The Seven Daughters of Eve (13 page)

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. How, 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.

There are no clear signs of interbreeding, no convincing remnants of earlier genes from these subdued species anywhere. But, as with the Europeans, so much remains untested. Who knows what the next sample will bring? Who can be sure that in the remote mountains of Bhutan, the lonely deserts of Arabia, the forests of central Africa or the crowded streets of Tokyo there is not a single person who holds the evidence of a different history embedded somewhere in his or her genes?

Although Cro-Magnon stone technology was a significant advance over the existing apparatus of the Neanderthals, life in the Old Stone Age was still based on hunting. Archaeologists divide the Stone Age into three phases, based on the style of stone tools used. It is not a hard and fast classification and is fuzzy at some of the boundaries, but it has endured as a useful way of referring to the main features of an archaeological site where the only evidence to go on is the artefacts that are found there. A trained archaeologist can tell at a glance whether he or she is dealing with an Old, Middle or New Stone Age site by the features of the stone tools and other artefacts found there and without needing to find any human bones to help.

The Old Stone Age, or
Palaeolithic
(from the Greek for
old
and
stone
) covers the time from the first appearance of stone tools about two million years ago up until the end of the last Ice Age about fifteen thousand years ago. There are huge differences between the crude hand axes that come from the beginning of this period and the delicately worked flint tools that are found at the end. To differentiate the various phases of this development, the Palaeolithic is divided into Lower, Middle and Upper phases. The Lower Palaeolithic roughly coincides with the time of
Homo erectus
, the Middle Palaeolithic corresponds approximately with the time of the Neanderthals, and the most recent, the Upper Palaeolithic, refers to the period beginning in Africa about a hundred thousand years ago when
Homo sapiens
finally arrived on the scene. In Europe, the Upper Palaeolithic doesn't begin until the first
Homo sapiens
, the Cro-Magnons, appear with their advanced stone technology between forty and fifty thousand years ago.

After the end of the last Ice Age, the Middle Stone Age, or
Mesolithic
, takes us up to the beginnings of agriculture. The boundary between the Upper Palaeolithic and the Mesolithic is very indistinct. There is an increase in the sophistication of worked stone tools and characteristic styles of implements made from bone and antler. Many more sites are found around coasts. However, there is no entirely new stone technology on the scale of that which divides the Middle and Upper Palaeolithic. At the other end of the Mesolithic, though, the transition is dramatic. The New Stone Age or
Neolithic
is the age of farming, and it is associated with a whole new set of tools – sickles for cutting stands of wheat; stones for grinding the grain – and, almost always, the first evidence of pottery.

The Cro-Magnons of the European Upper Palaeolithic lived in small nomadic bands following the animals they hunted, shifting camp with the seasons. Although a vanishingly few people around the world still make a living like this, for most of us (certainly for most of you reading this book) the fundamental basis of life has changed dramatically. This is due to the one technical revolution which eclipses any refinements to the shape and form of stone tools in its importance in creating the modern world. That revolution is agriculture. Within the space of only ten thousand years, human life has changed beyond all recognition, and all of these changes can be traced to our gaining control of food production.

By ten thousand years ago, our hunter–gatherer ancestors had reached all but the most inaccessible parts of the world. North and South America had been reached from Siberia. Australia and New Guinea were settled after significant sea crossings, and all habitable parts of continental Africa and Europe were occupied. Only the Polynesian islands, Madagascar, Iceland and Greenland had yet to experience the hand of humans. Bands of ten to fifty people moved about the landscape, surviving on what meat could be hunted or scavenged and gathering the wild harvest of seasonal fruits, nuts and roots. Then, independently and at different times in at least nine different parts of the world, the domestication of wild crops and animals began in earnest. It started first in the Near East about ten thousand years ago and, within a few thousand years, new centres of agriculture were appearing both here and in what are now India, China, west Africa and Ethiopia, New Guinea, Central America and the eastern United States. This was not a sudden process but, once it had begun, it had an inexorable and irreversible influence on the trajectory of our species.

There has never been a completely satisfactory explanation of why agriculture began when it did and how it sprang up in different parts of the world during a period when there was no realistic possibility of contact between one group and another. This was a time when the climate was improving, though fitfully, after the extremes of the last Ice Age. It was becoming warmer and wetter. The movement of game animals became less predictable as rainfall patterns changed. Even so, none of these things in themselves explain the radical departure from life as a hunter to that of a farmer. Why hadn't it happened before? There were several warm interludes between Ice Ages during the course of human evolution where the climate would have favoured such experimentation. What must have been lacking was the mind to experiment.

Whatever the reasons behind the invention of agriculture, there is no doubting its effect. First of all, numbers of humans began to increase. Very roughly, and with wide variations depending on the terrain, one hunter–gatherer needs the resources of 10 square kilometres of land to survive. If that area is used to grow crops or rear animals, its productivity can be increased by as much as fifty times. Gone is the necessity for seasonal movements to follow the game or wild food. Very gradually camps became permanent, then in time villages and towns grew up. Soon food production exceeded the human effort available to keep it going. It was no longer essential for everybody to work at it full-time; so some people could turn to other activities, becoming craftsmen, artists, mystics and various kinds of specialists.

But it was not all good news. The close proximity of domesticated animals and dense populations of humans in villages and towns led to the appearance of epidemics. Measles, tuberculosis and smallpox crossed the species barrier from cattle to humans; influenza, whooping cough and malaria spread from pigs, ducks and chickens. The same process continues today with AIDS and BSE/CJD. Resistance to these diseases slowly improved in exposed populations, and here they became gradually less serious. But when the pathogens encountered a population which had not previously been exposed to them, they exploded with all their initial fury. This pattern would be repeated throughout human history. The European settlement of North America which followed on from the voyage of Christopher Columbus in 1492 was made easier by the accidental (or sometimes deliberate) infection of native Americans by epidemic diseases, like smallpox, which killed millions.

The first nucleus of domestication that we know about appeared about eleven thousand years ago in the Near East, in what is known as the Fertile Crescent. This region takes in parts of modern-day Syria, Iraq, Turkey and Iran, and is drained by the headwaters of the Tigris and Euphrates rivers. Here or hereabouts the hunters first began to gather in and eat the seeds of wild grasses. They still depended on the migrating herds of antelope that criss-crossed the grasslands on their seasonal migrations, but the seeds were plentiful and easy to collect. This was not agriculture, just another aspect of gathering in the wild harvest. Inevitably some seeds were spilled, then germinated and grew up the following year. It was a small step from noticing this accidental reproduction to deliberate planting near to the camps, which had by then already become more or less permanent in that part of the world thanks to the local abundance of wild food. Over time, the plants which produced the heavier grains were deliberately selected, and the natural genetic variants that produced them increased in the gene pool. True domestication had begun.

The same process was repeated in other parts of the world at later times and with different crops: rice in China, sugar-cane and taro in New Guinea, teosinte (the wild ancestor of maize) in Central America, squash and sunflower in the eastern United States, beans in India, millet in Ethiopia and sorghum in west Africa. Not only wild plants, but wild animals too were recruited into a life of domestication. Sheep and goats in the Near East along with cattle, later separately domesticated in India and Africa; pigs in China, horses and yaks in central Asia, and llamas in the Andes of South America were all tamed into a life of service. Even though most species resisted the process – for example, no deer have, even now, been truly domesticated – the enslavement of wild animals and plants for food production was the catalyst that enabled
Homo sapiens
to overrun and dominate the earth.

But how was this accomplished? Was there a replacement of the hunter–gatherers by the farmers, just as the Neanderthals had been pushed aside by the technologically advanced Cro-Magnons? Or was it instead the
idea
of agriculture, rather than the farmers themselves, which spread from the Near East into Europe? This seemed like another case of rival theories that could be solved by genetics – so we set out to do just that.

By the summer of 1994, by which time I had secured the three-year research grant I needed to carry on, I had collected together several hundred DNA sequences from all over Europe, in addition to the samples we had acquired on our Welsh trip two years previously. Most of them had been collected by the research team, or through friends, as the opportunity arose. One friend of mine was engaged to a girl from the Basque country in Spain, so he surprised his future in-laws by arriving with a box of lancets and setting about pricking the fingers of friends and family alike. A German medical student who was spending the summer in my lab on another project went para-gliding in Bavaria and tucked the sampling kit into his rucksack. Other DNA samples came from like-minded colleagues in Germany and Denmark who sent small packages containing hair stuck to bits of sellotape. Hair roots are a good source of DNA, but they are fiddly to work with and a lot of people, especially blondes, have hair that breaks before the root comes out. And pulling out hair hurts.

Another year on, and by the early summer of 1995 a few papers were beginning to appear in the scientific literature on mitochondrial DNA, from places as far apart as Spain, Switzerland and Saudi Arabia. It is always a precondition of publication in scientific journals that you deposit the raw data, in this case the mitochondrial sequences, in a freely accessible database; so with the help of these reports we were able to build up our numbers of samples further. The papers themselves were not encouraging. Their statistical treatment of the data was largely limited by the computer programs available at the time to comparisons of one population average against another, and drawing those wretched population trees. Given this treatment, the populations looked very much like one another, and the authors inevitably concluded with pessimistic forecasts about the value of doing mitochondrial DNA work in Europe at all. Compared to the genetic dramas being revealed in Africa, where there were much bigger differences between the DNA sequences from different regions, Europe was starting to get a reputation for being dull and uninteresting. I didn't think that at all. There was masses of variation. We rarely found two sequences the same. What did it matter if Africa was ‘more exciting'? We wanted to know about Europe, and I was sure we could.

When we had gathered all the European data together, we started by trying to fit the sequences into a scheme which would show their evolutionary relationship to one another. This had worked very well in Polynesia, where we saw the two very distinct clusters and went on to discover their different geographical origins. We soon found out that it was going to be much more difficult than that in Europe. When we plugged the data into a computer program that was designed to draw evolutionary trees from molecular sequences, the results were a nightmare. After thinking for a very long time, the computer produced thousands of apparently equally viable alternatives. It couldn't decide on the true tree. It looked hopeless. This was a very low point. Without a proper evolutionary scheme that connected the European sequences, we were going to be forced to publish our results, the results of three years' hard work and a lot of money, with only bland and, to me, pretty meaningless population comparisons that might conclude, say, that the Dutch were genetically more like the Germans than they were like the Spanish. Wow.

Before going down that miserable route – and we had to publish something soon to have any hope of securing yet more funding – we went back to the raw data. Instead of feeding it into the computer, we started drawing diagrams on bits of paper. Even then we couldn't make any sense of the results. For instance, we would have four obviously related sequences but couldn't connect them up in an unambiguous evolutionary scheme. Figure 4a shows an example. Sequence A was our reference sequence, sequence B had one mutation at position 189 and sequence C had one mutation at position 311. That's easy enough. Sequence A came first, then a mutation at 189 led to sequence B. Similarly, a mutation at 311 turned sequence A into sequence C. No real problem there. No ambiguity. But what to do with a sequence like D, with mutations at 189
and
311? D could have come from B with a mutation at 311, or from C with a mutation at 189 (see Figure 4b). Either way it was obvious that the mutations, on which everything depended, were happening more than once. They were recurring at the same position. No wonder the computer was getting confused. Unable to resolve the ambiguity, it would draw out
both
trees. Another ambiguity somewhere else would force the program to draw out four trees. Another one and it had to produce eight trees, and so on. It was easy to see that it wouldn't take many recurrent mutations in such a large set of data for the computer to produce hundreds or even thousands of alternative trees. How were we going to get over this? It looked as if we were really stuck. For the next week I would think I'd solved it, get out a piece of paper and start drawing, then realize whatever idea I'd had wasn't going to work. Finally, I was sitting down in the coffee room one day doodling on napkins when the solution dawned on me. Don't even try to come out with the perfect tree. Leave the ambiguities in there. Instead of trying to decide between them, just draw it as a square (Figure 4c). Freely admitting that I didn't know which route led to D, I could leave it at that. Once I had unhooked myself from this dilemma, the rest was easy. I could relax. I was no longer seeking the perfect tree from thousands of alternatives. There was just one diagram, not a tree but a network, which certainly included some ambiguities but whose overall shape and structure was full of information.