Tuesday, August 24, 2004

Men are Pigs II

Nothing like a nice bit of controversy to start the day: here's news of a study which casts doubt on a notion cherished both by the Traditional Values crowd and the "Sexual Behavior is Culturally Determined" school of feminism.

Now comes a fascinating new paper in press at Molecular Biology and Evolution. Scientists at the University of Arizona suspected that some of the confusion over Adam and Eve might be the result of comparing the results of separate studies on the Y chromosome and mitochondrial DNA. One study might look at one set of men from one set of ethnic backgrounds. Another study might look at a different set of women from a different set of backgrounds. Comparing the studies might be like comparing apples and oranges. It would be better, the Arizona team decided, to study Y chromosomes and mitochondrial DNA all taken from the same people. Obviously, those people had to be men. The researchers collected DNA from men belonging to three populations--25 Khosians from Southern Africa, 24 Khalks from Mongolia, and 24 highland Papuan New Guineans. Their ancestors branched off from one another tens of thousands of years ago.

The results they found were surprisingly consistent: the woman who bequeathed each set of men their mitochondrial DNA was twice as old as the man whose Y chromosome they shared. But the ages of Adam and Eve were different depending on which group of men the scientists studied. The Khosian Adam lived 74,000 years ago, and Khosian Eve lived 176,500 years ago. But the Mongolian and New Guinean ancestors were both much younger--Adam averaged 48,000 years old and Eve 93,000 years.

You wouldn't expect these different ages if a single Y chromosome had been favored by natural selection, the Arizona team argues. Instead, they are struck by the fact that Khosians represent one of the oldest lineages of living humans, while Mongolians and New Guineans descend from younger populations of immigrants who left Africa around 50,000 years ago. The older people have an older Adam and Eve, and the younger people have a younger one. The researchers argue that some process has been steadily skewing the age of Adam relative to Eve in every human population.

Now here's where things may get a little sticky for the "one-man-one-woman-is-traditional-and-natural" camp. The explanation the Arizona scientists favor for their results is polygyny--two or more women having children with a single man. To understand why, imagine an island with 1,000 women and 1,000 men, all married in monogamous pairs, just as their parents did, and their grandparents, and so on back to the days of the first settlers on the island. Let's say that if you trace back the Y chromosomes in the men, you'd find a common ancestor 2,000 years ago. Now imagine that the 1,000 women are all bearing children again, but this time only 100 men are the fathers. You'd expect that the ancestor of this smaller group of men lived much more recently than the common ancestor of all 1,000 men.

Scientists have proposed that humans have a history of polygyny before (our sperm, for example, looks like the sperm of polygynous apes and monkeys, for example). But with these new DNA results, the Arizona researchers have made a powerful case that polygyny has been common for tens of thousands of years across the Old World. It's possible that polygyny was an open institution for much of that time, or that secret trysts made it a reality that few would acknowledge. What's much less possible is that monogamy has been the status quo for 50,000 years.

People are perfectly entitled to disagree over what sort of marriage is best for children or society. But if you want to bring nature or tradition into the argument, you'd better be sure you know what nature and tradition have to say on the subject.
One can't say that this argument is completely conclusive - indeed, arguments for scientific theses never can be - and it still is a possibility that some powerful mutation occurred that triggered one or more selective sweeps throughout the human male population in the recent evolutionary past, but this finding, in combination of what we know both from studying human behavior today and from traits like the differences in stature between men and women, makes a powerful case against the idea that one-man one-wife has ever been the norm amongst our species.

One is tempted to speculate whether the high rate of divorce in modern Western societies might not have something to do with the fact that for the first time in history, monogamy is being seriously enforced not just on the women of a community but also on the men; in every single historical monogamous society that I'm aware of, what was expected was that though a man could have only one wife, he was free to take on as many concubines or slave girls as he chose, or failing that, he could visit brothels with impunity.

It goes without saying that none of the above implies that polygamy is "right" or that monogamy is "wrong"; one should never make the elementary mistake of concluding ought from is.