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, one African and a second containing both some African and all the people from outside Africa before coalescing on a single matrilineal ancestor – ‘Mitochondrial Eve’. The Wayne dog DNA tree consisted of four main branches, each with a different, but still closely related, ancestor. Most dog breeds were placed in the major branch, which Wayne called branch I, and included many of the common breeds as well as some so-called ‘ancient breeds’ like the dingo, New Guinea Singing Dog, African Basenji and Greyhound. Branch II contained two Scandinavian breeds, the Elkhound and the Jämthund, while branch III included a variety of breeds such as the German Shepherd, Siberian Husky and Mexican Hairless. Finally, branch IV included Wirehaired Dachshund, a Flat-coated Retriever and an Otter Hound. This last branch also contained a few wolves, one of which, from Romania, was the only wolf in the whole study whose sequence exactly matched that of a number of dogs including a Toy Poodle, a Bulldog and, surprisingly, another Mexican Hairless.
The simplified diagram (see here) only shows the major mitochondrial groups. Within each circle are a number of breeds. They are not shown here but can be inspected in the original,3 where there are many examples of exactly the same mDNA sequence being found in several different breeds. For example, a Norwegian Buhund, a Border Collie and a Chow Chow had precisely the same mitochondrial DNA sequence. Equally, the same breeds could have different mDNA sequences and appear on different branches of the tree. For instance, the eight German Shepherds had five different sequences between them. We will consider what this means a little later.
Wilson’s Human Tree (simplified). (Image courtesy of Professor Bryan Sykes)
Wayne’s Dog Tree (simplified). (Image courtesy of Professor Bryan Sykes)
Had Darwin been alive to read it, he would have been itching to know where wolves, coyotes and jackals fitted into the tree, if at all. The answer was very clear. The coyote and jackal fell out of the main wolf/dog tree, as it were, immediately. Their DNA sequences were clearly quite different to all the dogs, and none made it into any of the four major branches. When it came to placing the wolf DNA sequences, the answer was equally striking, not because they were outside the dog tree but because they were deeply embedded within it. There was no doubt, from the mitochondrial DNA analysis, that all dogs were descended from wolves and from no other species. It was the first triumph of molecular genetics as applied to dogs – and by no means the last.
Darwin wasn’t wrong about much, but, by means he could never have foretold, his statement on dogs that ‘We shall probably never be able to ascertain their origin with certainty’ would turn out to be one of those rare exceptions. I am sure he would have been utterly delighted to be proved wrong.
Turn the clock forward another ten years to the present day and the Wayne dog tree is still alive. But, like the technical improvements we have considered in the decade between the Wilson and Wayne papers, there have been great strides in DNA analysis in the last ten years, which have led to some radical pruning of the original tree, while leaving the major branches intact.
Before we turn to the effect of these improvements in filling in the blanks in our knowledge of dog evolution there is one other important genetic system to consider. This is the Y-chromosome, the mirror image of mDNA in a genealogical sense in that it traces not the maternal but the paternal genealogy through time. Again the reason is simple enough. Only males have Y-chromosomes and they pass them on exclusively to their male offspring. In many species it is a less reliable witness than the mitochondrial equivalent because of the very variable mating success of males. In most species, including our own, males have the potential to father virtually unlimited numbers of offspring, or none at all, but females are restricted to just a few. This has major implications when we come to look at pedigree dogs.
Just as any conclusion about evolution based on mitochondria should carry the caveat that it can only reveal patterns based on females, so the Y-chromosome only traces the origins of males. Of course, ultimately they both have to tell more or less the same story, but there are fascinating twists and turns along the way.
In any sort of genetic analysis it is vital to be able to detect inherited variation, which is the lifeblood of genetics. Variation comes in many different forms – blood groups, hair colour, height or DNA sequence. You simply can’t do any genetics without it. For DNA the variation in sequence can be read directly, as it is for most mDNA comparisons, or it can use what are known as genetic markers. These are places where the sequence between, in this case, different Y-chromosomes, is known to differ. You can then test for the markers directly without having to sequence the whole chromosome, which saves a lot of time and money. But before you can use them you have to find them, which used to be an enormous bore. It is much better now, as we shall see.
The tedious process of discovering dog and wolf Y-chromosome markers was slow to get going and the first studies used a panel of only four markers. Luckily Y-chromosomes alone are spared the process of shuffling with other chromosomes, something else I will explain as we go along, and so the markers can be combined as blocks. So four markers (A–D), with two versions at each one (1 or 2), can differentiate sixteen Y-chromosomes (A1, B2, C1, D2; A2, B1, C2, D1 and so on), meaning that you can do a lot with just four markers and sixteen combinations.
A group from Sweden was the first to publish any wolf and dog results from this kind of analysis, having studied both Y-chromosomes and mitochondria
