We came into this discussion from the realization that sexual mixing messes up the historical record. We distinguished two ways in which the effects of sex could be escaped. We've just dealt with one of them, following from the fact that sex does not mix genes between species. This opens up the possibility of using DNA sequences to reconstruct remotely ancient family trees of our ancestors that lived long before we became recognizably human. But we've already agreed that if we go back that far we humans are all definitely descended from the same single individual {45} anyway. We wanted to find out how recently we could still claim common descent with all other humans. To discover that, we have to turn to a different kind of DNA evidence. This is where African Eve comes into the story.

African Eve is sometimes called Mitochondrial Eve. Mitochondria are tiny, lozenge-shaped bodies swarming by the thousands in each one of our cells. They are basically hollow but with a complicated interior structure of membranous baffles. The area afforded by these membranes is much larger than you'd think from the outside appearance of mitochondria, and it is used. The membranes are the production lines of a chemical factory – more precisely, a power station. A carefully controlled chain reaction is strung out along the membranes – a chain reaction involving more stages than those in any human chemical factory. The result is that energy, originating in food molecules, is released in controlled steps and stored in reusable form for burning later, wherever it is needed, anywhere in the body. Without our mitochondria, we'd die in a second.

That's what mitochondria do, but we are here more concerned with where they come from. Originally, in ancient evolutionary history, they were bacteria. This is the remarkable theory championed, by the redoubtable Lynn Margulis of the University of Massachusetts at Amherst, from heterodox origins through grudging interest to triumphant near-universal acceptance today. Two billion years ago, the remote ancestors of mitochondria were free-living bacteria. Together with other bacteria of different kinds, they took up residence inside larger cells. The resulting community of (“prokaryotic”) bacteria became the large (“eukaryotic”) cell we call our own. Each one of us is a community of a hundred million million {46} mutually dependent eukaryotic cells. Each one of those cells is a community of thousands of specially-tamed bacteria, entirely enclosed within the cell, where they multiply as bacteria will. It has been calculated that if all the mitochondria in a single human body were laid end to end, they would girdle the Earth not once but two thousand times. A single animal or plant is a vast community of communities packed in interacting layers, like a rain forest. As for a rain forest itself, it is a community seething with perhaps ten million species of organisms, every individual member of every species being itself a community of communities of domesticated bacteria. Not only is Dr. Margulis's theory of origins – the cell as an enclosed garden of bacteria – incomparably more inspiring, exciting and uplifting than the story of the Garden of Eden. It has the additional advantage of being almost certainly true.

Like most biologists, I now assume the truth of the Margulis theory, and in this chapter I mention it only to follow up a particular implication: mitochondria have their own DNA, which is confined to a single ring chromosome, as in other bacteria. And now for the point that this has all been leading up to. Mitochondrial DNA does not participate in any sexual mixing, either with the main “nuclear” DNA of the body or with the DNA of other mitochondria. Mitochondria, like many bacteria, reproduce simply by dividing. Whenever a mitochondrion divides into two daughter mitochondria, each daughter gets an identical copy – give or take the odd mutation – of the original chromosome. Now you see the beauty of this, from our point of view as long-distance genealogists. We found that where our ordinary DNA texts are concerned, in every generation sex scrambles the evidence, confusing the {47} contributions from paternal and maternal lines. Mitochondrial DNA is blessedly celibate.

We get our mitochondria from our mother only. Sperms are too small to contain more than a few mitochondria; they have just enough to provide the energy to power their tails as they swim toward the egg, and these mitochondria are cast away with the tail when the sperm head is absorbed in the egg at fertilization. The egg is massive by comparison, and its huge, fluid-filled interior contains a rich culture of mitochondria. This culture seeds the child's body. So whether you are female or male, your mitochondria are all descended from an initial inoculum of your mother's mitochondria. Whether you are male or female, your mitochondria are all descended from your maternal grandmother's mitochondria. None from your father, none from either grandfather, none from your paternal grandmother. The mitochondria constitute an independent record of the past, uncontaminated by the main nuclear DNA, which is equally likely to come from each of four grandparents, each of eight great-grandparents and so on back.

Mitochondrial DNA is uncontaminated, but it is not immune to mutation – to random errors in copying. Indeed, it mutates at a higher rate than our “own” DNA, because (as is the case with all bacteria) it lacks the sophisticated proofreading machinery our cells have evolved over the eons. There will be a few differences between your mitochondrial DNA and mine, and the number of differences will be a measure of how far back our ancestors diverged. Not any of our ancestors, but our ancestors in the female female female… line. If your mother happens to be a purebred native Australian, or a purebred Chinese, or a purebred!Kung San of the Kalahari, there {48} will be rather a lot of differences between your mitochondrial DNA and mine. It doesn't matter who your father is: he can be an English marquess or a Sioux chieftain, for all the difference it makes to your mitochondria. And the same goes for any of your male ancestors, ever.

So there is a separate mitochondrial Apocrypha, handed down alongside the main family Bible but with the great virtue of going down the female line only. This is not a sexist point; it would be just as good if it came down through the male line only. The virtue lies in its intactness, in its not being chopped and merged in every generation. Consistent descent via either sex but not both is what we, as DNA genealogists, need. The Y chromosome which, like a surname, is handed down the male line only, would in theory be just as good, but it contains too little information to be useful. The mitochondrial Apocrypha is ideal for dating common ancestors within one species.

Mitochondrial DNA has been exploited by a group of researchers associated with the late Allan Wilson in Berkeley, California. In the 1980s, Wilson and his colleagues sampled the sequences from 135 living women drawn from all around the world – Australian aborigines, New Guinea highlanders, Native Americans, Europeans, Chinese and representatives of various peoples in Africa. They looked at the numbers of letter differences separating each woman from each other woman. They gave these numbers to a computer and asked it to construct the most parsimonious family tree it could find. “Parsimonious” here means doing away as much as possible with the need to postulate coincidence. This requires some explaining.

Think back to our earlier discussion of horses, pigs and {49} yeast, and the analysis of cytochrome c letter sequences. You remember that horses differ from pigs in only three such letters, pigs differ from yeast in forty-five letters, and horses differ from yeast in forty-six letters. We made the point that, theoretically, since horses and pigs are connected to each other by a relatively recent common ancestor, they should be exactly the same distance from yeast. The difference between forty-five and forty-six is an anomaly, something that in an ideal world would not be there. It may be due to an additional mutation on the route to horses or a reverse mutation on the route to pigs.