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2007

High-Achieving Genes
Forbes, June 2007 (originally published on Forbes.com March 2, 2007)
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You make partner. You get tenure. You conquer the known world. You achieve greatness in your lifetime. But in the great scheme of things, how much does any of that really matter? After you die, they come to empty your desk. They take down your plaques from the wall. The grand statues you had built for yourself crumble in the desert winds.

One way to overcome your own mortality is to produce a dynasty. A thriving flock of descendants can sustain themselves, generation after generation, passing down your name€¦ or at least your DNA. That's what Genghis Khan did, and did with astonishing success. An estimated 16 million men today, plus an uncounted number of women, are his direct descendants.

Khan's great flock came to light in a survey of DNA. When we pass down our genes to our children, they also inherit some distinctive genetic markers. Over the past 20 years, scientists have learned how to recognize these markers and use them to study human history. For example, the markers preserve a record of the spread of our species out of Africa some 50,000 years ago.

Geneticists have also used genetic markers to learn more about the ancestry of people in particular parts of the world. Genghis Khan's genetic achievements turned up in a study in which an international team analyzed the DNA of 2,123 men from Asia. Why just men? Because, unlike other chromosomes, the Y chromosome carried by each man is usually a carbon copy of his father's. (Other chromosomes come in pairs, and they get scrambled before we inherit them from our parents.)

In their survey of Asian men, the geneticists discovered one particularly remarkable genetic marker. It turned up in men in a vast region stretching from China across Mongolia and as far west as Uzbekistan. Eight percent of the men in that region carried it. Beyond those borders, they found the marker in just half a percent of Asian men. Closer study revealed that this marker probably originated in Mongolia roughly 1,000 years ago, plus or minus three centuries.

All of these lines of evidence pointed the geneticists to a dramatic conclusion: the men who carry this particular marker are all descended from Genghis Khan.

Khan was born around 1162 in Mongolia, and in his forties he began a campaign of conquest, ultimately creating an empire stretching from the Caspian Sea to the Pacific. Khan had a great many children, both with his wives and with other women. His sons, who expanded the Mongol Empire into Europe, had many children of their own. Although the empire broke up in the decades following Khan's death in 1227, his male descendants ruled large chunks of it for centuries. And like their ancestor, they had many children as well.

If the geneticists are right, Khan and his descendants spread his distinctive Y chromosome to about half a percent of the world's male population alive today, or some 16 million men. Strikingly, the region where almost all of those men live matches the boundaries of the old Mongol Empire.

Other kings appear to have achieved similar kinds of genetic success. Irish geneticists have discovered a marker carried by one in five men from northwestern Ireland. They also noticed something else these men shared in common: their last names.

People with certain Irish surnames, such as O'Neil, have long been thought to have descended from a dynasty of Irish kings known as U­ N©ill. And the U­ N©ill dynasty is traditionally thought to have been founded by a fifth-century warrior known as Niall of the Nine Hostages. Recent genetic studies suggest that Niall bequeathed his Y chromosome to over 2 million Irish men alive today.

While these reproductive achievements may seem impressive, there's actually less to them than meets the eye. Genghis Khan and Niall of the Nine Hostages were not somehow biologically superior to all the other men of their day. They did not carry genes on their Y chromosome that turned their sons into supermen. Their reproductive accomplishments came from quirks of history. They were in the right place at the right time with the right personal qualities to become kings, and their descendants continued to enjoy the privileges of royalty--including having lots of children. Instead of natural selection, they experienced historical selection.

But in the full sweep of human history, being the ancestor of a few million men is not much to brag about. After all, every man alive today descended from a single man who lived in Africa some 230,000 years ago, just about the time our species Homo sapiens was emerging.

This ancestral man was not the emperor of the world. Nor was he some genetically enhanced superman. He probably was just another face in the crowd of early hunter-foragers living in East Africa. He and thousands of other men of his generation passed their Y chromosomes to their sons. But thanks mainly to luck, his Y chromosome alone gradually spread through the entire species, while those of other men eventually reached dead ends.

It turns out that individual genes have their own genealogies as well. People who carry a certain version of a gene all inherited it from a common ancestor. Push back further in time, and you can find the common ancestor from which all versions of the gene came from.

While some chunks of DNA are common today thanks to the conquest of kings or historical flukes, others have become widespread thanks to good old natural selection. People from time to time have been born with mutant genes that gave them a slight reproductive edge, one that their offspring enjoyed as well. These lucky mutants might be less likely to die of malaria, for example, or be better able to tolerate lactose or handle the complexities of full-blown language. In any case, their versions of genes spread through the human population while others dwindled away.

We'll never know exactly who first carried those adaptive genes. But ultimately that doesn't matter. It's the genes, not the people, who have achieved this kind of greatness. The starkest proof of this comes from a gene called microcephalin, which is involved in brain development. All humans carry some version of microcephalin. One version is far more common than the others, found in 70% of all people.

Recently scientists at the University of Chicago compared the different versions of microcephalin to figure out how long ago they all originated from a single ancestral gene. The answer was startling: over a million years ago--long before our species emerged. But weirder still, the most common version of microcephalin only began to spread 37,000 years ago. What was that version of microcephalin doing in the intervening time?

The best explanation for this finding is that the most common version of microcephalin in our species came to us from Neanderthals. Neanderthals and humans evolved from a common ancestor that lived in Africa about half a million years ago. The ancestors of Neanderthals moved out of Africa and arrived in Europe about 300,000 years ago. These rugged, barrel-chested people survived the vast flux of Ice Age rhythms, hunting and building shelters. They had Europe to themselves until about 45,000 years ago, when modern humans arrived from Africa.

The slender, clever Africans came to stay. Over the next 15,000 years or so, Neanderthals shrank back into remote mountain refuges, while modern humans spread across the continent. And then the last true Neanderthal died--yet another species hurled onto the ash heap of extinction.

Neanderthals and humans presumably could have interbred, just as closely related species of other mammals do today. And the microcephalin study suggests that they did. Their Neanderthal-human hybrid children carried genes from both species, but it appears that most of the genes from the Neanderthals gradually disappeared from the human gene pool.

Microcephalin was different, though. Humans who carried the Neanderthal version had more children than those who didn't, and the gene spread steadily.

Scientists don't yet know why this particular Neanderthal gene gave humans such a reproductive edge. But scientists do know that microcephalin helps build brains. Its name--which means "tiny head"--comes from the devastating birth defects that can be produced when the gene is crippled by a mutation. So it's possible that the human mind itself was reshaped by a Neanderthal gene.

In evolution, it seems, achievement is a very strange thing. A gene may break free from its ancestral species and go on to enjoy greatness, even as that species vanishes into extinction.

Copyright 2007 Carl Zimmer

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