New DNA testing spills the beans on those old family secrets...
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Who are you?
More and more people are trying to trace their ancestry with a quick DNA test. A new book -- and my own experiment -- show that science can reveal some interesting things about your past, but not necessarily what you want to know.
By Laura Miller
June 26, 2006 | Every family has its genealogical myths, legends and secrets. There's the Native American ancestor some clans like to talk about and the Jewish or black (or in the case of African-American families, white) great-great-grandparent that no one mentions or even knows for sure existed. Whole nations tell themselves similar stories about the past. Icelanders believe their country was settled by Norsemen and the British or Irish women they brought (often unwillingly) with them. British schoolchildren are taught that when the Anglo-Saxons invaded Britain in the 5th century, they pushed Britain's Celtic inhabitants out to the hinterlands of Scotland and Wales and made England an essentially Anglo-Saxon country.
Until recently, it's been impossible to prove or disprove any of these stories. DNA analysis has changed all that, and as New York Times science reporter Nicholas Wade explains in his new book, "Before the Dawn: Recovering the Lost History of Our Ancestors," in the process it has toppled more than one cherished belief. It turns out, for example, that most Icelanders are probably descended from Norsewomen and that a large proportion of the male population of Britain carries the Y chromosome of the Celtic speakers who were supposedly chased off the land by the Anglo-Saxons. Similar research has established that an astonishing 8 percent of the men living in the vast territory formerly controlled by the Mongol Empire are most likely direct descendants of Genghis Khan.
The power of DNA analysis to nab criminals, exonerate the wrongly convicted and determine a baby's true paternity has understandably impressed everyone and provided new fodder for trashy daytime talk shows. With "Before the Dawn," however, Wade goes further, offering a survey of how cutting-edge genetics has been combined with a variety of other sciences to solve, or at least further illuminate, some long-standing puzzles in humanity's distant -- and more recent -- past. As with any powerful new technology, it's easy to get carried away. (My own enthusiasm prompted me to send a sample of my DNA off to one of several new services that offer ancestral DNA analysis, hoping to learn something about the murkier corners of my own genetic heritage, but more on that later.) It's easy, in other words, to think that because DNA can tell us so much, it can tell us just about anything, and that would be a dangerous mistake indeed.
Here, greatly simplified, is how it works: When sperm and egg combine to create the embryo of a new organism, a whole lot of swapping of genetic code goes on. The DNA that spells you is a salad of code taken from both your mother and your father. Their DNA is a mix of each of their own parents, and so on back into time immemorial. However, a big portion of the DNA in each of our cells isn't used to make our bodies; this is what's often called "junk" or "filler" DNA. Both the active and the filler DNA is subject to small, random variations when the code is transferred from one generation to the next; that's mutation.
The DNA that's actively involved in making us what we are is subject to "selective pressure." If a new mutation makes a human being who's a little bit taller or faster and that change makes the individual more successful in his or her given environment or more attractive to the opposite sex, then that individual will be more likely to survive and to reproduce more plentifully. Over generations, the mutated gene will become more common, a process called natural selection, which (we can only hope) most of us learned about in school.
Filler DNA, however, because it has no input in shaping the physical organism that carries it, isn't affected by the natural selection process. It still undergoes the occasional mutation, but those mutations simply accumulate over time without doing anything. In particular, two sections of this DNA -- the Y chromosome, which men hand down to their sons, and mitochondrial DNA, which women hand down to all their offspring -- have been useful to those researching human ancestry. If, say, 10,000 years ago a particular woman was born with a certain mutation in her mitochondrial DNA, all of her children will carry the same mutation in their mitochondrial DNA, and her female children will pass it on to their children. The same is true for a man of the same period born with a mutation in his Y chromosome, although he will only pass it on to his male offspring, who will only pass it on to their male offspring, etc.
By looking at the Y chromosomes and mitochondrial DNA of various populations in various places, scientists can get a good idea of where these genetic lineages began and how they spread over the globe. The most prominent popularizer of this notion is the Oxford geneticist Bryan Sykes, author of "The Seven Daughters of Eve," in which he employed the fairly kitschy device of naming the seven women from whom all individuals of European origin are descended, and describing their lives as he imagined them. Yes, it sounds very "Clan of the Cave Bear," but at its heart there's an intriguing fact: All contemporary humans are part of one of 38 major "haplogroups," each founded by a single woman.
A few of those lineages stayed in Africa, where human life originated, but everyone else on the planet, Wade reports, is descended from a tiny group of people who migrated out of Africa 50,000 years ago. That group might have consisted of as few as 150 hunter-gatherers, but eventually their descendants spread as far as Australia and North America to populate the world. Researchers lean toward the theory that there was only one migration out of Africa, one little band of people who braved either the coastal edges of the Arabian desert peninsula or the perils of the eastern Mediterranean, which was then occupied by fierce Neanderthal tribes.
The idea of this exodus is irresistibly romantic, but it can't be said that Wade takes advantage of that. "Before the Dawn" is at best a workmanly account of what is, after all, a field rife with controversies, reversals, ambiguities and political land mines. Still, what the book lacks in vividness it doesn't exactly make up for in clarity. The descriptions of how DNA works are more difficult to understand than they should be (although the processes are fundamentally hard to grasp). Genetics, unlike, say, quantum physics, is new enough that it hasn't yet found a great, nonpolemical popular science writer to do it justice.
Nevertheless, Wade has collected many fascinating stories about the creativity and (occasional) hubris of those scientists who have used DNA analysis to study the past. When did human beings first begin to wear clothes? That used to be impossible to estimate, since the clothes themselves have long since crumbled to dust. But when Mark Stoneking, a researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, got a notice from his kid's school about a problem with lice, he had a brainstorm. Body lice, unlike the head lice from which they evolved, have claws "specialized for living in clothing, not grasping the shafts of hair." Since the lice can't live for more than 24 hours away from the warmth of a human body, they must have evolved shortly after human beings (whose hairiness has drastically decreased over the millennia) began to wear clothes. Comparing the DNA of head lice to body lice gave Stoneking a date for when body lice emerged.
But there's a catch. The kind of analysis Stoneking performed provides only a ballpark figure: 42,000 to 72,000 years ago. That 30,000-year spread may not be much on the scale of the history of life itself, but in human terms, it's pretty wide. Did this happen before or after our ancestors left Africa, a date estimated at 50,000 years ago? Your guess is as good as mine.
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