Adam and His Eves

Studies of mitochondrial DNA and the Y-chromosome show that our female common ancestor is twice as far back in time as our common male ancestor.

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.