AN INTERVIEW WITH SPENCER WELLS
Geneticist Spencer Wells spends his life traveling the globe taking blood samples from men and women in order to unravel the secrets of the human story: Where did humans come from? How did they spread over the globe? How did different races evolve? In THE JOURNEY OF MAN: A GENETIC ODYSSEY (Princeton University Press), Wells answers those questions for the first time using the latest discoveries of human genetics. We talked to Spencer as he sat for a moment between trips to Lebanon and Tunisia:
You say that there really was an Adam--a common male ancestor for all humans. How did you find that out?
We study a historical document carried in the blood of everyone alive today - DNA. Tiny spelling mistakes - changes in the DNA sequence - that occurred in the past can give us clues about genealogical relationships. If two people share a change, then they are likely to share an ancestor. If we look at the spelling mistakes carried by people all over the world, we find that ultimately all of us share a common ancestor. In the case of the male line, defined by a piece of DNA known as the Y-chromosome, this analysis allows us to trace back to a common male ancestor for everyone alive today. In other words, Adam.
Where did Adam live and what did he look like?
The unequivocal answer is that he lived in Africa. Every piece of DNA in our bodies can be traced back to an African source. The Y-chromosome traces back to eastern or southern Africa, around 60,000 years ago. The present-day inhabitants of Ethiopia, Sudan and southern Africa carry the clearest signals of our earliest ancestry, signals that have been lost in the rest of us. So they give us a glimpse of our 60,000 year-old Adam. Adam would have been fully modern, both in terms of his appearance and his brain function. It is speculation, of course, but perhaps the San Bushmen of the Kalahari - who in many ways are a composite model of facial features from people all over the world - give us a portrait of Adam and his fellow early humans.
Why do you focus on men? What about Eve?
It turns out that the Y-chromosome gives us two very important clues to the question of how we populated the world. First, it shows us our most recent common ancestor (Adam). This man lived in Africa around 60,000 years ago. The significance of this date is that it means that all modern humans were living in Africa until at least that time. In other words, within the past 60,000 years - only about 2,000 generations - our species has populated the entire planet. Clearly, we are all very closely related. The second clue provided by the Y-chromosome concerns the routes we followed in our migrations around the planet. Due to something I describe in the book as 'sexual politics', the male line gives us the best view of the routes followed. So, the Y - a piece of DNA that really doesn't do much more than to make men men - is one of the best historical documents ever written. Women also have a female history written in their mitochondrial DNA, showing the path to Eve around 150,000 years ago. For reasons explored in the book, the mitochondrial signal turns out to provide less resolution for studies of population history than the Y. Again, it comes down to a long history of sexual politics.
How does the genetic Adam relate to the Adam of the bible?
It's interesting that both genetics and the Bible show that there is a common origin of humanity. According to genetic data we come from a single male ancestor. In the Bible too it is mentioned that there is a single male Adam and single female, Eve. I don't equate our results one-to-one with the biblical story, of course, because if you count back through the generations described in the Bible, Adam should have existed in 4004 BC, and our Adam existed 60,000 years ago. Also, our Adam and Eve weren't the only people alive at the time, just the lucky ones who left descendants down to the present day. But it is nice to know that we arrive at the same general conclusion: we're all related.
If we all came from a black man, how did men and women of different colors come into being?
The accepted explanation for skin color differences is that we first evolved in a tropical region, in Africa. The tropical sun is quite strong, so the skin needed the protection provided by the natural sunscreen, melanin, which makes skin dark. When we started moving into the Northern Hemisphere 40,000 years ago, the sun was not as strong. Anyone who's been to London in February can tell you that! And because the sun helps us to synthesize Vitamin D, which we need to grow strong bones, we had to lose some of our pigmentation to allow enough sunlight through.
So what do our genes tell us about the biological differences between, say, Europeans and Africans?
They are literally only skin deep. We are all African cousins separated by - at most - 2,000 generations.
Has research on genes told us something about the first people to arrive in America?
Yes. Our data tells us that we could not have been in the Americas prior to 20,000 years ago, and the most likely date of entry was around 15,000 years ago. This is because the oldest Y-chromosome lineage in the Americas originated in Central Asia 15,000-20,000 years ago, and then migrated to the northeast, across the Bering Strait. If we were still in Central Asia 20,000 years ago, we couldn't have been in the Americas until after that date.
How do other scientists and the public react to your research?
In general, there is more and more agreement among paleoanthropologists, archaeologists, geneticists and historians about the details of our past. I suppose one thing that some people still find hard to accept is that we left Africa so recently, and blitzed our way around the world, but it does seem to have happened like that. I urge them to read the book, where I discuss the archaeological, linguistic and climatological clues that fill in the details of our journey. It is a synthetic look at the past, not simply a genetic tale.
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