The Seven Daughters of Eve (10 page)

I needed to get the permission of the cabinet and the co-operation of George Koteka at the health department to collect a substantial DNA sample from Rarotonga and the other islands. I met the cabinet in the Prime Minister's office above the post office, and they could not have been more helpful. Within a few weeks I had collected five hundred samples from Rarotonga, Atiu, Aitutaki, Mangaia, Pukapuka, Rakahangha, Manihiki and even from the tiny atoll of Palmerston (population sixty-six). I packed them carefully in ice and took them back to Oxford.

The Institute of Molecular Medicine, where my laboratory is based, is built around the pioneering work of its first director, Professor Sir David Weatherall. His research over the past twenty-five years has been focused on the inherited diseases of the blood, in particular those involving the main component of red blood cells – haemoglobin. These diseases are not particularly common in northern latitudes, but have a quite devastating effect on public health in parts of Africa, Asia and Mediterranean Europe. The main diseases, sickle cell anaemia in Africa south of the Sahara and thalassaemia in Asia and Europe, kill hundreds of thousands of children every year. The causes of all this misery are tiny mutations in the haemoglobin genes, which very slightly alter the oxygen-carrying properties of the red blood cells. In sickle cell anaemia, the usually circular red blood cells visibly change shape, as the name implies, and can no longer slide past each other in the narrowest of blood vessels. This clogs up the flow of blood to vital tissues. In thalassaemia the haemoglobin itself forms clumps inside the red blood cells, which are then destroyed in the spleen. Either way, the anaemias can be fatal if left untreated; and still the only effective remedy is repeated blood transfusions which – quite apart from the side-effects these cause by overloading the body with iron – are beyond the public health budgets in most of the affected regions.

Why do these diseases occur in some places and not in others? The answer is – malaria. Sickle cell anaemia and thalassaemia are found principally in parts of the world where malaria is, or has been, endemic. Both diseases, in order to develop, require a double dose of the mutant haemoglobin gene, one from each parent. Many inherited diseases follow the same pattern; among Europeans the most familiar is cystic fibrosis, where the parents are carriers with one copy each of the mutant gene but no symptoms of the disease. For a reason that even now is not entirely clear, the parasite that causes malaria finds it difficult to infect the red blood cells of sickle cell anaemia and thalassaemia carriers, who as a consequence become at least partially resistant to the disease. Over many generations, this resistance leads to a spread of the haemoglobin mutations in the malarial regions through the forces of natural selection. However, while the mutations are good for carriers, they can be devastating for their children, because some of the offspring of two carrier parents get the double dose of haemoglobin mutants and develop the potentially fatal anaemias. This cruel balance of carrier advantage and offspring elimination keeps the haemoglobin mutants at a high frequency wherever malaria is found. Malaria does not cause these diseases directly, but does so indirectly by allowing, indeed encouraging, the haemoglobin mutations – which are the real cause – to survive and prosper. So, even if you eliminate malaria, you do not at once eliminate these diseases. In Mediterranean Europe – Sardinia, Italy, Greece, Cyprus and Turkey – programmes to eradicate the mosquitoes which carry the malarial parasite have virtually eliminated malaria – but not thalassaemia. Tens of thousands of people still carry the haemoglobin mutations, and only an entirely different programme, built around the genetic testing of prospective parents to see if they are carriers, is reducing the incidence of the disease.

Many people from the Mediterranean have emigrated to different parts of the world, in particular to the United States and Canada, Australia and Britain. With them, literally inside them, travel the thalassaemia genes, so that the disease is also encountered in these communities. For the same reason, forced immigration on slave ships from west Africa introduced the sickle cell gene to North America, where sickle cell anaemia is still encountered, even though there is no malaria. Gradually, over many generations, it will fade from these populations as the mutations are eliminated either by active counselling programmes or simply by the deaths of those who have the disease. Without malaria to help it along it will suffer the ultimate fate of all disease genes – extinction by natural selection.

Unravelling the roots of sickle cell anaemia and thalassaemia has had a major influence on genetics. It is no exaggeration to say that without the examples of these two diseases to guide researchers, very few of the great advances that have been made since the mid-1980s in finding the causes of genetic diseases would have happened. It was studies of the inherited anaemias that convinced scientists and doctors that simple mutations in genes did indeed cause disease.

The advantages of all this work for me, in my search for the origins of the Polynesians, were far more prosaic. It was field work in the islands of south-east Asia and Oceania, mainly Papua New Guinea, Vanuatu and Indonesia, that finally proved the connection between thalassaemia and malaria. The thalassaemia genes were found only in the low-lying, swampy areas near the coast, where malaria was common, while in the mountainous interiors, where the mosquitoes could not survive the high altitude, the troublesome genes were virtually absent. As a result of this research, the freezers in the Institute of Molecular Medicine were full of DNA samples from these islands. I needed to look no further than the first floor of the Institute where I worked to augment my own samples from Polynesia with a fabulous collection which covered more or less the entire route from south-east Asia into the remote Pacific. If the Polynesians had come that way, surely we would find their mitochondrial DNA scattered along the route.

Over the summer of 1992 I sequenced over 1,200 mitochondrial DNAs. The first thing to do was to see whether we could find any with the small deletion. Nineteen out of the twenty Rarotongans were missing this tiny segment and it was very easy to test for it. And there it was: very common in Samoa and Tonga; less common further west in Vanuatu and the coast of New Guinea. The deletion was even less frequent in Borneo and the Philippines, but still there far to the west among the native Taiwanese. This looked like good evidence for an Asian origin. But remember that we knew from published work that the same tiny deletion was also to be found in North and South America. Were we going to find ourselves in the same frustrating situation as everyone else who had tried to use genetics to solve the puzzle, unable to differentiate between a gene that had arrived in Polynesia directly from Asia and one imported indirectly via the land bridge to America? Our only hope was that the control region sequence itself would be able to tell the difference.

The common sequence in Rarotonga, and from the lab in Hawaii, had variants at 189, 217, 247 and 261 as well as the tiny deletion. The other, less frequent but obviously related sequence had variants at 189, 217 and 261 but not 247. As film after film peeled itself out of the developing machine over the next few weeks, I got very good at recognizing the particular pattern of bands that meant we had found the Polynesian sequences. There they were, spread back along the island trail to Polynesia. The further west we went and the closer we got to the Asian mainland, the rarer the full sequence with 247 became, while a new type with just 189 and 217 began to appear, reaching its highest concentration among the Ami, Bunum, Atayal and Paiwan from Taiwan. The record of the whole amazing journey was there. I rang as many people as I could think of who might have new mitochondrial sequences from native Americans. I had to be sure that 247, the defining variant of Polynesian mitochondrial DNA, was not abundant in the Americas. No-one had seen it. Not even once. Heyerdahl was wrong.

I could not help feeling a tinge of disappointment that I had been unable to vindicate the man who had inspired a generation with his voyage in
Kon-Tiki
. But there it was. His theory had wilted under the fierce spotlight of genetics. The majority opinion had been proved right: the Polynesians had come from Asia and not America. I never got to know what Heyerdahl himself thought about this. I am sure that, at eighty-three, he has better things to do than defend himself against the awesome power of modern genetics. There was a ripple of applause from the anthropology establishment when we published our results; but these academics were already so sure of themselves and convinced by the weight of evidence for an Asian origin that they were not notably excited by this new information. To agree with the prevailing consensus is unlikely to disturb the peace. To disagree with it, as I was to find out before long in another part of the world altogether, was anything but peaceful.

The genetic trail into the scattered islands of the vast Pacific was now crystal clear. The ancestors of the Polynesians began their epic journey in either coastal China or Taiwan. This is where the highest frequencies of what we can safely assume to be the ancestral mitochondrial DNA sequence of most Polynesians are found today, with variants at 189 and 217 and the small deletion. We also found in the samples from Taiwan other sequences with extra variants on top of the core 189, 217 pattern but at positions we didn't find in other parts of the region. These are the mutations that have happened in Taiwan since the ancestors of the Polynesians left. By counting up the mutations and multiplying by the mutation rate we can estimate the length of time since the ancestral sequence itself first arrived in Taiwan. As we shall see when we come to explore the genetic landscape in Europe, this is a controversial area in contemporary research. None the less, it was pretty clear from the great diversity of variation on the basic theme of 189, 217 in Taiwan that the sequences had been there a very long time indeed, probably as long as twenty to thirty thousand years.

There are many archaeological signals of a very sudden population expansion in the islands of south-east Asia around three to four thousand years ago, defined by a range of artefacts associated with an agricultural economy. The most significant of these is pottery of a distinctive style called Lapita, with a red surface glaze and tooth-like decorations stamped into the clay in horizontal lines. For archaeologists, pottery with an identifiable style is a real bonus. It survives for thousands of years in the ground, and a similarity of ceramic style can connect settlements that are geographically far apart. It doesn't automatically mean that the people who used the pottery were biologically related, but it is a certain sign of contact between the different places. Within a period of only five hundred years, beginning three and a half thousand years ago, Lapita settlements appeared on the coast of many of the islands in the western Pacific, from the Admiralty Islands north of New Guinea to Samoa in western Polynesia. Supporters of the Asian origins of Polynesians had always associated this rapid expansion, which implied a sophisticated seagoing capacity, with the people who ultimately colonized the whole of Polynesia. The mysterious absence of Lapita pottery on the islands to the east of Samoa was explained by the lack of a suitable clay. Now that the genetics had come down firmly in favour of an Asian rather than an American origin for the Polynesians, could we say anything new about where this remarkable expansion of people and pottery might have begun?

First of all, the complete absence of the variant at position 247 in Taiwan made it extremely unlikely that it had started there. If it had, then I would have seen plenty of variant 247 in Taiwan. In fact, I never see variant 247 west of Borneo. So the rapid Lapita expansion is only supported by the genetics if it began somewhere to the east of Borneo. I did see 247 in the Moluccas, an island group in Indonesia, and it has been there long enough to accumulate additional mutations. My best estimate for the place of origin of the remarkable Lapita Polynesians would be somewhere in that island group. From there, the mitochondrial trail leads out into the Pacific, to Hawaii in the north, to Rapanui (Easter Island) in the east and to Aotearoa (New Zealand) in the far south.

All this is clear from the main Polynesian type. But what of that strange, rare sequence that I had found in the blood from a single outpatient in Avarua hospital and Koji Lum had found in one native Hawaiian? Could this be the faint echo of Heyerdahl's American Polynesians? We had certainly found this sequence all over Polynesia after our extensive sampling, though it was never common; but none of my contacts had seen anything like it in North or South America. Then we found a single example in Vanuatu and two more from the north coast of Papua New Guinea. However, only when I tracked down some old samples from the mountainous interior of New Guinea did I find this sequence in abundance. This was mitochondrial DNA that had been handed down to the present-day inhabitants from the earliest settlers of that huge island – settlers who, according to the dating of early archaeological sites, had made their way there at least forty thousand years ago in the same ancient migration that had carried the first Australians to that vast continent. So the direct maternal ancestors of the mysterious outpatient from Avarua hospital had spent almost forty thousand years on the island of New Guinea before joining a Lapita voyaging canoe heading east into the unknown.

From the north coast of New Guinea a line of islands, each visible from the previous one, stretches out into the Pacific as far as the Solomon Islands. These are high islands with mountain peaks which can be seen on the horizon either before setting out or, at the very least, before losing sight of your departure point. This comparatively safe navigational technique had already taken the earliest settlers of New Guinea up past New Britain and New Ireland and down the main chain of the Solomon Islands as far as San Cristobal thirty thousand years ago. But this was the end of the pier. Beyond that was the open sea with the nearest land, the islands of Santa Cruz, still three hundred kilometres away far beyond the horizon. There is no archaeological evidence of any settlement beyond the Solomons until the arrival of the Lapita people twenty-seven thousand years later.

Two crucial developments enabled the new wave of colonists to launch into the unknown. The first was the development of the double-hulled voyaging canoe. These magnificent vessels reached enormous sizes. The first Europeans to reach Polynesia saw canoes over 30 metres long, and smaller versions are still used today. The double hull prevents capsizing in the same way as the outrigger on a catamaran. The vessels had a prow at each end, and so could be tacked across the wind and then reversed without turning round. These were the vessels that carried the Polynesians into the Pacific; the complementary and equally crucial development was a highly sophisticated set of navigational skills. Whereas the earliest settlers had managed to reach Australia, New Guinea and the Solomon Islands by steering to visible targets, the Polynesians sailed off into a void, not only unable to see land but not knowing if there was any. Their progress can be followed through the dating of archaeological sites. They quite quickly settled Santa Cruz and the islands of Vanuatu, paused before the 750 kilometre crossing to Fiji and beyond to Samoa and Tonga, then paused again before pushing on to the limits of Polynesia. They reached Easter Island and Hawaii about fifteen hundred years ago and, last of all, New Zealand about twelve hundred years ago. They had reached every island in this vast ocean in a little over two thousand years. How did they do it?