The Seven Daughters of Eve (14 page)

Figure 4a

Figure 4b

Unknown to our team in Oxford, a German mathematician, Hans-Jürgen Bandelt, had been working on the theoretical treatment for just such a scenario. He was looking for the best way of incorporating DNA sequences with the sort of parallel mutations we were finding into an evolutionary diagram. He contacted us because he needed some real data to chew over, and we at once realized that we were both thinking along the same lines and solving the problem in the same way, drawing networks and not trees. The big difference was that Hans-Jürgen was able to apply a proper mathematical rigour to the process of constructing the networks, an advantage which was crucial to their acceptance as a respectable alternative to the traditional trees.

Figure 4c

With this important obstacle overcome, we could now concentrate on the picture that was slowly emerging from the European sequences. Whereas in Polynesia we saw two clearly differentiated clusters, in Europe the networks were sorting themselves out into several related clusters, groups of mitochondrial sequences that looked as though they belonged together. These were not so obviously distinct or so far apart as their Polynesian equivalents, in the sense that each cluster had fewer mutations separating it from the others. We had to look hard to make out the boundaries, and Martin Richards and I spent many hours trying to decide how they best fitted together. Were there five or six or seven clusters? It was hard to decide. At first we settled on six. We found out later that we had missed a clue which would have divided the biggest of the six clusters into two smaller ones to give us the seven clusters that we now know form the framework for the whole of Europe.

What mattered to us at the time was not so much precisely how many clusters there were, but that there were clusters at all. This was not the homogeneous and unstructured picture presented by the scientific articles which had been published by the summer of 1995, leading their authors to despair that anything useful could be found out about Europe from mitochondrial DNA. The clusters might have been hard to see, in fact impossible to distinguish without the clarifying summary of the network system, but there was no mistaking their presence. Now that we had our seven defined clusters we knew what we were dealing with, and could start looking at where they were found, and how old they were. Because we had a figure for the mutation rate of the mitochondrial control region we could combine this with the number of mutations we saw in each of the seven clusters to give us an idea of how long it had taken each cluster to evolve to its current stage of complexity. This had worked beautifully in Polynesia, where the two clusters we found had accumulated relatively few mutations within them for the simple reason that humans had only been in Polynesia for three to four thousand years at most. When we worked out the genetic dates for the two Polynesian clusters in the different island groups by factoring in the mutation rate, they corresponded very well to the settlement dates derived from the archaeology. The earliest islands to be settled, Samoa and Tonga in western Polynesia, had the most accumulated mutations within the clusters and a calculated genetic age of three thousand years, very similar to the archaeological age. Further east, the Cook Islands had fewer accumulated mutations and a younger date. Aotearoa (New Zealand), the last Polynesian island to be settled, had very few mutations within the clusters and the youngest date of all.

When we applied exactly the same procedure to the clusters in Europe we got a surprise. We had been expecting relatively young dates, though not as young as in Polynesia, because of the overwhelming influence of the agricultural migrations from the Near East in the last ten thousand years that were so prominent a feature in the textbooks. But six out of the seven clusters had genetic ages much older than ten thousand years. According to the version of Europe's genetic history that we had all been brought up on, a population explosion in the Near East due to agriculture was followed by the slow but unstoppable advance of these same people into Europe, overwhelming the sparse population of hunter–gatherers. Surely, if this were true, the genetic dates for the mitochondrial clusters, or most of them at least, would have to be ten thousand years or less. But only one of the seven clusters fitted this description. The other six were much older. We rechecked our sequences. Had we scored too many mutations? No. We rechecked our calculations. They were fine. This was certainly a puzzle; but still we didn't question the established dogma – until we looked at the Basques.

For reasons discussed in an earlier chapter, the Basques have long been considered the last survivors of the original hunter–gatherer population of Europe. Speaking a fundamentally different language and living in a part of Europe that was the last to embrace agriculture, the Basques have all the hallmarks of a unique population and they are proud of their distinctiveness. If the rest of Europe traced their ancestry back to the Near Eastern farmers, then surely the Basques, the last survivors of the age of the hunter–gatherers, should have a very different spectrum of mitochondrial sequences. We could expect to find clusters which we saw nowhere else; and we would expect not to find clusters that are common elsewhere. But when we pulled out the sequences from our Basque friends, they were anything but peculiar. They were just like all the other Europeans – with one noticeable exception: while they had representatives of all six of the old clusters, they had none at all of the seventh cluster with the much younger date. We got hold of some more Basque samples. The answer was the same. Rather than having very unusual sequences, the Basques were as European as any other Europeans. This could not be fitted into the scenario in which hunters were swept aside by an incoming tide of Neolithic farmers. If the Basques were the descendants of the original Palaeolithic hunter–gatherers, then so were most of the rest of us.

But what about the cluster that was absent from the Basques – the cluster that was distinguished from the rest by having a much younger date compatible with the Neolithic? When we plotted the places where we found this cluster on a map of Europe, we found a remarkable pattern. The six old clusters were to be found all over the continent, though some were commoner in one place than in others. The young cluster, on the other hand, had a very distinctive distribution. It split into two branches, each with a slightly different set of mutations. One branch headed up from the Balkans across the Hungarian plain and along the river valleys of central Europe to the Baltic Sea. The other was confined to the Mediterranean coast as far as Spain, then could be traced around the coast of Portugal and up the Atlantic coast to western Britain. These two genetic routes were exactly the same as had been followed by the very first farmers, according to the archaeology. Early farming sites in Europe are instantly recognizable by the type of pottery they contain, just as Lapita ceramics identify the early Polynesian sites in the Pacific. The push through central Europe from the Balkans, which began about seven and a half thousand years ago, is recorded by the presence at these early sites of a distinctive decorative style called Linear pottery, in which the vessels are incised with abstract geometric designs cut into the clay. The Linear pottery sites map out a slice of central Europe where, even today, one branch of the young cluster is still concentrated. In the central and western Mediterranean, early farming sites are identified by another style of pottery, called Impressed ware because the clay is marked with the impressions of objects, often shells, which have been pressed into the clay before firing. Once again, the concordant distribution of Impressed ware sites and the other branch of the young cluster stood out. This didn't look like a coincidence. The two branches of the young mitochondrial cluster seemed to be tracing the footsteps of the very first farmers as they made their way into Europe.

There was one further piece of evidence we needed before we could be confident enough to announce our radical revision of European prehistory to the world. If the young cluster really were the faint echo of the early farmers, then it should be much commoner in the Near East than it is in Europe. At that time, the only sequences we had available from this region were from the Bedouin of Saudi Arabia. While only about 15–20 per cent of Europeans belonged to the young cluster – depending on which population was being studied – fully half of the Bedouin were in it.

We now had the evidence that most modern Europeans traced their ancestry back, far beyond the Neolithic, to the hunter–gatherers of the Palaeolithic, including the first Cro-Magnons that had replaced the Neanderthals. Certainly there had been new arrivals from the Middle East in the Neolithic; the correspondence between the geographical pattern of the young cluster and the archaeologically defined routes followed by the early farmers was good evidence of that. But it was not an overwhelming replacement. The young cluster makes up only 20 per cent of modern Europeans at the very most. We were ready to go public.

Professor Luigi Luca Cavalli-Sforza is a man whose eminence is matched only by his elegance. Erect of posture, even in his late seventies, his silver hair always immaculately groomed, he is equally at home in the busy conference rooms of the academic circuit by day and the exclusive restaurants that welcome the most distinguished delegates by night. His contributions and influence in the field cannot be over-estimated. Scientists who once studied under him, either in Italy or later at Stanford University in California, today hold many of the important academic positions in the discipline of human population genetics. It was Luca who first formulated the theory which had come to dominate European prehistory over the preceding quarter-century. According to this theory, or at least the version believed by archaeologists, farmers from the Near East had overwhelmed the descendants of the Cro-Magnons, who themselves had replaced the Neanderthals. This was a large-scale replacement which meant that most Europeans traced their ancestry back not to hunter–gatherers but to farmers.

Having collected together the records of thousands of blood and other genetic tests from all over Europe, Luca had amalgamated the results into a gradient of gene frequencies that summarized this mountain of data. These gradients were organized into simple vectors, called principal components, which were projected as lines on a map. The most striking, the first principal component, led diagonally across Europe from Anatolia in Turkey to Britain and Scandinavia in the north-west. To Luca and his colleagues, this was the signature of a massive influx of people into Europe from the Near East. The fit between the south-east/north-west axis of this genetic slope and the routes followed by the early farmers according to the archaeology available at the time was convincing. The farmers had overrun Europe.

The influence of Cavalli-Sforza's conclusion spread far beyond the narrow bounds of human genetics, through archaeology and related disciplines. Although there were some archaeologists who did not share his conclusion and saw in the record signs of only minimal population movement, they had a hard time making their views heard. Like any academic discipline archaeology has its fashions, and the fashion in Europe was for a large-scale settlement by incoming farmers. It had not been so when Cavalli-Sforza and his colleague, the American archaeologist Albert Ammerman, first put forward their ideas in the 1970s. At that time the contemporary taste was for entirely indigenous development; for the gradual adoption of agricultural methods and practice by the mesolithic hunter–gatherers of Europe without a large-scale movement of people. The original argument put forward by Ammerman and Cavalli-Sforza was for at least some movement, some migration from the Near East. Launched in a hostile intellectual atmosphere, this process was described in a term which sounded unchallenging. It was called ‘demic diffusion'. Demic means ‘to do with people', and diffusion is a gentle phrase implying the gradual inching outward of the farmers from their stronghold in the Near East. However, demic diffusion was not just a descriptive idea; it had a strong mathematical basis. It took as its foundation a mathematical model developed by Arthur Mourant's mentor, the great statistical geneticist R. A. Fisher, who produced equations to describe the spread of anything – animals, people, genes, ideas – outwards from a growing centre. This mathematical model was given the dramatic title the ‘wave of advance'.

Over the past twenty-five years, the ‘wave of advance', the name of the mathematical model, has gradually taken over from ‘demic diffusion' as the description of the spread of farming. I don't entirely understand the reasons for this. It may be that as the model became more widely accepted there was no need to present it in a tone conciliatory to the intellectual atmosphere into which it was introduced, resistant to any theories which suggested large-scale movements of people; or it may just have been that archaeologists were beguiled by the power of the phrase ‘wave of advance'. In any event, somehow the dramatic had taken over from the gentle. The idea of a gradual influence of incoming agriculturalists had been replaced in the collective psyche by the image of an unstoppable tidal wave of land-grabbing farmers that swept away everyone and everything in its path. The notion that the farmers overwhelmed the original inhabitants became the prevailing lore among archaeologists.

Not only had this tsunami of people brought agriculture to Europe, it was also responsible, according to the distinguished Cambridge archaeologist Colin Renfrew, for the introduction and dissemination of the language family to which most European languages belong. Although it is not readily apparent to any but professional linguists, there is no doubt that, with only a few exceptions, the languages spoken in Europe today all stem from a common root. They belong to a family of languages called Indo-European. The way in which sentences are constructed and many of the words they share betray a relationship among them that may not be obvious to most of us as we struggle with our phrase books. It takes a linguist to connect English and Portuguese, Greek and Gaelic. The exceptions are the Basques' Euskara, Finnish, Estonian, Lapp and Hungarian. While Euskara is unique among living European languages and cannot be reliably linked to any other (though some linguists see a connection with languages of the Caucasus mountains), the other four are members of the Uralic language family which has its origins further east.

The Indo-element in Indo-European is there because there is a strong connection, again visible only to linguists, between the European languages and Sanskrit. This link was discovered by William Jones in 1786 while he was working as a judge in India for the British Raj. It was an amazing piece of amateur scholarship; indeed, Jones invented the concept of language families that is still a feature of comparative linguistics today. The essential idea of a language family is that all the different languages within it have evolved from a common root, almost certainly a language that is by now extinct. This raises the question of where the original Indo-European language was spoken and, importantly, how it spread out from there. Renfrew deduced that the original Indo-European language was spoken in Anatolia in central Turkey, and was then spread to Europe by the first farmers. A massive replacement of the hunter–gatherers by the agricultural ‘wave of advance', as demic diffusion had surreptitiously become, was just what was needed to spread the language from its base in Anatolia.

There was now a powerful confederation of genetics, archaeology and linguistics in support of the argument that the mesolithic hunter–gatherers of Europe had been overwhelmed by the neolithic farmers. So, by the time we produced our startling results, the received wisdom was that most native Europeans today were descended not from the people who had endured the rigours of the last Ice Age but from the farmers who had walked in only ten thousand years ago with a bag of seeds and a few animals. But it just didn't fit with the ages of our DNA clusters. We were sure that the strongest signals from the mitochondrial DNA in today's Europeans were from much further back in the past than ten thousand years. We saw these signals as the genetic echo of the hunter–gatherers. These were not the faint whispers of a defeated and sidelined people but a resonant and loud declaration from our hunter–gatherer ancestors: ‘We are still here.'

I decided to present our work at the Second Euroconference on Population History, held in Barcelona in November 1995. I knew very well that the main proponents of the ‘wave of advance' theory would be there, so at least what I had to say would be noticed. I was given a twenty-minute slot. The conference room was vast, with four hundred delegates and room for many more. I was introduced by the convenor, Sir Walter Bodmer, Fellow of the Royal Society, a long-time associate of Luca Cavalli-Sforza and co-author with him of two influential textbooks on genetics. Walter is not widely known for his conciliatory remarks, but I did think ‘And the next speaker is Bryan Sykes who is talking about mitochondria. I don't believe in mitochondria' was a less than gracious introduction. I began to lay out the basis for our revision of European prehistory.

Walter and Luca were both sitting below the podium, side by side in the front row. It is surprising how much you can take in when addressing even a large audience such as this. As I went from one point to another I could see that Walter was getting agitated. He began to mutter to himself, then to Luca; at first inaudibly, then louder and louder. ‘Rubbish,' ‘Nonsense,' I thought I heard him say. He began to fidget, to half raise himself in his seat then sit back down, as one slide followed another in my presentation. As I came to the concluding slide, I could almost see the steam coming out of his ears.

No sooner had I finished talking than Walter and Luca were on their feet, throwing questions at me. I have known Walter for ages and seen him in action many times. I have watched him crush young researchers by his aggressive questioning, and I was determined the same would not happen to me. There is only one effective remedy with Walter, and that is to argue back. I had been expecting fireworks, and as I stood there under this barrage, I began to see it all as a piece of theatre – like a cross-examination in the High Court or a fierce exchange at the Despatch Box in the House of Commons. I began to enjoy myself.

At one point Walter insisted that they (he and Luca) had never said that the farmers had overwhelmed Europe and replaced the hunter–gatherers. I had brought along a copy of their jointly written textbook
Genetics, Evolution and Man
against just such an assertion. In response, I opened it at a page I had already marked with a yellow sticker and read out: ‘If the population of Europe is largely composed of farmers who gradually immigrated from the Near East, the genes of the original Near Easterners were probably diluted out progressively with local genes as the farmers advanced westward. However, the density of hunter–gatherers was probably small and the dilution [of Near Eastern genes, that is] would thus be relatively modest.' There it was in black and white, in their own words. This was massive replacement in all but name. Walter puffed one last time and sat down. The chairman closed the session. I had survived the first charge: but the fuse had been lit on a fierce debate that would not be resolved for another five years.

In science these days, international conferences like the one in Barcelona are useful for announcing new findings and getting an initial reaction. But work presented at a conference has no real validity until it is published in a scientific journal. Publication involves close scrutiny of the data, the methods and the conclusions by expert reviewers working unpaid and under an obligation to declare any conflict of interests. Though a conference presentation has to be truthful, it is only during the review process prior to publication that the assumptions, results and interpretations are thoroughly checked. Considering the fierce reaction that our radical revision of European prehistory had provoked in Barcelona, it came as no surprise to us when we submitted our manuscript to the
American Journal of Human Genetics
, the leading international journal in the field, that the reviewers were even more demanding than usual. They insisted that the evolutionary network method, which we had published in 1995 as an intensely mathematical and opaque article, be explained once more in an appendix. They asked for additional tables of, to my mind, old-fashioned population comparisons. But finally, they published it. ‘Palaeolithic and Neolithic lineages in the European mitochondrial gene pool' appeared in the July 1996 issue. It was now in print. We had set out our stall; now we waited for the reaction.

For a while, nothing happened. Then we started hearing from friends that the work was being discussed as at best irrelevant or at worst just plain wrong. Surprisingly, the main target of the whispering campaign was not us but mitochondrial DNA itself, which had distinguished itself so well in solving the puzzle of the Polynesians. Suddenly it was portrayed as being unreliable, too unstable, with too many parallel mutations in the section that we had chosen to use. The mutation rate estimates were attacked as being wildly out. This meant that the dates for the clusters were much younger than we thought and thus perfectly compatible with the ‘wave of advance' model of an essentially farming-derived gene pool. Lastly, mitochondrial DNA was accused of being just one marker, just a single witness to events whose account of prehistory could not be substantiated.

When a controversial paper is published it is not unusual for the scientific journal in which it appears to receive and publish a criticism from others in the field. This takes the form of a ‘Letter to the Editor'. The authors of the original paper are given the opportunity to respond, and if they do, both letters appear next to each other in the same issue of the journal. It was no surprise to learn that Cavalli-Sforza had composed such a criticism of our paper and that it had been accepted by the
American Journal of Human Genetics
. The editor sent us a copy of Luca's letter with an invitation to reply to it.

The letter was a withering attack on mitochondria in general and on our interpretation of the control region sequence data in particular. It did, however, contain one very interesting statement that we had been waiting to hear. Although the overwhelming influence of the neolithic farmers on the make-up of the European gene pool was the main feature of Luca's ‘demic diffusion/wave of advance' model, no figures had ever been put on their overall genetic contribution. While we had estimated that roughly 20 per cent of modern Europeans traced their mitochondrial ancestors back to these early agriculturalists, there was no comparable figure from Luca's work that we could use as a contrast. The assumption which most people had made was that the farmers had ‘overwhelmed' the hunters. That was certainly how a generation of archaeologists had interpreted the ‘wave of advance' model. But the scale of the immigration had never been quantified. There was probably no need. The model had gained its own momentum and everybody knew what it meant, or thought they did. But now, for the first time, Luca put a figure on the proportion of modern European genes contributed by farmers from the Near East. It was, according to the letter, roughly equal to the proportion of the genetic variation that contributed to the first principal component which tracked the cline of genes across Europe from the south-east to the north-west. And this was 26 per cent. No mathematical proof whatsoever accompanied the statement, but we weren't going to complain about that. It was close enough to our estimate of about 20 per cent, derived from mitochondrial DNA analysis, that it looked as if there was little left to argue about.