The Seven Daughters of Eve (17 page)

Back in the brightly lit office, the teeth looked even better. Could it be that, inside the teeth, the dentine and the pulp cavity would be much better protected even than the bone? Could the few molecules of DNA, which were all we needed to test our theory, be hiding inside the teeth encased in an unbroken shield of enamel? Even though we had failed with Cheddar Man's toe, we agreed it would be worth a shot. But no-one had any experience of extracting DNA from teeth, especially teeth still embedded in the jaw, and there was no question of being allowed to remove them to make it easier. I promised to go away and devise a method of drilling into teeth in a way that did not mark the enamel and allowed them to remain in the jaw. If I could do that, then Chris would allow me to take a sample from the Gough's Cave specimen.

I was back within a fortnight, having practised on some teeth given to me by my dentist, Mr Miller. I had perfected a way of drilling into a molar tooth and getting the dentine out with the tooth still embedded in the jaw, and I brought with me some samples of my handiwork for Chris to inspect. After trying and rejecting a straightforward dental drill (the compressed air blew the powder all over the place) I had found a small modelling drill which had been recommended by a colleague at work and which I bought from an ironmonger on the Tottenham Court Road in London. It was just perfect for making the small entry hole just below the enamel. Once I had got inside the tooth, another, longer drill bit could be attached and wiggled to and fro, reducing the soft dentine to a fine powder. I rigged up a suction device and, using this, it was very easy to remove the powder from inside the tooth into a small test tube. The hole then only needed to be filled with a colour-matching cement and the tooth looked as good as new – as it were. And the dentine, at least in my trial teeth, was full of DNA.

To avoid the ever-present possibility of contamination with modern DNA I needed to drill the teeth from the Cheddar fossils in my own laboratory, where we had recently installed a filtered-air clean room. We had bought it as a ready-made unit constructed for the silicon chip industry. The incoming air was filtered and maintained at a positive pressure, which meant that there was no chance of dust or flakes of skin getting into the room when you went in through the air lock. It was an expensive and elaborate precaution, but well worth it. So I had to take the jaw back with me to Oxford – which was a nightmare. I had come into London on the bus, and it was on the bus that I returned with this priceless and irreplaceable specimen in its box on the seat beside me. Every few seconds I would turn to make sure it was still there, trying to imagine what I could say if I lost it. Thank God, I didn't; and by late afternoon it was safely locked away in the specimen cabinet back in Oxford.

The next day I started the extraction. It couldn't have gone better. The drill sank into the second molar easily, but not too easily – that would have been a sign of bad preservation – and there was a slight smell of burning in the air. This was the collagen being vaporized by the speed of the drill, a smell I used to hate during my own visits to the dentist but one I had now come to love as a sign that there was plenty of protein left in the specimen – and where there is protein there is usually DNA. When I switched on the suction pump, the pale cream powder came flying out of the tooth into the tube. There was lots of it – just under 200 milligrams. I took 50 milligrams, so as to leave plenty for a repeat, and started up the extraction process.

By the following evening I knew I had mitochondrial DNA from the tooth. Over the next two weeks I read through the sequence, checked it again and confirmed it by a second extraction. I was looking at the DNA sequence of the oldest human fossil, by far, that had ever been successfully extracted anywhere in the world. But that wasn't the most important thing. The crucial piece of information we were looking for was embedded in the details of the DNA sequence itself. Was this the same sequence as a thoroughly modern European, or was it an obscure relic that was now extinct?

The answer was crystal clear. The ancient DNA from Gough's Cave was also completely modern. The sequence lay at the centre of the largest of the seven mitochondrial clusters. It is by far the commonest sequence in modern Europe; and here we had found it in the tooth of a young man who had lived fully seven thousand years before the arrival of farming in Britain. Here was the proof that this sequence, this cluster, and, by extension, the others of a similar estimated age were well and truly established in Europe long before the farmers. The Upper Palaeolithic gene pool had not been fatally diluted by the Middle Eastern farmers. There was more of the hunter in us than anyone had thought.

Though I had got no further than drilling into his big toe, this was not the last encounter I had with Cheddar Man. We were re-introduced, so to speak, as part of a television documentary. Philip Priestley, an independent producer, was setting up a series of archaeology-based programmes for a west country TV station, and one of them was built around the excavation of a Saxon palace in Cheddar. By now our work on the genetic continuity between the Palaeolithic and the present day was reasonably well known, and it occurred to Philip that it would make good television if he could relate, through DNA, some of the present-day residents of the town with Cheddar Man himself. This seemed both fun and worthwhile; but I explained that we had already had a go at getting DNA from the Cheddar Man remains without success. If he could get permission from Chris Stringer, I was willing to try again, this time with the teeth, not the toe-bone, but only on condition that if nothing came of it we would not be filmed. I always work on that basis. I have seen too many programmes that begin with a big build-up anticipating a great scientific discovery at the finale, only to peter out in an inconclusive or unsuccessful experiment. So, with all the ground rules agreed, and after another nerve-racking journey on the bus, this time with an even more famous fossil in a box beside me, I drilled into Cheddar Man's first molar.

Out came the powder – not quite as clean as the earlier Gough's Cave material, but in sufficient quantity for an extraction. We found enough DNA for a reasonable sequence and were not surprised when it fitted comfortably into one of the seven clusters. Philip, understandably nervous as the deadline for filming got closer, was delighted and immediately organized the second strand of the piece, the sampling of the Cheddar residents. The site of the Saxon palace, featured in another programme in the series, is in the grounds of the local secondary school, and it made good sense to approach the school to see if they would agree to their pupils taking part in the programme. By now we had refined our DNA sampling procedure. We no longer used blood samples; instead we found that a small brush rubbed gently against the inside of the cheek picks up enough cells from the surface to give us plenty of DNA. After a short visit to the school, we had twenty samples from the sixth form volunteers and some of the teachers. Knowing how often we had found Cheddar Man's sequence in modern Britain, I reckoned there was a fifty–fifty chance of getting a close match in the twenty samples we had taken. Within four days we had the results. We knew the names, and (crucially, as it turned out) the ages of the volunteers. Philip was on the phone.

‘We've got a match,' I told him.

‘Who is it?' was his first question.

This wasn't part of the deal. While we had agreed to see if we could find a match among the twenty residents, I had not agreed to identify any individuals, for a very good reason. Although the children, and their parents, had signed forms consenting to have their DNA sampled and to take part in the television programme, I felt there was a risk that they might not have realized what they were letting themselves in for if the story broke in a big way. Though there is no way of knowing beforehand how big a story is going to become, the experience of Marie Moseley and the Iceman was an indication of its potential.

At this point Philip became distinctly agitated. He thought the story would be worthless without an individual identification. He immediately faxed me a copy of the consent form, but as far as I could see it was just a standard release – not, in my opinion, sufficient as a basis on which to claim consent to a possible worldwide media intrusion into the life of a teenager. I checked our list of sequences against the names and ages of the volunteers. There was not one match but three: two exact matches with Cheddar Man, and one with a single mutation; and while the two exact matches were children, the close match was a teacher, in fact the head of history who was organizing the filming in the school, Adrian Targett. I made the decision that I would identify Adrian Targett but not the two children. As it turned out, it was one of the best decisions I ever made. Unknown to me, Philip and his publicity team had organized a public ‘reveal' where Adrian Targett would be identified as Cheddar Man's relative in front of the cameras and in the presence of a television news crew. They, too, were beginning to sense the potential magnitude of the story. The next day, when I went to the newsagent, I could not believe my eyes. The story of Adrian Targett and Cheddar Man was in
all
the papers: from the London
Times
to the tabloid
Daily Star
, there was Adrian on the front page, posing beside his famous fossil relative. I bought the lot.

In the following days and weeks the story of Cheddar Man spread around the world. I met Adrian Targett on a TV chat show. He told me how one tabloid newspaper, famous for its pictures of topless women, had offered him a five-figure sum (so at least £10,000) to pose in a fur loincloth beside his ancient relative. Being a sensible man, conscious of his standing as a teacher, he declined. But it did make me wonder what the newspaper would have offered a teenage girl to wear the same outfit – or less. Even now, years later, people still remember the Cheddar Man story, if not always accurately. I was talking to an American audience in 2000 on something completely different when a woman asked me: ‘Are you the one who did the DNA from the Cheese Man?' At the time, not surprisingly, I had a full postbag for weeks after the story broke. Many letters were complimentary, including a very well-informed one from the inmates of San Quentin gaol in California, who were keen to discuss the findings at the next meeting of their anthropology study group. But the one that stood out came from the secretary to Lord Bath. It turned out that Cheddar Caves are part of Lord Bath's estate. Evidently he had read the story (though whether in the
Times
or the
Daily Star
I never discovered) and wanted to know if he too was related to Cheddar Man.

Alexander Thynn, Lord Bath, is the owner of Longleat, one of the most beautiful houses in England. It is famous for the safari park in the grounds, where visitors can watch the famous Longleat lions and other dangerous animals from the alleged safety of a car. Lord Bath himself, affectionately referred to as the Loins of Longleat, is well known for his idiosyncratic personal life. In addition to a legitimate wife and children, he has a stable of what he calls his ‘wifelets', many of whom live on the estate. This was definitely one to follow up, and the next weekend I was on the way to Wiltshire. I was led upstairs to the penthouse suite on the top floor of this magnificent Elizabethan house. Lord Bath himself, now in his sixties but with a youthful twinkle in his eye, was dressed in one of his collection of brightly coloured kaftans that bulged from a wardrobe close to an absolutely enormous wooden desk. The life clearly suited him. He poured out two large glasses of rosé from a tap on the wall as I went through the genetics with him. A few glasses later we got round to the test itself, and he brushed the inside of his cheek. During the course of the morning several other people passed through the penthouse, and each was encouraged to give a sample, which they cheerfully did. He was evidently very popular with his staff. By lunchtime we had half a dozen DNA brushes and it was time for me to leave.

When we got the results back it came as no surprise that Lord Bath was not closely related to Cheddar Man. There was no particular reason why he should be. But his butler, Cuthbert, one of the other people who had given a sample during my visit to Longleat,
was
an exact match. At a stroke he could claim an ancestry which stretched back nine thousand years, making the five-hundred-year pedigree of the Thynns look distinctly
nouveau
. I asked Lord Bath how Cuthbert had received this piece of news. Had it made him reassess his attitude to the aristocracy? ‘Well,' he replied with a smile, ‘he has been feeling very confident lately.'

We had now done about as much as we could to establish our claim that the maternal ancestors of the majority of modern Europeans were already living in Europe well before the arrival of farming. We could not say anything about other genes, only about mitochondrial DNA; but on this basis we had a clear picture of European prehistory, built up from both modern and fossil DNA, not of a massive replacement of the hunter–gatherers by the farmers but of a strong continuity back to the days of the Palaeolithic. There was only one of Cavalli-Sforza's criticisms that we could not answer. Whatever way you look at it, mitochondrial DNA is only one gene and, as such, subject to statistical fluctuations that might make it unrepresentative of the human genetic legacy as a whole. I did not think this very likely; but what was needed to substantiate our version of European prehistory was confirmation from another gene altogether.

The story I have narrated in this book is a history of the world recorded in the gene that is the easiest to read, mitochondrial DNA. So far, then, it is the gospel according to Eve. The beauty and simplicity of viewing the record of the past through mitochondrial DNA derive from its unique genetics, and in particular from the clear message that passes virtually unchanged from generation to generation, modified only by the slow ticking of the molecular clock as mutations gradually build up one at a time.

It would be strange indeed if a second, completely different, history were to be encrypted in the other genes that we carry. All these other genes are found on the chromosomes of the cell nucleus. According to the latest estimates, there are just under 30,000 of them. Are there 30,000 different versions of the human past waiting to be read? In one sense there are, because each of these genes could have a different history. Each of them might have a different common ancestor somewhere in the course of human evolution. However, while our nuclear genes have percolated down through time, it is quite impossible to trace all these lines back along a known pathway of descent in the way that we were able to do with mitochondrial DNA. The reason is that, unlike mitochondrial DNA, the nuclear genes are inherited equally from both parents. While you have only one mitochondrial ancestor in the last generation, your mother, you have two nuclear ancestors, your mother and your father. That doesn't sound too complicated. But go back one more generation. Now you have four nuclear ancestors, your grandparents; but still only one mitochondrial ancestor, your mother's mother. Go back another generation and there are eight nuclear ancestors, your great-grandparents; yet
still
only a single mitochondrial ancestor, your grandmother's mother. At each generation the number of nuclear ancestors doubles. Go back twenty generations, to about
AD
1500, and there could be, theoretically, over one million ancestors who could have contributed to your nuclear genes. In practice, many of these potential ancestors will actually be the same individuals, whose lines of descent have come down to you along different pathways, crossing between males and females through the generations in an unpredictable way.

Tracing the genealogy of all 30,000 genes through this maze of interconnections would be quite impossible. Add to that the confusion introduced by recombination, and the magnitude of the task becomes mind-numbing. The shuffling of chromosomes at each generation means that any one gene might itself be a combination of one part from one ancestor and another from someone else. Reading the different individual versions of human history from these genes, and bits of genes, in the cell nucleus is impossibly complicated at the moment. It will take a long time to advance beyond the kind of crude summaries of human history that we already have from the days of gene frequency comparisons.

However, one gene – or, more correctly, one chromosome – is immune from these ghastly complications. It is called the Y-chromosome, and it has only one purpose in life: to create men. By comparison with the other human chromosomes it is small and stunted, and it carries only one gene which really matters. This is the gene that stops all human embryos from turning into little girls. Without a Y-chromosome, the natural course of events is for the human embryo to develop into a female. If an embryo has a Y-chromosome, and if the gene, which has been given the undistinguished name SRY, is working properly, then it will trigger a number of other genes on different chromosomes to steer the development of the embryo away from becoming a female and towards becoming a male. The SRY gene activates genes on other chromosomes which suppress the development of ovaries and instead promote the growth of testes and the production of the male hormone testosterone.

Two observations pinpointed the key part played by the SRY gene in sex determination. Very rarely, in something like one in 20,000 births, a girl is born with a Y-chromosome. These girls look normal, they have normal intelligence and they develop normally, though they are usually slightly taller than average. But at puberty their ovaries and uterus do not develop properly, and they cannot have children. Genetic analysis of the Y-chromosomes of these girls shows that the SRY gene is either missing altogether or contains a mutation that stops it working properly. The other piece of graphic evidence that the SRY gene is itself sufficient to make a male came from research on mice. Male mice have Y-chromosomes too, and they carry the mouse equivalent of the human SRY gene – called, in a burst of imaginative classification, Sry. In a very elegant genetic engineering experiment, the Sry gene was cloned from a male mouse and transplanted into a fertilized mouse egg that would otherwise have turned into a female. Despite the fact that the mouse embryo had only the cloned gene to work on, rather than a complete Y-chromosome, it turned into a male.

So this is how the sex of a baby is determined. Fathers, being male, have a Y-chromosome. Half of their sperm contains his Y-chromosome, carrying the SRY gene, and the other half carries another chromosome – the X-chromosome – instead. The sex of the baby depends entirely on whether or not the particular sperm that fertilizes the mother's egg contains an X-or a Y-chromosome. If the successful sperm carries an X-chromosome, then the child will be a girl. If it carries a Y-chromosome instead, the child will be a boy. The woman has no influence whatsoever on the sex of the child. How many women in past centuries would have loved to know this simple fact? How often was the ‘failure' to produce sons attributed to a failure, deliberate or not, on the part of wives to conceive boys?

Just as mitochondrial DNA follows a maternal genealogy through the generations, the inheritance of Y-chromosomes by sons from their fathers should trace the mirror-image paternal pathway from one generation to the next. If the Y-chromosome could be genetically typed, and if it were not involved in recombination that would scramble the message, then there was good reason to believe that it would be the perfect complement to mitochondrial DNA in reading the history, not of women, but of men. The Y-chromosome, in common with all the chromosomes of the nucleus, is a very long, linear molecule of DNA. While mitochondrial DNA has just over sixteen and a half thousand bases in its DNA circle, the Y-chromosome stretches for about sixty million bases from one end to the other. It might be the runt among human chromosomes, but it still packs more than four thousand times as much DNA as mitochondria. Moreover, there is some gene shuffling within it. At the tips of each end of the Y-chromosome there is a section of DNA that recombines with the X-chromosome; but since these sections involve less than 10 per cent of the whole chromosome, this doesn't present a great problem. Genes that are on the recombining part of the Y-chromosome will trace a mixed genealogy, swapping unpredictably from males to females just like all the other nuclear genes. However, the remaining 90 per cent of the Y-chromosome, between the recombining ends, is not scrambled. This long segment travels intact through the generations. But are Y-chromosomes different from one another, and if so how do they differ? Only if there were variety and diversity in the Y-chromosome would it be of any value at all for reading human history. If all Y-chromosomes were exactly the same, they would be no use for our purposes.

Chromosomes are intensively studied under the microscope by trained cytogeneticists in medical genetics laboratories who are on the lookout for abnormalities that can diagnose inherited diseases like Down's syndrome or explain the cause of infertility. With all this activity going on, cytogeneticists had noticed that some Y-chromosomes stood out as being much longer than the average. This was promising; but it was not a very precise way of differentiating between Y-chromosomes on a large scale. Besides, the lengths were unstable and changed between one generation and the next. What was needed was the same kind of testing involving Y-chromosome DNA that had identified mitochondrial DNA as such a star. Then it would be straightforward to fingerprint Y-chromosomes from hundreds or thousands of volunteers easily and cheaply. But how were the segments of Y-chromosomes that were going to show the biggest differences among people to be found?

The rich diversity of the mitochondria is concentrated in a small DNA circle of only a few thousand bases. Better still, the control region compresses about a third of the diversity of the whole mitochondria into just five hundred bases that can be sequenced in a single run on an automated sequencing machine. Would something similar be found in the Y-chromosome? The answer was not long in coming. Several labs, hoping for the best, began to look for differences between Y-chromosomes by sequencing through the same segment of Y-chromosome DNA from volunteers who were as distantly related to one another as possible. In one of the first studies, 14,000 bases were sequenced from the Y-chromosomes of twelve men from widely different geographical origins. Only a single mutation was ever found. If an equivalent 14,000 bases had been taken from mitochondrial DNA instead of the Y-chromosome, they would have shown dozens of mutations in the same number of people. Another lab sequenced a 700 base segment of one gene from the Y-chromosomes of thirty-eight different men without finding a single difference in any of them!

This was all rather depressing for the scientists involved (thankfully, I wasn't one of them). There was a lot of head-scratching. Why were Y-chromosomes so similar all around the world? Since Y-chromosomes didn't carry any genes to speak of, and were full of ‘junk' DNA which had no obvious function, the expectation was that there should be more, not less, variation on the Y-chromosome than on regular, gene-rich chromosomes. Mutations are free to accumulate in ‘junk' DNA because it doesn't do anything, so its precise sequence doesn't really matter. Most mutations that occur in genes which do have important functions interfere with their proper working and are soon eliminated by natural selection. It was certainly a puzzle to find that there were so few mutations on the Y-chromosome.

The most popular theory advanced to account for this lack of variation was that it had to do with the fact that, under the right circumstances, men can have a lot more children than women. If, in the past, only a few men had lots of children, and therefore lots of sons, their Y-chromosomes would spread around quickly at the expense of the Y-chromosomes of their unfortunate male contemporaries who had few children or none at all. If this had happened a lot, the theory went, there would be far fewer different Y-chromosomes around today than if all men had roughly the same number of children. It's true that there have been some particularly prolific males. The world record holder is Moulay Ismail, Emperor of Morocco, who is alleged to have had 700 sons (so presumably as many daughters) by the time he was forty-nine in 1721. He died in 1727 – so there was another six years to have some more. The most prolific woman comes way behind this. She is Mrs Feodora Vassilyev, a Russian woman who produced sixty-nine children between 1725 and 1765. They were all multiple births – sixteen pairs of twins, seven sets of triplets and four lots of quadruplets – so she was a remarkable woman in that respect as well. The capacity of women to produce large numbers of children is limited by their biology, which restricts them to one pregnancy a year at most. Men, on the other hand, are not restricted by this timetable and can, in theory, have thousands of children. But the fantasy of enormously prolific males seeding the entire world, thereby reducing the diversity of Y-chromosomes by their prodigious feats of polygamy, turned out to be just that. A fantasy. A hard slog in laboratories around the world over the past ten years has found that there are plenty of mutations on the Y-chromosome after all.

These mutations come in two main types. The first is exactly the same as those we are already used to seeing in mitochondrial DNA: the simple change from one base to another. However, unlike in mitochondria, where they are neatly compressed into the control region, these mutations are spaced out at irregular intervals right along the length of the Y-chromosome. This is a practical nuisance because each one has to be tested individually, but it is not an insuperable obstacle. The other type of mutation is very uncommon in mitochondria, although we did encounter one example in the Polynesian samples. That is where there was a deletion of nine bases from the mitochondrial DNA circle. A careful look at the DNA sequence around that region revealed that in fact this wasn't so much a deletion from the Polynesian mitochondrial DNA as a doubling up, a duplication, of that nine-base segment in the rest of us. This type of mutation, where short segments of DNA are repeated over and over again, is remarkably common in the nuclear chromosomes and, thank heavens, in this respect the Y-chromosome is no exception. Dozens of these repeated segments have been discovered on the Y-chromosome, and the difference between individuals lies in the number of repeats. Fortunately, this is an easy thing to measure. This rich source of variation suddenly revealed that there are thousands of different Y-chromosomes around that can be distinguished from one another on the basis of these two sorts of mutation. Genetic fingerprinting of Y-chromosomes has become a reality.

Because it has been such a struggle for the scientists involved to find the useful mutations, laboratories have been very careful about whom they tell when they find a new one. As a consequence, labs have organized themselves into rival cliques which have used different sets of mutations to fingerprint Y-chromosomes; there is not yet a common standard. This means that there are different evolutionary networks being produced by the different confederations of laboratories. This is only a temporary situation, and I hope and expect that in the near future these will be reconciled into a scheme which everyone can accept. But how is it looking up to now? In particular, does the history of Europe revealed by the Y-chromosome bear any resemblance to the one read from mitochondrial DNA which forms the basis for this book? Does the Y-chromosome version of events agree or disagree with the mitochondrial DNA in placing such a great emphasis on the Palaeolithic as the source of our genetic legacy? In other words, does the history of men agree with the history of women? The answer came in an article published in the 10 November 2000 edition of the journal
Science
.