After You, Eve
Natural History, March 2001
My wife, Grace, and I are expecting our first child in July, so I’ve had a lot on my mind recently. Most of it has been pretty mundane stuff. What’s the fastest route from our apartment to the hospital? How exactly do you swaddle a baby? But sometimes loftier thoughts invade. I think about our child as the union of two heritages. My wife’s flows back to Ireland, to County Kerry in the south and County Derry in the north. My own heritage is more farflung, encompassing Wales, England, Germany, and Hungary, as well as countries in eastern Europe that no longer exist, having been bisected and trisected by countless wars.
Both our family trees extend back only a few generations, at which point written records and the memories of relatives fail. But we, like all other humans, also carry a genetic genealogy. Encrypted in our DNA is a history of our species. Scientists are learning how to decode that history, and they find that if you go back far enough in time, my wife’s heritage and my own eventually fuse, along with that of the rest of humanity.
There’s an apparent paradox in our molecular genealogy, however: different genes tell different stories. Research on one set of genes indicates that all humans on earth descend from an African woman who lived 170,000 years ago. Scientists studying a different group of genes recently concluded that all humans descend from an African man who lived 59,000 years ago. Even allowing for a healthy margin of error in these estimates, it would seem that Eve lived more than 100,000 years before Adam.
You might think that the scientists involved need to find the mistake that produced two such different conclusions. But as contradictory as the results may appear, they are perfectly compatible. Genealogy is much stranger than most of us realize.
The quest for humanity’s genetic genealogy began in the early 1980s, when researchers were just starting to decipher the genetic code. DNA is shaped like a twisted ladder, and each of its rungs is made of a pair of building blocks called bases. When scientists began to read the sequence of these base pairs, some of the first genes they chose to decode were a peculiar sort. Nearly all of our 30,000 genes are located within the cell nucleus. But 37 genes reside outside the nucleus, in sausage-shaped structures known as mitochondria, which act as the powerhouses of the cell.
These outlying genes are also unusual because of the way they are inherited. The genes in a human cell nucleus are arranged on twenty-three pairs of chromosomes. At the time an egg or a sperm cell is being formed though cell division, the chromosomes in each pair swap parts of their genetic material (with one important exception, which I’ll get to later). Each egg or sperm then receives only one from each pair of rearranged chromosomes. When a sperm fertilizes an egg, it contributes its chromosomes, and a new set of twenty-three pairs is established. Thanks to the swapping episode, it’s a unique combination of genes.
Mitochondria are different. Mitochondria themselves do not engage in sexual reproduction. Moreover, a father cannot contribute mitochondrial genes to a child, because mitochondria in sperm can’t enter the egg cell. My child inherited only my wife’s mitochondrial genes, my wife inherited her mitochondrial genes only from her mother, and so on, back through thousands of generations of women.
In the mid-1980s Allan Wilson, a geneticist at the University of California, Berkeley, recognized that the genes in mitochondria were loaded with historical information that other genes lack. Our chromosomes go through a complex shuffle in every generation, but mitochondrial genes create a clean, unmuddled pedigree. The only way a difference between the mitochondrial genes of a mother and her child can arise is if they mutate. In some cases, a mutation will cause a genetic disorder with symptoms such as weakness, respiratory problems, or deafness. Natural selection steadily removes the most harmful of these mutations from the human gene pool. On the other hand, mutations can alter mitochondrial genes without causing harm (and, in very rare cases, may even bestow some benefit). If a woman acquires a harmless mutation in her mitochondrial DNA, she will pass it on to her children, and her daughters will pass it on to their own children. It will mark her descendants as distinct from other people—distinct even from the descendants of her own sister.
Wilson and his students studied some of these markers by gathering samples of mitochondrial DNA from people around the world and comparing specific stretches from each individual. They found that different groups of people shared certain markers, suggesting that these groups had descended from a common ancestor. For example, Europeans all shared markers that no one else did, while Asians had unique markers of their own. Wilson’s team also found that Asians and Europeans shared certain other markers that Africans lacked. By grouping people on the basis of their markers, Wilson’s team was able to construct an evolutionary tree for humanity. The branches that sprouted closest to the base of the tree were all African lineages, suggesting that mitochondrial DNA in all humans living today descended from an African woman.
Wilson’s team named not only the place, but also the time, of human origins. “All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa,” they announced in Nature in 1987. To come up with their age estimate, the researchers compared the variations in mitochondrial DNA and then calculated how long it must have taken for so many mutations to build up in different lineages. Their estimate of 200,000 years was shockingly recent. It was already clear from the fossil record that human relatives lived in Europe and Asia at that time; now Wilson’s work suggested that when ancestors of living humans migrated out of Africa, these other humans-with their own, different mitochondria-went extinct.
As it turned out, Wilson’s statistics weren’t as sound as he had believed, but later studies on mitochondria have come to essentially the same conclusions. Wilson worked only with fragments adding up to about 9 percent of the mitochondrial genome; in the December 7, 2000, issue of Nature, Swedish and German scientists reported on the results from examining all the mitochondrial genes-the entire sequence of about 16,000 base pairs. Comparing the sequences of fifty-three individuals, these researchers, too, found that their genes had come from an African woman. But instead of living 200,000 years ago, they estimated, she lived 170,000 years ago.
Newspaper reports on Wilson’s 1987 paper dubbed this African woman “mitochondrial Eve.” But despite the biblical overtones, she was not the sole female progenitor of all living humans. She was simply the most recent female ancestor to whom we can all trace this particular genealogical connection. Mitochondrial Eve existed alongside thousands of other women in Africa, all of whom had mitochondrial genes of their own. Many of those other women had children who inherited their genes, and some of their descendants had children with mitochondrial Eve’s descendants. But over the course of thousands of years, the other mitochondrial lineages gradually disappeared. A lineage would have vanished if the women carrying it died without having children or gave birth only to sons. As these different mitochondrial genes were dropping out, Eve’s were becoming more widespread.
The way her genes came to dominate our species is similar to the way in which a patrilineal surname can take over an entire community. In a village where many different family names were in use thousands of years ago, everyone alive now could conceivably share just one. The fact that everyone in the village is named Chen, however, doesn’t mean that one man named Chen was their sole male progenitor. Similarly, we descend from thousands of other women who were alive at the same time as mitochondrial Eve. We just don’t carry their mitochondrial genes.
just as mitochondrial genes contain a record of female lineages, men have a set of genes that can tell a story about the other half of our species. Of the twenty-three pairs of chromosomes carried by men in the nucleus of their cells, twenty-two consist of partners identical in length, shape, and sequence of genes. The remaining pair is different. From his father, a boy inherits a chromosome called Y, and from his mother, a chromosome called X. During the formation of sperm, as the other chromosomes shuffle their genes, only a small section of the Y chromosome exchanges bits of material with its partner. Most of it remains aloof, providing a clean, unmuddled pedigree passed down from father to son-a male counterpart to mitochondrial DNA.
The story embedded in the Y chromosome has been much harder to extract than that of mitochondrial DNA. The mitochondrial genome is small, only 16,000 base pairs long, making it easy to sequence, and every human cell contains an average of 1,700 mitochondria, offering plenty of targets for genetic probes. By contrast, each of a man’s cells contains a single Y chromosome, and it is big, measuring 60 million base pairs long. Searching for markers on the Y chromosome is like looking for a few typos in a thirty— volume encyclopedia. By 1994, scientists had managed to find a grand total of only two markers on the Y.
The Y chromosome has finally been tamed by investigators at the laboratory of Stanford University geneticist L. Luca Cavalli-Sforza. In 1995 Stanford researchers Peter Underhill and Peter Oefner found a way to speed-read through the Y-chromosome encyclopedia. Soon they and their colleagues were discovering a new mutation every month or so (they’re now up to 167). It was then relatively easy for them to study Y chromosomes from various parts of the world to see if they shared any particular mutation. If they did, it would mark them as descendants of a common ancestor.
Like Wilson’s group, the Stanford team has used their markers to draw an evolutionary tree of the human race, and they find that the oldest branches are exclusively African, confirming that Africa is the motherland of us all. Some Africans spread out to the other continents, a journey that the Y chromosome records in exquisite detail. It reveals individual waves of migration from Asia to Europe, from Asia to Polynesia, and from Asia to the New World.
But there’s one important disparity between the findings based on the Y chromosome and those based on mitochondrial DNA. Underhill and his colleagues established a clock for the Y chromosome, and in the November 2000 issue of Nature Genetics they estimated that Y-chromosome Adam, the man from whom all living men descend, lived only 59,000 years ago.
How can we all descend from two people who lived thousands of generations apart? These scientists are calculating the antiquity of the various genes we share, not attempting to reconstruct the complete family tree of the people who carry them. Only our mitochondrial genes descend from a common female ancestor who lived 170,000 years ago, and only our Y chromosomes descend from a single Y chromosome dating back 59,000 years.
The men who were alive in mitochondrial Eve’s day carried a number of different versions of the Y chromosome. They passed their chromosomes down to their sons, and over time, each version of Y went through its own ups and downs. Finally, about 59,000 years ago, a man was born with a newly mutated Y chromosome that would eventually dominate our entire species. Other versions of the Y gradually disappeared as men died without children or had only daughters.
It’s intriguing that Y-chromosome Adam appears to have lived so much later than mitochondrial Eve. His Y chromosome apparently needed less time to overwhelm the human gene pool. One possible explanation for this speed is that one of the genes on Adam’s Y chromosome had a mutation that gave it an evolutionary edge, and natural selection then drove its spread. But natural selection of a gene is not the only force capable of making it more widespread-culture might have been responsible. Perhaps it was common 59,000 years ago for only a few men in each band to earn the privilege of fathering children. If that was the case, Y-chromosome lineages might have gone extinct quickly, because most men would have been unable to pass on their genes. The Stanford team is now exploring these two possible explanations.
The findings from research on the Y chromosome and mitochondrial DNA are only a taste of the genealogical feast that will be served up in the next few years. Last year, government and private-sector scientists made a joint announcement that they had sequenced the entire human genome. Researchers will now be able to find markers far more quickly-and not just on the Y chromosome or on mitochondrial DNA but on any gene they want to study. Some of these genes will turn out to be hundreds of thousands of years old. Others will turn out to be much younger, having evolved in response to recent epidemics or similar challenges.
All this new research makes me think differently about our child. We are not simply giving him or her my nose or my wife’s eyes. We are giving our child tens of thousands of histories combined into a single genome. And she or he will carry this record of human existence another generation into the future. Somehow, the mysteries of swaddling don’t seem like such a big deal anymore.
Copyright 2001, Carl Zimmer. Reproduction or distribution is prohibited without permission.