The Seven Daughters of Eve

by Bryan Sykes

  In the early spring of 1992 we set off in two cars, having mapped out a complicated pincer movement whereby two pairs (we were joined by Catherine Irven, who took a week off from another project) would take different routes round the country, meeting up halfway to see how the other was getting on. My car at the time was a thirty-year-old Mk II Jaguar/Daimler that I had bought in a moment of utter madness from a garage fore-court in New Zealand the previous year and had had shipped over. It had a tendency to dislodge its water hoses every so often, causing the coolant to flood out and sending the engine temperature rocketing skywards before eventually conking out. So as well as all the blood sampling equipment on board, I was forced to carry a full toolkit – which was just as well. As we swept into the school in Bala, in central north Wales, there was a loud bang and a foul smell of burning oil filled the car. We pulled up in the car park at one side of the playground and, with the children watching from the classroom windows, I looked under the bonnet to see what had happened this time. There was black oil all over the place and clouds of acrid grey smoke were billowing up from where the oil had hit the exhaust pipes. This wasn’t the best way to arrive. I couldn’t tackle the job without getting covered in oil; hardly the best way to appear if you want to take blood samples. I shut the bonnet and walked into the school.

  Sometimes the problems didn’t stop outside. We had let the schools know that we would be happy for them to tell their local papers that we were coming if they wanted to. This had seemed a good idea – until I got to Ysgol-y-Gader in Dolgellau. Sitting with the head teacher, Catherine James, in her office was a reporter from the Caernarvon and Denbigh Herald.

  ‘So you are here to do blood tests on the children?’ he asked, opening the interview innocently enough.

  ‘Well, yes,’ I replied. ‘But only as a source of DNA, the genetic material.’

  ‘Why have you come to Dolgellau?’ he asked.

  I gave a short description of the background to our project and what we wanted to do. I explained that, because of their settled population over the last few centuries, we were particularly interested in the areas of Wales, like Dolgellau, where the Welsh language was still spoken. He didn’t look as if he believed me.

  ‘You’re really here because of the power station, aren’t you?’ He looked right at me. ‘You want to test the children for mutations, don’t you?’

  I was stunned. Dolgellau is just twelve miles south of the Trawsfynnyd nuclear reactor. A few months before, news reports had linked mutations found in children living near the nuclear reprocessing plant at Sellafield in Cumbria with their fathers working at the plant. The expression on the head teacher’s face rapidly changed from one of mild interest to intense suspicion. Was her school, was she, being used by undercover agents for the nuclear power industry posing as academics engaged on an innocent-sounding study of Celtic genes?

  ‘Of course not,’ I stammered and embarked on a stream of denials and reassurances. I repeated an account of the scientific background, a description of mitochondrial DNA, a summary of our work on ancient bone, and finally what I thought would be an irrefutable certificate of our integrity: ‘Anyway,’ I said confidently, ‘I’ve just come back from doing the same research in the South Pacific.’ That would do it. Or so I thought.

  ‘But isn’t that where they test the atom bombs?’ he replied, quick as a flash.

  I groaned, took a deep breath, and launched myself on another twenty minutes of explanation. Eventually they were both persuaded of our innocence and we could get on.

  At the end of my talk to the sixth form, the time came to ask for the blood samples. This was the point where I anticipated some further difficulty. Taking DNA from older schoolchildren (they had to be over sixteen to be able to give legal consent) ruled out taking a large blood sample, and we settled on a drop of blood taken from a finger-prick. This did involve some slight discomfort, and we were worried that no-one would want to do it. At first, to demonstrate how painless it was, I pricked my own finger and dabbed the little drop of blood on to a special absorbent card. Next, the teacher tried it; and one by one the pupils followed. For youngsters who haven’t done it before, it does require a little bit of courage. What happened next was an unexpected bonus. Precisely because they had done something brave, as soon as they had finished the children shot out of the classroom and round the school – it was the lunch break by then – daring their friends to do the same. A line of suppliants appeared, all swearing they were over sixteen, begging to be sampled not so much because of their intense interest in the project but because they wanted to prove themselves just as courageous as their friends. This wave of bravado spread to the staff room and the kitchens, so that by the start of afternoon classes we had blood samples from all the children old enough to take part, the teachers, the janitors and the dinner ladies.

  By the end of the week we had over six hundred blood samples, dried on to cards, from all over Wales – a remarkable haul that far exceeded our expectations. Even though it may not sound a lot, and is only a tiny proportion of the total Welsh population of almost three million, six hundred mitochondrial DNA sequences would be more than enough to get a good idea of the general genetic structure of the principality. Back in the lab we punched out the circles of dried blood from the cards and set about extracting the tiny amount of DNA they contained. Though there are a lot of cells in blood, most of them were no use to us. The red corpuscles, which carry the oxygen and make blood red, are so specialized that they do not need a nucleus or mitochondria; so these superfluous components are evicted early on in the life of the cells, which consequently do not have any DNA in them. Only the white blood cells, whose job it is to seek out and destroy invading bacteria and viruses, retain their own nuclear and mitochondrial DNA. White blood cells make up only 0.1 per cent of the cells in blood, so that while a drop of blood might have fifty million cells in it, only fifty thousand of them contain any DNA. But this is still plenty for the exquisitely sensitive DNA amplification method to work on. We followed the same recipe for getting DNA out of the blood spots as forensic laboratories use on blood-stained clothing prior to taking a genetic fingerprint. This involved boiling the dried blood spots in dilute alkali, which splits the cells open and dissolves the DNA, then adding a resin to absorb the iron which has been leached out from the red blood cells and which would otherwise interfere with the DNA amplification reaction. It worked very well indeed, and before long we had our first 100 Welsh mitochondrial DNA sequences.

  Compared to the relative simplicity of the Polynesian sequences, the Welsh results were all over the place. There was no sign of a clear-cut distinction in Wales analogous to what we saw in Polynesia, where the two separate clusters were so clearly the result of a mixture of people from very different origins. It looked as if we had a small number of little clusters which were all quite closely related to each other, rather than two big clusters separated by a large number of mutations. This did not look like the mixture of two very different types of mitochondrial DNA that we would have expected if the people of Wales had a mixed Neanderthal and Cro-Magnon ancestry. If Wales was going to be representative of Europe as a whole, then we were looking at a comparatively recent shared ancestry for everybody.

  Along the 500 base segment of the mitochondrial DNA control region, the average distance between any two people among the volunteers from Wales was three mutations. Remembering the rate at which the mitochondrial DNA clock ticks, so that two people with a single mutation between them can be said to have shared a common maternal ancestor about ten thousand years ago, the result from Wales showed that the average length of time it was necessary to go back in the past to connect any two people from Wales was only thirty thousand years; and even the most extreme difference between two of our volunteers, which was eight mutations, meant that they shared a common ancestor only about eighty thousand years ago. Although this is an enormously long time, it is nowhere near long enough for one of them to have been the descendant of a Nea
nderthal and the other of a Cro-Magnon. Unless the palaeontologists of the replacement school were way off the mark, Neanderthals and Cro-Magnons last shared a common ancestor at least two hundred and fifty thousand years ago. That means that the mitochondrial DNA of a Neanderthal descendant and that of a Cro-Magnon descendant would differ, on average, by at least twenty-five mutations. The biggest difference we saw in Wales was only eight. This was not a mixed population of ancient and modern humans. Either the Welsh were all Neanderthal or they were all Cro-Magnon. But which?

  The few sequences coming in from other parts of western Europe did not suggest to us that the Welsh were completely different from the rest. The stark alternatives of 100 per cent Neanderthal or 100 per cent Cro-Magnon ancestry seemed to apply throughout Europe. The acid test to distinguish which of the two competing ancestries was the real one would be a comparison between the European sequences and the corresponding data available from other parts of the world, which included our data from Polynesia. If there were big differences, of the order of twenty-five mutations or more, between the Europeans and the Polynesians, then the votes would go to a Neanderthal ancestry for all modern Europeans. If the differences were far less than that, it would mean a 100 per cent Cro-Magnon ancestry for Europeans, and a victory for the replacement school at the expense of the multi-regionalists.

  When we looked at the data, the biggest number of mutations we found between two people was the fourteen that separated Teri Tupuaki, a fisherman from Mangaia in the Cook Islands, and Mrs Gwyneth Roberts, who cooks the school lunches in Bala, north Wales. These two people, half a world apart, between them solved a puzzle that had divided scholarship for most of the twentieth century. Europeans were not that much different from the rest of the world; certainly nowhere near different enough to justify believing that they were all descended from Neanderthals. And since it was all or nothing, the Neanderthals must have become extinct. All modern Europeans must today trace their ancestry back to much more recent arrivals – to the Cro-Magnons, with their lighter skeleton, their much improved flint technology and their wonderful art. This was an absolute replacement of one human species by another. Whether it was an active and violent process, with the newcomers, our own ancestors, evicting or even killing the resident Neanderthals, or whether it was their technological and mental superiority that gradually marginalized the older inhabitants, the genetics alone cannot say. It is clear from the fossil record that the Neanderthals hung on for at least fifteen thousand years after the first Cro-Magnons reached western Europe some forty to fifty thousand years ago. When the last Neanderthal expired – probably in southern Spain, where the most recent skeletons have been found – his or her death drew a line under another phase in the human occupation of Europe. An era that had lasted for a quarter of a million years ended, finally and irreversibly, in a cave in southern Spain about twenty-eight thousand years ago.

  I confess to some surprise, and some disappointment, that the replacement was so complete. Even though we have now sequenced the mitochondrial DNA of more than six thousand Europeans, we have never yet found a single one that is even remotely credible as a Neanderthal survivor. We certainly haven’t sequenced everybody, nor have we had a chance to receive samples from every corner of the continent. I retain the hope that one day, when I look at a batch of read-outs from the sequencing machine, I will find a sequence so different from the rest that it calls out as the faint echo of a meeting between Cro-Magnon and Neanderthal which led to the birth of a child. If we ever did find one, we could not miss it. In 1997, DNA was sequenced from the very first Neanderthal skeleton from the original find in the Neander valley. It had twenty-six differences from the average modern European, more or less exactly as predicted for a species that last shared a common ancestor with Homo sapiens a quarter of a million years ago. The DNA sequence of a second Neanderthal, this time from the Caucasus mountains, was reported in the scientific literature in 2000. It was equally different from modern humans. These were not our ancestors.

  In 1998, the partial skeleton of a child with anatomical features intermediate between Neanderthal and Cro-Magnon was found in Portugal. Could this be evidence of interbreeding between the two types of humans? Perhaps. The child’s DNA has yet to be tested. But if this interbreeding were a frequent occurrence, then surely we would see the evidence in the modern mitochondrial gene pool, and we just don’t. If the interaction between Neanderthal and Cro-Magnon resembled more recent historical encounters between new arrivals and the original inhabitants of a territory, then we might expect the matings to be between Cro-Magnon males and Neanderthal females rather than the other way around. In that case, mitochondrial DNA would be an excellent reporter of these encounters, since while the offspring would have an equal mixture of nuclear DNA from both parents, their mitochondrial DNA, inherited from their mother, would be 100 per cent Neanderthal. As a geneticist it is very hard for me to imagine that social and other taboos were so strong that this never happened; but we must continually return to the evidence and the complete absence of any Neanderthal mitochondrial DNA in modern Europe.

  Could it be that the matings did occur but did not produce viable and fertile offspring? There are many examples from the animal world of hybridization between different species leading to perfectly healthy yet sterile offspring. The textbook example is the mule, the fruit of accidental or intentional matings between a male donkey and a female horse. The horse and donkey genes must be mutually compatible because mules are strong, healthy and fully functional, except when they come to breed. That’s because donkeys and horses have different numbers of chromosomes. Horses have 64 chromosomes, donkeys have 62. All mammals, including humans, inherit a half set of chromosomes from each parent to make up their full complement. So a mule gets 32 chromosomes from its horse mother and 31 from its donkey father – and so ends up with 63 chromosomes. That is not a problem for the body cells of the mule, because both horse parent and donkey parent genes can be read irrespective of which chromosome they are on. It’s only when the mule tries to breed that the confusion starts. For one thing, being an odd number, it is impossible to get a half set from 63 chromosomes. For another, the scrambling of the chromosomes that occurs at each generation leads to mule sperm and mule eggs with two copies of some genes and none of the others. For both these reasons, mules cannot produce offspring.

  Were the encounters between Neanderthal and Cro-Magnon also doomed to produce only one generation of infertile hybrids because they had different numbers of chromosomes? Our nearest primate relatives, the great apes (gorillas, chimpanzees and orang-utans) have one more chromosome than we do. At some point in the six million years since humans and great apes split away from our mutual common ancestor, two chromosomes that are still separate in the great apes fused together in the human lineage to produce our chromosome number 2. There is no knowing at which point along our own lineage this chromosome fusion occurred, but if it happened after the split between the lines that became Cro-Magnon and Neanderthal then there would be a chromosome imbalance, with Neanderthals having forty-eight chromosomes and Cro-Magnons only forty-six. The offspring of a mating between a Cro-Magnon and a Neanderthal would have forty-seven chromosomes and, although it may have been completely healthy, it would find itself in the same difficulty as the mule when it came to producing sperm or eggs. No-one knows how many chromosomes the Neanderthals had, but I suspect one day we will be able to find out. I think the experiment could be done. Until then we won’t know whether the complete absence of Neanderthal mitochondrial DNA in modern Europe is attributable to a fundamental biological or social incompatibility between our Cro-Magnon ancestors and the other human species with which they shared the continent.

  The publication of our genetic conclusion about the extinction of the Neanderthals met with a tongue-in-cheek chorus of disbelief from the British tabloids. The Daily Express published a picture of a Neanderthal alongside a photograph of a characteristically sullen Liam Gallagher, the Oasis singer. H
ow, it asked, could geneticists possibly claim that the Neanderthals were extinct when faced with such overwhelming evidence that they were alive and well in late twentieth-century Britain? Of course, they were predictably playing on the stereotype of Neanderthal as brutish and subnormal, for which there is no evidence at all. It was this kind of prejudice which dissuaded me from following up the several calls and letters I had from people who were sure that someone they knew (never themselves, of course) was definitely a Neanderthal. I still remember the letter from Larry Benson from Santa Barbara in California who wrote to tell me that a checkout clerk at his local supermarket had all the features of a Neanderthal. Apparently he was a really nice man, who (my correspondent assured me) would be only too pleased to provide a sample for DNA testing. I didn’t take it up.

  So the Neanderthals are extinct: completely replaced in Europe, and throughout their range, by the technologically and artistically superior new species Homo sapiens, represented in Europe by the Cro-Magnons. What happened in Europe, as far as we can tell from the genetics, also happened throughout the world, with Homo sapiens becoming first the dominant then the only human species, completely eliminating earlier forms. The Neanderthals, or Homo neanderthalensis as we are justified in calling them now that we are satisfied they constitute a separate species from our own, disappeared from Europe, and Homo erectus vanished from all of Asia. Whether Homo sapiens and Homo erectus ever overlapped in Asia is uncertain. In China there is a gap in the fossil record between 100,000 and 40,000 years ago. Perhaps Homo erectus had already died out before Homo sapiens arrived. There is no fossil evidence that Homo erectus ever reached Australia or the Americas, suggesting that Homo sapiens may have been the first humans to inhabit these two continents. In Africa, where Homo sapiens as a species first evolved, the equivalent replacement of other humans may have been sudden or gradual. Whatever the mechanism and whatever the reason, Homo sapiens has completely replaced other human species throughout the world. When the last Neanderthal died, twenty-eight thousand years ago, there was only one human species left to rule the planet. Ours.