Yours, mine and Eire’s

The Washington Post, Sunday March 18th

Like other Irish émigrés around the world, I see this weekend's St Patrick's Day celebrations as an opportunity to reach back into a greener past. With shallow roots in our adopted countries, it is comforting to reaffirm the deeper ones that stretch across seas.

This is not a new phenomenon. Even the writers of the Bible provided long lists of genealogies that anchored their prophets and kings in earlier days that were closer to God. The exploding popularity of genealogy, with thousands of Internet sites offering family tree searches, betrays our continuing need to underpin the uncertainties of the modern world with a surer past.

And now geneticists, who can track our roots back through the dark recesses of prehistory, are constructing the ultimate in family trees to uncover the origins of us all. But as fascinating as these studies certainly are, with their fresh evidence of humankind's peregrinations around the world, they will not tell us what it means to be Irish, Jewish or American. As we track our ancestors back through time, the strands of your family tree and mine become a tangled web of relationships, which makes a nonsense of such ideas as bloodlines, genetic purity or even race. Our cultural roots are not to be found in our genes but in an entirely different kind of inheritance that was invented by our early ancestors and passed down not just from parent to child, but from cousin to cousin and from friend to friend. We are born into a culture, rather than born with one.

My own roots in Ireland have been traced by a relative to a Cornelius McFadden who, in mid-18th century Donegal, was caught stealing a sheep (an easy theft to prove in 18th-century Ireland since the English owned all the sheep). The usual penalty was hanging, but Cornelius's wife, Nancy, was heavily pregnant, prompting the authorities to be merciful and impose on Cornelius the lesser punishment of having his ears cut off, his wounds bound and then being pushed out to sea on a raft with his young wife. The pair rowed along the coast, so the story goes, until they beached on the bleak Atlantic island of Innishirther, where they thrived and raised a family of 11 children. Their fate is remembered in a Gaelic phrase, Thug siad Oidhe Concubhair air, which is roughly translated as "the justice of Cornelius."

St. Patrick's Day gives me an excuse to raise a glass and drink to the fortitude of Nancy and Cornelius and to our shared history. But several Guinnesses into the evening and my mind usually wanders to older migrations. Who are the Irish with whom I claim kinship, and what brought them to Ireland -- or their ancestors to Europe? Nancy and Cornelius's voyage was just one step in the long line of journeys that spread people across the globe. What paths did their forebears tread before history recorded those wanderings? And where, in the end, does this leave my feeling of kinship with old Cornelius and the people who call themselves Irish?

Stories, surnames and genealogies penetrate just a few centuries. Archaeology goes further, but its artifacts are rare and ambiguous. Our past has also left traces in material that is both durable and accessible -- our genes -- and scientists who unravel human genetic codes from across the globe have begun building family trees that go back millennia, revealing striking facts about where we came from and how we got to where we are today.

Take the female line. Using the mitochondrial DNA we inherit only from our mothers, one researcher, Rebecca Cann of the University of California at Berkeley, was able to build a genetic tree that goes back to the mother of us all -- a mitochondrial Eve who lived in Africa about 200,000 years ago and whose descendants trotted (or paddled) out to colonize the rest of the world. Mysteriously, of all the females who must have inhabited the globe at that time, this one Eve appears to have been the source of all of our mitochondrial DNA.

Studies of the Y chromosome have revealed similar secrets of our male ancestors. Contrary to the myth of the male warrior searching out new territory, in most populations our male ancestors seem to have been homebodies, while the female did the traveling -- a finding that fits with marriage practices in many traditional societies such as the Bedouins, where the young bride travels to live in the house of her new spouse.

Love was, of course, not the only reason to migrate. With the domestication of plants and animals about 9,000 years ago came an agricultural revolution that eventually brought wheat cultivation into Europe and then the British Isles. Genetic data indicates that it was not simply the practice of farming that spread, but the people -- and their genes. In 1997, Luigi Luca Cavalli-Sforza of Stanford University plotted gene frequency patterns across Europe and the Near East that bore a striking similarity to the spread of agriculture. Traveling with their crops, farmers and their genes eventually reached Ireland some 5,000 years ago.

Later migrations, many of which were more warlike, coincided with the expansion of the Indo-European languages, including Latin, German and Irish. Ever since an English judge serving in India in the 18th century pointed out the similarity between modern European languages and the ancient Indian Sanskrit tongue, linguists have sought a homeland for the Indo-Europeans, ranging from Spain to Anatolia. The genetic data favors an origin on the Russian steppes with a group of nomadic farmers known as the Kurgan who domesticated the horse and rode it out to invade India and Europe. Studies of Irish Y chromosomes indicate that the Emerald Isle represents the western edge of that migration.

Although the spread of genes and languages are often similar, they do not always coincide exactly. Take the case of the Lemba, a tribe of Bantu-speaking black Africans who have long believed that they are descended from Jews. Lost tribes of Israel are commonplace, but the Lemba have a stronger claim than most since they practice semitic traditions such as circumcision, kosher food preparation and the keeping of the Sabbath. Analysis of the Lemba's mitochondrial DNA did not find any differences from their Bantu neighbors but lurking in their Y chromosomes was a genetic marker found only among Jews. The Lemba tradition that a high priest named Buba led them out of Judea may indeed be based on a real event that has left its impression on their genes.

But there is a danger in getting carried away with these studies and seeing gene migrations as the source of human culture and national identity. The Jewish gene in the Lemba tribe is found in only about 10 percent of the men, yet the whole tribe practices Jewish traditions. Similarly, although Kurgan genes may have spread westward from an Indo-European homeland, the Kurgan swapped lots of genes with the indigenous populations along the way. By the time they reached Ireland, their genetic inheritance was heavily diluted, though the language they spoke remained thoroughly Indo-European.

The differences between populations are nearly always just a matter of the varying proportions of a few (probably unimportant) genes. Germans, Celts, Latins, Jews, Polynesians, Africans and Americans are all blends of genes from diverse sources. My passport claims that I am Irish, but examine my genes and you will find that I, like the rest of humanity, ama mongrel.

Cavalli-Sforza's genetic studies have led him to hypothesize that the invention of modern language was the spark that ignited human expansion. Perhaps what set Cann's mitochondrial Eve apart from her contemporaries in Africa 200,000 years ago, what made her the source of our mitochondrial DNA, was that she was the inventor of language. And language allowed a new form of heredity -- cultural inheritance -- whose letters were not written in our genes.

So although my Irish ancestors have left me something of their DNA, they also, and more significantly, left me their tales of Finn mac Cumail and his Fenian warriors, their stories of mermaids who tempted young men into the sea and their warnings of fairies who might steal an untended child. Language and culture provided mankind with a means of exploring the world in stories, tales and songs that could fill the dark winter nights.

So on this St. Patrick's Day weekend, by all means drink a toast to your Irish ancestors. But let's not forget that although their wanderings have left traces in our genes, our real cultural inheritance is in the language, traditions and customs that we learned from parents, relatives and friends. Being Irish is about a shared liking for sad songs, magical tales, fiddle music and Guinness, rather than shared genes.

Johnjoe McFadden, a professor of molecular biology at the University of Surrey in southeast England, is the author of "Quantum Evolution" (Norton).