, however, cells could squeeze up to nine times as much energy from the same amount of food. In the early atmosphere, oxygen was toxic but mitochondria turned it into the life-giving gas upon which every animal species depends upon today.
The other unusual feature of mitochondria is that they are inherited only through the female line. The reason is that animal eggs are crammed full of mitochondria, while sperm don’t have any to speak of. To be entirely accurate, those few they do have don’t survive in the fertilised egg. This was the feature that appealed to Wilson and his team. Everyone inherits their mitochondrial DNA from their mother, who got it from her mother, who inherited it from her mother and so on back through time. Males and females have mitochondrial DNA – after all they both need to breathe oxygen – but only females pass theirs on to their offspring.
In complete contrast to mitochondria, the DNA in the cell nucleus is inherited more or less equally from both parents. This nuclear DNA controls most of the body’s functions, with the important exception of aerobic metabolism, which remains the responsibility of the mitochondria and its DNA. Unfortunately, ancestral connections traced backwards by nuclear DNA soon become extremely complicated. We all have two parents, four grandparents, eight great-grandparents, sixteen great-great-grandparents and so on. The number of ancestors doubles with each past generation, so by the time we go back only twenty generations, that’s about four hundred years for humans, we have over a million ancestors. It’s very unlikely that we have inherited DNA from all of those thanks to the random mixing of nuclear DNA with each generation, something I shall explain later in the book. Even so, we will have inherited DNA from a great many of them, but from whom we will never know. In comparison with this genetic muddle, there was only ever one woman in each generation who is our mitochondrial ancestor and whose DNA we have inherited. It is that simplicity which drew Allan Wilson to investigate mitochondrial DNA (or mDNA for short) rather than nuclear DNA in his representative sample of the world’s population.
The striking conclusion of this work was that if you went back far enough, everyone on the planet has inherited his or her mDNA from just one woman. In ways that we will come on to, Wilson estimated that she lived in Africa about 200,000 years ago. Unsurprisingly, she was immediately dubbed ‘Mitochondrial Eve’. The results also showed a clear connection between Africans and everyone else, suggesting that modern humans spent a long time in Africa before some of them left to populate the rest of the world. It’s as well to remind ourselves here that we are only considering strict female–female matrilineal inheritance, with no consideration, for now, given to the DNA from men.
It was a delightfully simple conclusion, although some people still find it confusing. Eve was certainly not the only woman alive at the time, just the only one to have direct matrilineal descendants living today. As now, couples can have only sons or no children at all, but it is only daughters who can pass mitochondrial DNA to the next generation. It follows that in the 10,000 or so generations since Eve, the only mDNA to survive to the present day has been passed along unbroken matrilineal lines, while that from Eve’s many contemporaries has been eliminated at some point along the way.
Though there have been some modifications in the ensuing thirty years, this overall concept of Mitochondrial Eve has stood the test of time. Wilson’s 1987 paper became a model for all future molecular genealogies, which have completely revolutionised our view of human origins. I analyse mitochondrial DNA samples from all over the world, and marvel at every one of them. They have each travelled unseen for tens of thousands of years in the cells of a continuous line of ancestors from ancient times until today when, at last, they reveal their secrets in the laboratory.
It took ten years before the Los Angeles-based biologists Robert Wayne and Carles Vilà published an equivalent genetic analysis for the dog.2 Like Wilson, they used mitochondrial DNA, but with a more advanced technique that examined the DNA sequence itself rather than the limited summary that was all that had been available to Wilson a decade earlier. I will say more later on about DNA sequences, including what they are and how to read them, but for now we will concentrate on the dogs.
Wayne and his team collected an impressive set of samples. In addition to 140 domestic dogs from 67 different breeds, Wayne also included wolves, coyotes and jackals in his analyses. The wolf collection came to a total of 162 animals from 27 locations worldwide. In addition, because they had been mooted as possible ancestors of modern dogs, Wayne included 5 coyotes and 12 jackals – 2 golden, 2 black-backed and 8 simien. When the mDNA sequences from all these animals were displayed in a molecular tree (referred to as Wayne’s tree) in the same way that Wilson had portrayed the human mitochondrial genealogy, the resemblance between the two was clear to see.
Wilson’s human tree (see here) divided the world population into two main branches,