Where We Come From
Recent advances in genetics are starting to illuminate the wanderings of early humans
Indeed, in recent months, two groups of geneticists have published sweeping chronicles of the peopling of Europe, one tracing maternal DNA lineages, the other, paternal. These findings portray the majority of European forebears arriving from the Middle East as hunter-gatherers 25,000 to 40,000 years ago. During the last Ice Age, these first Europeans fled south to Iberia, Ukraine, and the Balkans. As the ice retreated, the Ice Age survivors spread out and flourished. The last major migration from the East 9,000 years ago brought agriculture and domestic animals but did not displace the earlier settlers, as some researchers had thought.
Genetic clock. The European studies are among the first to capitalize on a new ability to compare the migrations of males and females, which don't always follow the same path through history. Over the past 20 years, researchers have been able to track women's wanderings through mitochondria--tiny energy-producing bodies that cluster by the hundreds in human cells. Mitochondria have very odd DNA. They contain genetic material only from the maternal line, unlike the cell nucleus, which is a mix of DNA from both parents. This means that all children, male and female, carry copies of their mother's mitochondrial DNA.
That peculiarity gave geneticists a key tool for learning the movements of ancient populations. That's because as mitochondrial DNA is passed along, tiny, harmless mutations occur. By comparing the mutations among people, it's possible to calculate how closely they're related. And by calculating the mutation rate, researchers can deduce how far back in time different groups split apart. Douglas Wallace, director of the center for molecular medicine at Emory University Medical School, says: "You literally have a genetic clock." Wallace proved that point in 1980, when he was able to differentiate people from Europe, Asia, and Africa by comparing their DNA.
The realization that there is a map and a clock of human history in every cell completely transformed the small, highly technical field of population genetics. Scientists had been searching for human history in the genes at least since World War I, when two Polish immunologists discovered that different armies had very different proportions of various blood types. (Type B blood, for example, is more common in East Asians and Africans than it is in Europeans. Since blood type is hereditary, controlled by a single gene, a blood type can be used as a crude form of genealogy.) Blood types were used to prove that the Romany, or Gypsies, were correct when they claimed they originally came from the Indian subcontinent, not Europe.
But although researchers kept cataloging genetic markers in blood proteins, the number identified was far fewer than the millions of inherited mutations that must exist. "There just weren't enough data to answer the interesting questions," says Kenneth Kidd, a genetics professor at Yale University School of Medicine.
Times changed. Since the mid-1980s, technology has unleashed a flood of new data, so much that researchers struggle to keep pace. Restriction enzymes allow scientists to snip DNA into tiny, easy-to-read bits. The 1983 invention of the polymerase chain reaction, or PCR, made it possible to make unlimited copies of a DNA strand in a test tube. PCR made it possible to decode the human genome. And for students of human history, it is opening the window to the past further than anyone imagined.
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