Out of Eden: The Peopling of the World Read online

  I cannot praise and recommend my agents Julian Friedmann and Carole Blake highly enough to anyone who is lucky enough to get on their list. They went over and above the call of duty.

  Constable & Robinson, my publishers, have put much patient and good-natured effort into transforming something written with half an eye to academics into something with more sparkle and accessibility. I think they did a good job. At the top of the list are the editorial team: Pete Duncan (my editor), John Woodruff, Anna Williamson, Sara Peacock and Jane Anson. Bill Smuts, the illustrator, translated my numerous sketches into great maps. But there are others, including Gary Chapman, Sallie Robins and Jennifer Duthie.

  There are a number of others not in the above groups who have helped me. Georgina Harvey who researched on the Toba literature and put me in touch with Eelco Rohling oceanographer, who illuminated the pre-history of Red Sea salinity for me. An old friend, George Wells, brought me up to scratch on the 200-year history of the two language origin paradigms. Roger and Geeta Kingdon introduced me to Jonathan Kingdon’s work, John Robinson was a great source of references and support. Finally of course, Freda my wife, who has always taken better photos than me, has supported me throughout.

  Acknowledgement of the above does not necessarily indicate their agreement with the views expressed in this book.

  PREFACE

  IMAGINE AN AIRLINE CHECK-IN QUEUE in Chicago or London. Seven people stand there, looking in different directions. One is a solicitor of Afro-Caribbean origin, another a blonde-haired girl whose family come from northern Europe, another a computer expert who was born in India. The fourth is a Chinese teenager listening to music on a Walkman. The fifth, sixth, and seventh are all attending a conference on rock art and come respectively from Australia, New Guinea, and South America. All seven are quiet, and avoid eye contact because they neither know one another nor feel related in any way. Yet it can be proved they are all related and ultimately all have an African female and male ancestor in common.

  In all our cells we have genes. Genes are made up of DNA, the string-like code of life that determines what we are, from our fingernails to our innate potential for playing the piano. If we analyse the genes of any one of these seven people, we can trace the geographic route taken by their ancestors back to an ultimate birthplace in Africa, at the dawn of our species. Further, if we take any pair of them and compare their genes, we will find that they share a more recent ancestor – living, in all probability, outside Africa (see Figure 0.3). What is more, I believe that we can now prove where those ancestors lived and when they left their homelands. This remarkable proof has become fully possible only within the last decade, as a result of pioneering work by a number of people.

  Many of us have wondered what we would find if we could perhaps board a time machine and travel back through the generations of our ancestors. Where would it take us? Would we find ourselves to be distantly related to some famous or notorious person? How many generations would we pass through before we arrived at the first humans? Does our line continue back to monkeys, and beyond to worms and single-celled creatures, as Darwin maintained? We know from dry biology lessons at school that this ought to be so, but as with the uncertainty of what happens to us after we die, it is hard to grasp.

  We are now so used to the pace of technical advances that the sense of wonder fades with each new one. Yet, until very recently, geneticists could only dream of using our genes to trace the detailed history of how we conquered the world. The reason for their pessimism was that the majority of the genes they examined shuffled themselves around at each generation and were common to most populations anyway. Their task was like trying to reconstruct a previously played card game from the pack of cards after it has been shuffled. So it was nearly impossible to draw an accurate genetic family tree going back even a few hundred years, let alone back to the beginning of our species. Most human populations look very similar beneath the skin, so where could one start?

  The use of gender-specific gene lines, the so-called Adam-and-Eve genes, has in the last ten years changed all that. In contrast to all other genes, mitochondrial DNA (a collection of genes outside the cell nucleus) is inherited only through our mothers, and the Y chromosome is inherited only by men. These two sets of gender-linked genes are passed on unchanged from generation to generation, with no shuffling, and can therefore be traced right back to our ancestors, to the first mammals, and even beyond to worms and worse. We can thus construct two family gene trees, one for our fathers and one for our mothers. As a result, in any population, of whatever size, we can trace any two individuals through one of these two gene trees back to a most recent shared ancestor on the tree. Such an ancestor may have lived 200, 5,000, or 150,000 years ago, but all ancestors can be assigned a place on the newly constructed Adam-and-Eve genetic trees. These are real family trees of modern human gene lines, with real branches. Each branch on each tree can be dated, although the accuracy of such dating still leaves much to be desired.

  Many regional human gene trees have now been fitted together, like a large jigsaw that is started by assembling the edges using certain clear landmarks. In this way, a picture of the Adam-and-Eve gene lines spreading from Africa to every corner of the world has been pieced together over the last decade. It has got to that satisfying point, as with jigsaws, when the whole structure suddenly links up and takes shape; the remaining pieces, though many, are now being placed on the tree and on the map with increasing ease and speed. The pace is now so rapid that people working at the cutting edge on one geographic region may still be unaware of breakthroughs in another region. The whole branching tree can now be laid flat on a world map to show where our ancestors and their gene lines travelled in their conquest of the world.

  The new knowledge has resolved some of the apparent paradoxes thrown up by the contrast between the cultural and biological stories of the last 150,000 years. We can now even start to hang the regional human fossil relics of that period in their correct places on the genetic tree of life.

  Many questions have been answered. It turns out that, far from the world being a common genetic melting pot with massive to-and-fro prehistoric movements and mixings, the majority of the members of the modern human diaspora have conservatively stayed put in the colonies their ancestors first established. They have dwelt in those localities since well before the last ice age. We can also trace the dates of specific migrations over the last 80,000 years. Thus, from a picture of great diversity and lack of definition, we have the opportunity to move to a highly specific and regional focus on the branching networks of human exploration.

  Several other obvious examples of long-standing archaeological questions have been resolved by the new gene trees. One is the ‘Out-of-Africa’ v. ‘Multiregional’ controversy. The Out-of-Africa view is that all modern humans outside Africa descend from a recent movement from Africa less than 100,000 years ago. This exodus wiped out all earlier human types around the world. The multiregionalists, in contrast, argue that the archaic human populations, Homo neanderthalenis (Neanderthals) in Europe and Homo erectus in the Far East, evolved into the local races we now see around the world.

  The Out-of-Africa view now wins the contest because the new genetic trees lead straight back to Africa within the past 100,000 years.1 No traces of Adam-and-Eve gene lines from older human species remain on our genetic tree, except of course at the root, from which we can measure our genetic distance from Neanderthals. Neanderthals have now been genetically typed using ancient mitochondrial DNA, and it seems that they are our cousins rather than our ancestors. We share with them another common ancestor, Homo helmei.

  Current Out-of-Africa proponents have usually hedged their bets, claiming that Australians, Asians, and Europeans came as separate migrations of Homo sapiens from Africa. Not so: the male and female genetic trees show only one line each coming out of Africa. This is my central argument in this book. There was only one main exodus of modern humans from Africa – each gender line had only
one common genetic ancestor that respectively fathered and mothered the whole non-African world.

  Other prejudices have also foundered. Some European archaeologists and anthropologists have long held that Europeans were the first to learn to paint, carve, develop complex culture, and even to speak – almost as if Europeans represented a major biological advance. The structure of the genetic tree denies this view. Australian aboriginals are related to Europeans, and share a common ancestor just after the exodus from Africa to the Yemen over 70,000 years ago. Thereafter they moved progressively round the coastline of the Indian Ocean, eventually island-hopping across Indonesia to Australia where, in complete isolation, they developed their own unique and complex artistic cultures. The first Australian rock art has been dated at least as early as the first European one. This must mean that humans came out of Africa already painting.

  Another old archaeological controversy concerns the spread of the Neolithic culture across Europe from Turkey 8,000 years ago. Did the farmers from the Near East wipe out and replace the European hunters, or did the new ideas spread more peacefully, converting the pre-existing Palaeolithic hunter-gatherer communities? The genetic answer is clear: 80 per cent of modern Europeans descend from the old hunter-gatherer gene types, and only 20 per cent from Near Eastern farmers. The old ones were not such a pushover.2

  Finally, moving to the other side of the world, there has always been colourful speculation over the origins of the Polynesians. Thor Heyerdahl was not the first. In fact, Captain Cook was nearer the mark in arguing for a Polynesian link to the Malay archipelago. For the past fifteen years archaeologists have thought that Polynesians came from Taiwan. The genetic tree discounts this now: the ancestors of the sailors of the great canoes started out further along the trail, in Eastern Indonesia.3

  Coming back to our airline queue, we should also remember that we are participants in this genetic story, since 99 per cent of the work of reconstruction of our ancient gene trees was carried out using modern DNA given voluntarily by people living in different parts of the world today. This is a story of relevance to each and every one of us.

  PROLOGUE

  MANY ANTHROPOLOGISTS NOW SAY that we came out of Africa, but how do they know? If we all have a single recent origin there, why do there appear to be different races of humans? How closely are these races related? Is ‘race’ a valid concept? Are we all part of one family, or do Africans, aboriginal Australians, Europeans, and East Asians all have different parallel evolutionary origins? And why us? What key forces in our evolutionary history took descendants of apes that had just left the trees to walk the African savannah and catapulted them onto the Moon within a couple of million years?

  Jacob Bronowski’s The Ascent of Man and Richard Leakey’s The Making of Mankind were landmark documentaries that stimulated public interest in human evolution. Yet, as with any consensus view, they left many questions unresolved. Lack of relevant evidence at the time meant that they also inevitably left what are now recognized as gaps in knowledge and understanding. Subsequent archaeological and biological findings have allowed us to pose new questions and plug some of those gaps. We are also now in a position to correct the distorted perspective we have of what really separates us from our ancestors and from our living cousins, the great apes, and to challenge the myths that have hindered past progress.

  DNA analysis has led to extraordinary advances in our understanding of the regional biological history of modern humans. As we shall see, the so-called Adam-and-Eve genes really do allow us to track back in time and space to follow the human family in its wanderings round the globe. But not all the advances have been in molecular biology. Palaeoanthropology, the study of our ancestors, has leapt forward in several important fields since Leakey’s presentation, over twenty years ago. First, a number of newly found early human skulls, both within and outside Africa, have resolved the timescale and geographical extent of the repeated out-of-Africa movements of the past 2 million years. Other skull finds have pushed the branch between our ancestors and those of chimps back even earlier. Second, the comparative analysis of skull shape has, with the advent of fast computers, been put on a more scientific basis, with the result that key skulls of prehistoric peoples from around the world can now be placed near branches on the human genetic tree, and today’s descendants on the twigs.

  This approach to defining the branches of the modern human biological tree by comparing skulls fell under the cloud of poor science linked to racism as a result of the activities of Nazi anthropologists in the middle of the last century. Since the 1960s, different motives, more objectivity, and better maths have allowed palaeontologists to resolve more detail in the hominid tree and even that of modern humans. The intense public interest in this field, and the persisting fear of it being hijacked to serve nationalist and racist agenda, have unfortunately led to a minefield of ‘politically correct’ euphemisms laid by Western anthropologists to protect us from our prejudices. Although such language has the expressed aim of defining differences between peoples more clearly, the result has been to obscure and proscribe discussion. ‘Race’, for instance, is now a politically incorrect term, and in some circles so is ‘ethnicity’. Such hypersensitivities should be regarded as a challenge rather than an obstacle. Discussion of human differences is racist when the agenda are competitive, exclusive, and derogatory. It can equally be a positive celebration of our diversity.

  In this book I am concerned mainly with questions about our recent story, over the past 200,000 years, which can be answered by tracing our way back through our genes. But to do that properly, and understand what it was that made us explore every corner of the Earth and then some, we do need to ask, ‘Why us?’ in the first place. The forces that drove our adaptive survival against overwhelming odds in the African savannah are the key to our nature and to our extraordinary story. We were not ‘put’ here fully formed, thinking, talking, and unique among animals. We were specially selected and moulded by a fierce, blind, unthinking environment. Like all evolving species, we had ancestors and cousins who shared some of our abilities but perished in adversity. Our physical and behavioural adaptations were focused on surviving the struggle with our greatest enemy and stern teacher, the climate.

  One of Rudyard Kipling’s immortal Just So Stories tells of the Elephant’s Child, who in those early days had no trunk and plagued his uncles and aunts with irritating questions. He made the mistake of going to the Crocodile to ask him what he normally ate for dinner. The Crocodile told him to bend down close to hear the answer. As soon as he did, the Crocodile clamped his jaws hard on the end of the elephant’s nose and tugged him towards the river. After a prolonged tug-of-nose, the Elephant’s Child managed to release himself – only to find that his nose had stretched into a fine, new trunk. This Kipling fable, like others in the anthology, cocks a crude snook at Jean-Baptiste Lamarck’s notion of evolution by the inheritance of acquired characteristics, but there is a hint of Darwinian reality in the result. Through non-benign means, the Elephant’s Child gained a uniquely useful modified nose, although he took a bit of time to realize it. Similarly, modifications to our brain, our free hands, our feet, and our family and social life have been selected by the recurrent cruel weather of the past few million years, and we have put them to unique new uses.

  One of the more surprising insights has come from a growing understanding of the effects of repeated glacial cycles during the past 2.5 million years on human evolution and expansions out of Africa. Whereas severe climatic change generally causes widespread megafaunal extinctions, the appearance of new and more successful human species seems to have coincided with severe glaciations and expansions of the African savannah. But climate may have been a major force behind hominid evolution for much longer than that. Primates have, in general, more dextrous hands, relatively larger brains, more varied diets, and more complex social lives than most other contemporary mammals. Ten million years ago, Africa was a lush paradise with vast open forests and
home to several species of ape. Even then, various primate species, not just apes, were experimenting with life on the ground and in the grassland around islands and tongues of forest. Africa’s grassland has expanded progressively since then, as the world’s climate has cooled and dried, but this has happened in cyclical fits and starts of increasing frequency and severity.

  Walking apes

  As we know from the effects of our recent ice age, the worst phase of the climate cycle, although brief, can cause widespread extinctions. Only the survivors of such climatic episodes can pass on their own genetic type, holding special features that may have been selected for by the environment. Around 7–8 million years ago, a dramatic reduction in the number of ape species coincided with several million years of global cooling and grassland expansion. Some have suggested that this short ice epoch already marks the time of the split between our ancestors and the ancestors of modern chimpanzees. The most important initial physical change in our ancestors, bipedalism (walking on two legs) may have its roots in that time. At present, however, the first clear evidence for bipedalism is seen only in skeletons of Australopithecus anamensis, a walking ape dating from 4 million years ago and found on the shores of Lake Turkana, in northern Kenya, in 1995.1

  Many think that there was a cause-and-effect relation between the spread of the grasslands and the change from quadrupedal forest-living to easy walking around the savannah. This may well be true but, judging by the toothed predators with which they shared their environment, the early bipedal apes probably did not stray too far into the plain and away from the protection of the wooded islands. In any case, other primates, such as the ancestors of baboons, managed to get along surprisingly well in the savannah on four legs (as do modern baboons).