The Journey of Man: A Genetic Odyssey

The aim of science is not to open the door to infinite wisdom,
but to set a limit to infinite error.

Bertolt Brecht,
Life of Galileo

Southern Asia

North-east Africa and the Middle East, showing the locations of desert and steppe

Eurasia, showing the mountain ranges of central Asia and the Eurasian steppe belt

1 The evolutionary ‘genealogy’ of two related molecules

2 Proof that modern humans originated in Africa

3 MtDNA genealogy, showing the split into M and non-M lineages outside Africa

4 Y-chromosome genealogy, showing the split into M130 and non-M130 lineages from an M168 ancestor

5 Mitochondrial DNA (mtDNA) mismatch distributions of two expanding populations

6 M89 defines the main Y-chromosome lineage in non-Africans

7 Descendant lineages of M89 characterizing the main geographic regions in Eurasia

8 M45 is the ancestor of most western Europeans and Native Americans

9 Genealogical tree showing the relationship among the Y-chromosome markers

10 The spread of Y-chromosome lineages around the world

Most of us can name our grandparents, many our great-grandparents, and some our great-great-grandparents. Beyond that, we enter a dark and mysterious realm known as history, through which we can only navigate with hesitant steps, feeling our way with whispered guidance. Who were the people that came before? Where did they live? What were their lives like?

In this book I will argue that the answers to these questions can be found in our genetic code, which makes us uniquely human – but also makes us unique individuals. Our DNA carries, hidden in its string of four simple letters, a historical document stretching back to the origin of life and the first self-replicating molecules, through our amoebic ancestors, and down to the present day. We are the end result of over a billion years of evolutionary tinkering, and our genes carry the seams and spot-welds that reveal the story.

It is not the code itself that delivers the message, but rather the differences we see when we compare DNA from two or more individuals. These differences are the historical language of the genes. In the same way that you wouldn’t include ‘water-dwelling’ in a classification of fish, because all fish live in water, the identical bits of our genetic code tell us nothing about our history. The story is in the differences, and this is what we study.

This is not a book on human origins. Rather, it is about the journey we have taken as a species, from our birthplace in Africa to the far corners of the earth, and from the earliest evidence of fully modern humans to the present day – and beyond. The argument pursued throughout is that genetics provides us with a map of our wanderings and gives us a rough idea of the dates – and it is up to us to reconcile
this data with the archaeological and climatological record in order to fill in the picture. Of course, every journey must have a beginning, and this one is no exception. It begins with the scientific effort to make sense of human diversity, which leads us to the birthplace of our species. The methods we use to infer our African origins are the same ones we then use to trace humanity’s global journey. It is the journey itself that is the main focus, and for this reason most of the details of our hominid ancestors have been left out.

This book was originally conceived as part of a documentary film project of the same name. It has since taken on a life of its own, and it stands alone as a unique entity, providing greater detail on the scientific background than is possible on television. The film, on the other hand, presents an (almost) first-hand experience, conveying the excitement and adventure of the journey in a way that only moving images can, so I hope that readers of this book will enjoy it equally.

Although it was often difficult to juggle the film and the book at the same time, this did provide some significant advantages. For me, the chance to retrace my own personal ‘journey of man’ and to meet people from around the world – to see how they live and to discuss the scientific results with them – were profound and wonderful experiences. I hope that a sense of this has come through in print.

The title was chosen for a reason – and it wasn’t sexism. The journey we trace is primarily one made by men, because it is the Y-chromosome, inherited from Adam down the male line, which gives us our keenest tool for deciphering the journey. The Y helps to place the stones, bones and languages in context better than any other part of our genetic code, and ultimately gives us the genetic answers we are looking for. Of course, in order to leave descendants, these early human groups must have included women; while the journey we follow may leave out some female-specific details, the resolution we can achieve only by following the male lineage is worth the omission.

What follows is a scientific detective story guided by the temporal order of events. We begin with a deceptively simple question: how do we decide if the concept of human ‘race’ has any validity? Are we indeed all one species, or are there discrete divisions among human groups? After all, we appear to be so different from one another. The answer to this question, first provided by my PhD adviser at Harvard,
Richard Lewontin, gives us a clue about the journey—but it doesn’t reveal the crucial details.

The second main question concerns our geographic distribution. How did we come to occupy every corner of the globe? The DNA markers are able to provide us with the details. The methods for doing this, developed over the course of half a century, have been greatly influenced by Luca Cavalli-Sforza, with whom I was lucky enough to work as a postdoctoral fellow at Stanford in the 1990s. It was Luca’s insight, as a geneticist with a passion for history and a talent for mathematics, which provided us with a time machine capable of resurrecting the stories of the past from people living in the present. This book could not have been written without his intellectual presence, and it is impossible not to feel humbled while taking in the view from his shoulders.

One of the most compelling things about being on an archaeological dig is the sense that you are actually seeing and handling implements that were last touched by human hands hundreds or thousands of years ago. Often the sense is so great that a feeling of
déjà vu
comes over you, and it seems as though you have somehow been transported back in time. When, as a child, I saw the Tutankhamun exhibition that toured the United States, I was struck by the combination of modern skill and ancient subject matter. It seemed as though the pieces, although fabulously exotic, could have been made the week before by a skilled craftsman. The fact that they were nearly 4,000 years old was extraordinary, and sparked in me a curiosity for the past that has never diminished.

Genetics, at least the branch of it that informs the subject of human origins and migrations, is necessarily less visually compelling than archaeology, despite the fascinating stories it tells. Mark Read’s wonderful photographic portraits included in the hardcover edition of the book go a long way towards correcting this imbalance. Because of the expense of including high-quality color photographs in the paperback edition, which we wanted to keep reasonably priced, we have chosen to make Mark’s images available online, along with the maps, at the following Web address:
www.atrandom.com/journeyofman
. As this book was going to press, this website was in preparation. I have been lucky enough to work with Mark on several occasions,
both in the course of sample-collecting expeditions in Asia and during the making of the film. He is a talented and dedicated artist, and his work adds enormously to the book. Mark’s photographs reflect the way people actually live today, because our knowledge of genetic history is inferred from the blood of people living in the present—it is their
living
genomes that give us our clues. Every one of us is carrying his or her personal history book around inside us—we simply need to learn how to read it.