I’m always a bit surprised at the number of people I meet who don’t realize that kinship systems are cultural constructs. There is a biological connection, of course, but order is imposed by systems that vary across cultures.

In some cultures, cousins of a certain type are assimilated into the same kinship class as brothers and sisters. In other cultures, a person might belong exclusively to the family of the father or the mother, while the other parent is only a relative by marriage. I imagine most of us have some idea of the astounding number of variations from what we remember of Anthropology 101.

If we’re not careful, we might think about these other systems and feel just a bit superior because we use don’t have any such quaint ideas. Our system is an irreducible universal, based on biology. Whatever other systems our ancestors might have used have been eroded away. We now use a simple system that divides relatives into the two categories of relatives by blood (consanguinal) and relatives by marriage (affinal). We calculate kinship bilaterally, taking paternal and maternal relatives equally into account, but use an elaborated system to weigh the distance of a biological relationship (1st cousins, 2nd cousins, and so on).

Forget all that.

The reality might come as a surprise to amateur anthropologists who are used to picking up second-hand reprints of 19th century texts at the local used book store. In fact, there is no evidence that the cultures who use all those quaint systems are any less sophisticated than we are when it comes to envisioning biological relationships. The difference between us and them, is that they have elaborated cultural systems overlaying biology, and those systems play significant roles in their culture.

Moreover, despite the erosion of our older Germanic kinship systems, we do have one vestigal survival of a cultural system — the surname. Each of us officially belongs to one, and only one, family. Our family name is our badge of membership in that lineage group. It’s not a coincidence that the surname is often called the family name. (Of course, since we use a bilateral kinship system, we end up with culturally significant relatives in other families.)

An easy way to see this point is to think about the inheritance of surnames. We all have eight great grandparents. All of them are equally our ancestors, but we inherit a surname from only one of them. There’s no biological reason to privilege one line of ancestry; it’s merely a cultural artefact. Such cultural systems always interact with other parts of a culture, however trivially. In our culture, some preference rule is arguably necessary if we are to have use surnames at all. It would become very cumbersome to use the surnames of all your ancestors, or even of your eight great grandparents — I would hate to go through life as Justin Howery-Alloway-Horn-Quillen- Swanstrom-Fyrsten-Luce-Wilson!

The residual significance of surnames becomes clearer when we look at our European cousins. In America, changing your surname is a relatively simple matter of appearing before a judge and making a case that there is nothing improper about the change. But, in some European countries it is illegal to adopt a surname that did not belong to one of your great grandparents. And, in Iceland, it is illegal to adopt an hereditary surname.

Although I’m primarily interested in surnames, it’s interesting to note that our supposedly logical kinship system does become a bit untidy at the edges, particularly when it comes to adoptions, relationships by marriage and relatives we don’t know. There is an interesting book, David M. Schneider, American kinship: a cultural account (University of Chicago Press, rev. 1980) that reports the results of a study of American kinship patterns. According to author, Americans become confused when asked about the cultural component of kinship. For example, is a woman your aunt if she is your uncle’s wife but you’ve never met her? Is she still your aunt if your uncle divorces her? If they get a divorce, does it make a difference if she is the mother of your cousins? Does it make a difference if she was your aunt when you were a child, or if she is someone your uncle married late in life? Is your sister’s husband’s sister your sister-in-law? Is her husband your brother-in-law? Where do you draw the line? What about a half-sibling that you’ve never met? Does it make a difference if the half-sibling is a child your mother gave up for adoption, or the by-blow of your father from a one night stand? What about the adopted child of a cousin? The collective answers to such questions show that, as a whole, Americans use a complicated and highly subjective system of biology, marriage, fosterage, adoption and acquaintance to define kinship, while stubbornly insisting that they are using only biology and marriage.

Everyone has trillions of ancestors. If you’ve never thought about this before, it might come as a surprise.

Nevertheless, the number of ancestors doubles each generation back. You have two parents, four grandparents, eight great grandparents, and so on. Assuming an average of 25 years per generation, if you were born about 1950 you would have 256 ancestors in the generation born about 1750, the generation that fought the American Revolution. The number quickly increases. Going back 20 generations to the generation born about 1450, the generation that fought the Wars of the Roses, you would have over a million ancestors. Going back 40 generations to the generation born about 950, when northern Europe was converting to Christianity, you would have over a billion ancestors — more than there were people in Europe. Seventy-eight generations back, the generation born at the time of Christ, you would have some 302,231,454,903,657,000,000,000 ancestors — far more than there have ever been humans on the earth. You can play with these numbers by assuming a different average length for each generation, but you won’t change the result; inevitably you end up with a particular number of ancestors in a given generation.

It should take only a moment to understand that one implication of the large number of ancestors we have is that each of us is descended from many marriages of distant cousins, so that the number of distinct persons in our ancestry is considerably fewer than the theoretical number of ancestors. Some experts believe that the vast majority of marriages throughout history have taken place between people no more distantly related than 2nd cousins. In fact, in about one-third of all human cultures the preferred marriage is between first cousins. Another third of cultures have no preference, and the final third disfavors marriages between cousins. In European cultures, the prejudice against cousin marriages goes back to the rules of the medieval Catholic church. Despite the church’s ban on marriages between relatives, some studies of medieval villages suggest that the normative marriage was between 3rd or 4th cousins; a relationship just distant enough to be deniable.

Because of the large number of theoretical ancestors, and necessary prevalence of marriages between cousins, some statisticians argue that every person in England must be descended from William the Conqueror; every person in western Europe must be descended from Charlemagne; and every person in continental Europe, western Asia and north Africa must be descended from Mohammed. Modern scholarship has even provided some conjectural links from Charlemagne back to the Anicii of the Late Roman Empire.

It is not at all uncommon for quite average people to have documented descents from these people. From doing genealogy, I’ve known hundreds of ordinary people with proven royal descents. (I have a few myself.) At the turn of the 20th century, there were an estimated 500,000 descendants of Edward III in Britain and America, and hundreds of thousands more descendants of Edward I. These English kings had the kind of diverse ancestry I’m talking about. Not only were they descended from William the Conqueror and Charlemagne (and perhaps from Mohammed), they were also descended from the earliest Norwegian kings, who lived in the 11th century and in turn had a legendary descent through the kings at Uppsala from the god Freyr.

In a lyrical passage, the historian Henry Adams wrote about the English, “If we could go back and live again in all of our two hundred and fifty million arithmetical ancestors of the eleventh century, we should find ourselves doing many surprising things, but among the rest we should certainly be ploughing most of the fields of the Cotentin and Calvados; going to mass in every parish church in Normandy; rendering military service to every lord, spiritual or temporal in all this region; and helping to build the Abbey Church at Mont-Saint-Michel.” I think it was Alex Shoumatoff who wrote that if we (people of English decent) could have a bird’s-eye view of England and western France in the years following the Black Death (1348), we would see that we are descended from every farmer tilling his field and every lord in his hall.

It should take only one further moment to realize that each of us also belongs to an extraordinarily large group of kin, more than we could ever keep track of. One expert asserted, “no people of English descent are more distantly related than 30th cousins.” Another study theorized that white Americans whose ancestors have been in America since before about 1850 are one of the largest, most closely related groups on earth – probably no more distant than 13th cousins to one another, because of their likely descent from a relative handful of early immigrants. For example, many of them (perhaps most of them) are descended from at least one of the the 30,000 immigrants to Massachusetts during the Great Migration 1620-1630. In short, we are all part of a huge inbred and interconnected kinship network.

Not only does our ancestry link us all together, it gives us an ancestral diversity far beyond what we see from looking only at our recent forbears. A person of English descent will probably descend from all the different ethnic groups that touched that country’s history, from the Celts, the Romans, the Anglo-Saxons, the Danes and the Normans. Each of those groups had its own cosmopolitan ancestry. The Roman soldiers stationed in Britain, for example, were drawn from all over the Empire. Modern genetic testing has now begun to provide proof of what statisticians have believed for years. For example, a man in Norway has mitochondrial DNA that shows his matrilineal ancestry to have been Korean. I have no doubt that he has some distant ancestor who came across the Indian trade routes, or perhaps came in the company of the Mongols in Russia. From Russia, it would have been an easy step to Sweden in the company of Norse traders.