DNA USA

by Bryan Sykes

  The strength of feeling connecting African Americans to their African cousins now has its ambassador, its go-between, in mitochondrial DNA, and thousands of African Americans have drawn great strength from the genetic testimony it carries. Though this phenomenon shares some of the ingredients of the deep-rooted desire of many Europeans to discover which of the “Seven Daughters of Eve” is their ancestral clan mother, it has the added dimension of revealing, if only dimly, a past that has been completely and deliberately hidden. It bolsters the feeling of identity that many African Americans feel for their ancestral continent. While there were hints of this in the early days, as my experience with Jendayi had shown me, it was only when public access to testing became widely available that the full extent was revealed.

  Public participation by African Americans began in earnest in the United States on February 21, 2003, when Dr. Rick Kittles and his business associate, Gina Paige, set up African Ancestry. Rick is a physician and geneticist, now at the University of Illinois, and as well as his work on the genetics of prostate cancer he has been instrumental in developing ways of estimating the proportion of African DNA in individual genomes, in ways that we will come to later. He told me when I visited him in Chicago that he remembers the start date of African Ancestry very precisely, as he and Paige had to rush to launch the company during Black History Week. Being an African American himself, like Paige, he realized the potential in linking individuals to Africa using their DNA, and appreciated the intrinsic qualities of mitochondrial DNA to do this. But as we have seen already, the potential is severely limited without large numbers of indigenous African DNA sequences for comparison. In 2002 there were not that many available, so he set about collecting his own. This is where the conflict between the academic and commercial worlds began, and it is one that I am familiar with myself.

  The natural inclination of academics is to make results freely available through publication in scientific journals. Indeed it is an obligation for anyone employed by a university. Academic research was the only source of mitochondrial sequences until a decade ago, and without it there would have been nothing on which to base any commercial service. And without that there would have been no widespread public access, and no one outside a university research department would have been able to have his or her DNA analyzed on request. I know some of my university colleagues shrink from the idea of commercialization, but I believe they are mistaken. It seems to me that academic research should do the trailblazing, but after a while, when the rules are understood and the field settles down, it doesn’t seem appropriate that public money or research charities should pay for all the implementation. Researchers should get on with something new, or explore avenues that are unsuitable for private funding.

  Kittles and I met in his office at the University of Chicago, and we both soon realized that we had shared similar experiences in opening the academic closet and letting people see inside—“taking the American Journal of Human Genetics to the street,” as he put it succinctly. Although Kittle’s main career has been centered on cancer genetics, he started out doing a graduate dissertation in biological anthropology. Being African American he naturally wanted to focus on Africa, but there were just no samples. There may be many shortcuts in research, but there is no getting away from the fact that to look into African genetics you need African DNA. However, there was an ongoing research collaboration in the department with Finnish scientists who were looking into the possible genetic basis for their country’s distressingly high rates of alcoholism. And for that reason the lab freezers were full of Finnish DNA. You couldn’t get much farther away from Africa, on the equator, than Finland on the Arctic Circle, but Kittles relished the thought of using the then newly discovered tools of mitochondrial and Y-chromosome DNA, albeit in much cruder ways than nowadays. In a nutshell he found that roughly half of Finnish Y chromosomes had come from Asia, while the mitochondrial component was determinedly European. As is often the case, the Y chromosomes were much more limited in their genetic diversity, a sure sign that a few men had more than their fair share of children. It is a picture I recognized only too well from Celtic Britain.

  After finishing his thesis, Kittles was able to focus on Africa when he got his first position at Howard University in Washington, D.C. Howard was founded in 1867, just after the end of the Civil War, with the express intention of admitting African Americans, a guiding principle it maintains to this day. The medical school, which Kittles joined, has a reputation for training doctors from Africa, so, as well as a good complement of African Americans, there was a constant flow of students back and forth. That was the means by which Kittles built up his collection of indigenous African samples. He soon found a match for his own mitochondrial DNA, which is in the clan of Lingaire (L2C), among the Hausa of northern Nigeria. His Y chromosome on the other hand is European. The natural assumption for an African American is that one of his enslaved maternal ancestors was impregnated by a white man, possibly her owner—an extreme version of proprietorial rather than aristocratic diffusion, in other words.

  However, I was fascinated to discover that there could be another explanation. Kittles told me that along the coasts of West Africa he had found that between 5 and 10 percent of fishermen also had a European Y chromosome. Coastal fishermen often have a different gene pool than do their inland neighbors. This was picked up a long time ago around the coast of Scotland when the blood-group composition of fishing communities was found to be quite different from that of the inland population nearby, and much more similar to that of fishermen and -women from other parts of coastal Scotland. The explanation of the European Y chromosomes among West African fishing communities is more about the flow of genes around the coasts of the Atlantic. The same flow has brought African DNA to the islands off the west coast of Scotland, as I discovered in my British research. It means, of course, that some African Americans may have gotten their European Y chromosomes from fishermen on the Atlantic coast of Africa, and not from white men in America. Some, but not all.

  Kittles is one of the many African Americans who have visited the homeland of their ancestors in so far as it can be identified through DNA. A few years ago he traveled to the Jos plateau in north-central Nigeria to visit the Hausa, the African tribe with the closest mitochondrial DNA matches to his own. It was both a frightening and a reassuring experience, he told me—frightening because the Jos plateau, in common with much of northern Nigeria, is embroiled in a war between Christians and Muslims. The roads were terrible, there were no lights, and no one dared to drive at night. There were potholes everywhere and larger craters with cars and even buses sticking out of them. The most profoundly disturbing images, however, were the unburied human bodies that littered the sides of the roads. Approaching a Hausa township one day, the car was pulled over and the driver gestured that he had better get out. Kittles was immediately surrounded by a hostile crowd, dressed in the long robes of Islam. However, what could have been a very dangerous moment was suddenly transformed. Among the men who surrounded the car was someone he recognized. Standing there was a man who was the spitting image of his uncle Clifford, his mother’s brother. They looked almost exactly alike. It wasn’t his uncle, but the mutual sense of recognition brought about by their shared ancestry immediately defused the tension. Rick suddenly felt the warm sense of kinship. It was, as he told me, “a very weird moment”. Not really scientific at all.

  Not being an African American, I don’t expect I shall ever be able truly to understand the strong feelings of not knowing my roots and the void that it leaves: knowing you come from Africa, but not which part. Claiming in your mind the whole of Africa until you get a call from the messenger in your cells and can return to your home and begin to shape your true identity. That being said, many African Americans realize only after they visit Africa that they have more in common with America than Africa and that they can never truly go “home” again.

  But things don’t always work out as expected. Around a thi
rd of Kittles’s African American customers turn out to carry European Y chromosomes. How do these men take the news? Not always well. About half of them are angry and frustrated and call up the company. They get mad and they want their money back. They know the general history of their race, but they look black and they certainly feel black. “How can I have a European Y chromosome?” they ask. “There must be some mistake.” So the tests get repeated, and the answers come back the same. By then a lot of them have had time to think about it and have quieted down. Maybe they have asked their grandmother on their father’s side, and she may have said there was talk of a white ancestor in the family a long time ago. Always ask a woman about these things, I have learned. Women always know more than the men.

  When African Ancestry started out, it wasn’t easy to know how to respond to the intensity of the reaction. Had Kittles anticipated it, he told me that he would have engaged a psychologist from the start. He has one now, who helps to manage the responses to unexpected or unwelcome news. With the experience of the years, Kittles has come to realize that a client’s response to a DNA test is a mixture of two things: motivation and expectation. Many of his clients have well-established oral histories, and when the genetics runs counter to these expectations, it gets hard.

  I feel a lot of empathy with Rick Kittles because at Oxford Ancestors we occasionally come across dissatisfied customers who just refuse to believe the implications of their DNA results. One of the services the company offers is to give our estimate as to whether a customers’s patrilineal ancestor was a Viking or a Saxon or a Celt. The “Tribes of Britain” test, as it’s called, is based on a large genetic survey of Britain and Ireland involving thousands of volunteers. Just recently I spoke with a man who was so sure his ancestor was Celtic that, when the “Tribes of Britain” tests showed his ancestor was probably a Viking, he called us. “I am certainly not a Viking,” he said before demanding his money back. (I secretly suspect that many of our clients would prefer to be descended from Vikings, but he clearly wasn’t one of them.) Usually the office staff can smooth things over, but he was so insistent that I called him myself. “I want my money back,” he said immediately. When I asked why, he replied, “Well, you said I was a Viking.” “Yes,” I replied. “But I’m not a Viking, I’m a Celt.” Thinking that sometimes you just can’t win, I asked, “How do you know?” “Because I am dark haired and short.” At that point I gave up, realizing once again that DNA always struggles to reverse the deepest of psychological perceptions or identity associations.

  11

  All My Ancestors

  Chromosome 11.

  We have traveled this far by listening to the clear music of just two solo instruments, free from the background rumble of the genome. The sharp and precise notes of mitochondria have traced the echo of our maternal ancestors back tens of thousands of years, following the journeys of women. The ferocious, warring blasts of the Y chromosome picked out the erratic history of men. The rest of the genome has been silent in narrating the story of our ancestors, leaving us free to concentrate on the separate melodies of men and women. Now is the time to turn up the volume on the rest of our genome, sit back, and listen to the sound of the whole orchestra.

  The fraction of our genome carried by the two principal soloists is tiny. Mitochondrial DNA carries only thirty-seven genes on its compact circle of precisely 16,569 bases. Although very much larger, at 58 million bases, the Y chromosome has fewer active genes than mitochondria, only twenty-seven, owing to its decayed and enfeebled state. The rest of the genome, on which we depend for virtually all our genetic instructions, is far larger again, with just over 3 billion bases spread over twenty-three, for the most part, healthy and robust chromosomes, containing about twenty-five thousand genes. Already you must feel how easily this could become a cacophony of different sounds, completely drowning the sweet music of our soloists. And you would be right, because interpreting the ancestral signals coming from the main bulk of our genome is far less straightforward.

  For a start, because we inherit the DNA in our genomes from both parents and it is shuffled at each generation, it is almost impossible to tell which ancestor is responsible for passing on which segment of DNA. We all have two copies of each gene, but without testing our parents directly, we cannot tell which copy we received from which parent. And that is just our parents. When it comes to more distant ancestors, who cannot be tested, then it becomes virtually impossible.

  Then there is the issue of the generation paradox. Just like us, both our parents have two copies of every gene. But they each pass on only one copy to us and so we, you and I, only ever get half of one parent’s DNA and half of the other. What happens to the rest of it? Some DNA may be passed on to our brothers or sisters, but the rest goes nowhere. The generation paradox arises because, for every generation back in time, the number of our ancestors doubles, but we still only inherit the same amount of DNA. To clarify this let us choose a particular gene, beta-globin, that controls one of the subunits of hemoglobin in our red blood cells. Thinking about our four grandparents, we will have inherited one copy of the globin gene from one of our paternal grandparents and the other copy from one of our maternal grandparents. But that leaves two grandparents whose globin genes we have definitely not inherited. Going back another generation, to our eight great-grandparents, we have inherited our globin genes from only two of them, leaving six grandparents whose globin genes did not get through to us. Likewise, going even further back and still with only two globin ancestors at each generation, fourteen out of our sixteen great-great-grandparents, and thirty out of thirty-two great-great-grandparents, will not have given us our globin genes. We will never know, without a lot of extra work, which of these thirty-two ancestors once carried the globin gene that we have inherited.

  The globin gene is only one of thousands, so even if we received our globin genes from only two of our thirty-two great-great-grandparents, we will have inherited the copies of plenty of other genes from all of them. However, because the number of ancestors keeps on doubling at every generation that we go back, there will come a time when there are ancestors from whom we don’t inherit any DNA at all. But when will that be? With 25,000 genes and two copies of each, that makes 50,000 separate DNA segments. So when the number of ancestors exceeds 50,000 there must be some from whom we get no DNA. It is a simple calculation, just doubling at each generation 2, 4, 8, 16, 32, and so on. After fourteen generations this mathematical series gets to 16,384, and exceeds the 50,000 mark by generation 16, when we have 65,538 ancestors. With a generation time of twenty-five years, that is only 400 years ago, which takes us back to the beginning of the seventeenth century, about the time of the first English settlements in America.

  However, this calculation assumes that we inherit our DNA in a neat and equitable way from our ancestors. In fact, which particular segments we inherit from which ancestors is completely random and therefore governed by the rules of chance. We get more DNA from some and less from others. This spread means that there are some much more recent ancestors, probably within only six generations, from whom we haven’t inherited any DNA at all. With the same 25-year generation time, that is only 150 years ago.

  While this is not so long back, the numbers of ancestors are doubling at each generation and growing at an alarming rate. Which is where the paradox shows itself, because at some point the number of ancestors will exceed the entire population of the world. What is the solution? It is this: Although the number of ancestors doubles at each generation, some of them will be the same people. Not our parents, obviously, but two of our grandparents could, theoretically, be the same person. It’s unlikely but possible. The chances increase as we go back until, at some point, it becomes inevitable. Where that point is depends on how our ancestors lived. If they were mainly endogamous—that is, marrying among themselves like Ashkenazi Jews, for example—these “double” ancestors may have lived quite recently. For more exogamous ancestries, they will have lived further
back in the past. However, whether from an endo- or exogamous ancestry, there will inevitably come a point when one person is the ancestor of everyone alive. This sounds absurd, but theoreticians have calculated that in an exogamous population of one million people, this person lived only twenty generations, or five hundred years, ago. Even when other factors, including a more realistic figure for the world population and the effects of migration and geographical isolation, were brought into the calculation in more sophisticated models, this “universal ancestor” still lived just seventy-six generations, or, assuming the same twenty-five-year generation time, only 1,900 years, ago.1 I do find it astonishing that, compared with the quarter-million-year history of our species, one individual, from whom everyone alive today can trace a line of descent, lived so recently.

  He or she was only our most recent “universal ancestor,” and as you go further back in time the same theorists predict that the population divides into individuals from whom everyone can trace at least one line of ancestry—and the rest, who were the ancestors of nobody alive today. That point is reached at five thousand years ago, about the time the Pyramids were built. So the slaves who built them would either have been the ancestors of everybody alive today, including you and me, or of nobody. Beyond that point everyone is descended from exactly the same set of ancestors, though along different lines. Even further back, the proportion of people with no living descendants increases until only one couple remains who were the ancestors of everyone living today through every line of descent, except two. As you might expect, these estimates are surrounded by caveats. However, even the most sophisticated models incorporating factors like the opening of sea routes do modify the timing, but not by much. The principle is still valid, and the conclusion, strange though it may seem, is inevitable.