Wednesday, August 24, 2011

African Eve

You've probably heard the story of "African Eve" or "Mitochondrial Eve," the matrilineal most recent common ancestor of all human beings. But do you know what that actually means?

I was reading this post at Slashdot about "evangelical scientists" who are trying to adjust their faith with reality and have come face to face with the fact that it's "unlikely that we're all descended from a single pair of humans."

Here's how the original article at NPR puts it:
According to the Bible (Genesis 2:7), this is how humanity began: "The Lord God formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul." God then called the man Adam, and later created Eve from Adam's rib. [*]

Polls by Gallup and the Pew Research Center find that four out of 10 Americans believe this account. It's a central tenet for much of conservative Christianity, from evangelicals to confessional churches such as the Christian Reformed Church.

But now some conservative scholars are saying publicly that they can no longer believe the Genesis account. Asked how likely it is that we all descended from Adam and Eve, Dennis Venema, a biologist at Trinity Western University, replies: "That would be against all the genomic evidence that we've assembled over the last 20 years, so not likely at all."

OK, there's nothing too unusual about this. Americans who are ignorant about science - often willfully ignorant - might believe almost anything, but when people start learning more about biology and genetics, they find it increasingly hard to believe ancient myths.

But then I saw this comment at Slashdot:
>it is unlikely that we all descended from a single pair of humans.

I thought that Lucy/African Eve was the one that we're all descended from. Or was that a single pair of humans ... Lucy and multiple males.

There seems to be considerable confusion there, and I suspect that it's widespread. So let's take a look at this. (Note that I'm just a layman, no particular authority on any of this stuff. But I think I can explain the errors fairly well.)

First, there's the problem of confusing "Lucy" with "African Eve." Lucy is just a particular fossil of an Australopithecus female from 3.2 million years ago. She really has no connection with this "African Eve" idea.

African Eve, also called Mitochondial Eve, is an estimate of the matrilineal most recent common ancestor of all humans, based on mitochondrial DNA, which is passed down in the egg from your mother only.

With nuclear DNA, you get half from your mother (in the egg) and half from your father (in the sperm which fertilizes it). That mixes things up so that even siblings aren't normally identical.

But an egg is a lot bigger than sperm, and it contains a lot more. In particular, it contains mitochondria, which "power cells," more or less. Since you get it only from your mother, your mitochondrial DNA is identical to your mother's, except for any mutations. Obviously, when there is a successful mutation, that's often passed down to succeeding generations, too.

Therefore, we can follow this process in reverse, using an estimate of how frequently mutations occur, to get an idea of when our matrilineal most recent common ancestor lived. (We can do the same thing with y-chromosomes to find "Y-chromosomal Adam." It's the same basic idea.)

But what does this actually mean? First, we're all related, so it's inevitable that we would have a "most recent common ancestor." You could say the same thing about human beings and dogs, for that matter. The common ancestor of humans and dogs is much further in the past, but if you go back far enough, it's inevitable that we do have a most recent common ancestor.

So there's nothing unusual in having a most recent common ancestor. It's inevitable. And "Mitochondrial Eve" almost certainly isn't even it. She's just our most recent common ancestor entirely in the matrilineal line. We all got our mitochondrial DNA from our mothers, and they got it from their mothers, and so on - back to this so-called "Eve."

But that's not to say that there aren't more recent common ancestors, not just in the patrilineal, but in descent that wasn't entirely in one gender. In fact, that's far more likely to be the case.

So this also tells us that we have many, many common ancestors, some of whom almost certainly lived at the time of "Eve." We are not descended just from Eve and her mate(s), not at all. Most likely, we are all descended from many people in Eve's own village, and certainly many people who lived at the same time.

I think that "African Eve" has just been hyped by too many people. Yeah, that sells newspapers, no doubt. But it's led to widespread confusion about what it actually means. It's not really all that significant. In fact, it's inevitable that a "Mitochondrial Eve" would exist at some point. I think the only scientific interest in it is the estimate of dating.

And if you want an example of how inconsequential this really is, look at those dates. According to Wikipedia, Mitochondrial Eve, the matrilineal most recent common ancestor of all human beings, lived approximately 200,000 years ago. But our most recent common ancestor lived perhaps only 2,000 to 5,000 years ago.

Now that's quite remarkable, don't you think? (And no, that doesn't imply that Adam and Eve were real, not at all. We aren't descended just from that ancestor.)

* PS. I've actually known people who thought that men had fewer ribs than women, based on that old myth in Genesis about God creating Eve from one of Adam's ribs. Funny, huh? But I suppose it's a logical assumption if you actually believe that stuff.

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