In a recent Panda’s Thumb comment thread, Pam asked (among other things) about our human species genetic Adam and Eve:
I have been reading for the last few years now, that there is a consensus among the majority, that humans have been genetically traced to a two human ancestory: A genetic “Adam and Eve”.
This is a relatively common misconception, and a very understandable one. There have been published studies that have looked at the most recent common mitochondrial DNA ancestor of all humans, and other studies that have looked at the most recent common Y-chromosome ancestor of all humans. Since mitochondrial DNA is only inherited from the mother, the most recent mitochondrial DNA ancestor is frequently referred to as “mitochondrial Eve.” Similarly, since the Y-chromosome is passed on exclusively from father to son, the most recent Y-chromosome ancestor gets called “Y-chromosome Adam” a lot. The use of those two terms is not entirely inappropriate, but it can be very misleading – particularly for those who haven’t taken a bunch of college-level biology.
Let’s start with the biggest misconception, and move on from there. When it comes to Adam and Eve, there’s good news and bad news. The good news is that I can unequivocally state that they never got divorced. The bad news is that they never married. That’s understandable, of course, since Eve died more than 50,000 years before Adam was born.
Right about now, I’m guessing that we’re hitting the point when confusion sets in. After all, if every man on earth is descended from Y-chromosome Adam, and if we’re all descended from our fathers, and if Y-chromosome Adam was married, why wasn’t his wife “Eve?” We all clearly must be descended from her, too, right? (And if confusion hadn’t set in already, it almost certainly has now that I wrote that.) Let’s see what (if anything) I can do to clear things up.
It’s probably a good idea at this point to step back for a minute or two and review some really basic genetics.
When we talk about a gene, we’re usually referring to a specific stretch of DNA. Each stretch of DNA has a location on a chromosome. Humans (with some rare and typically pathological exceptions) have 46 chromosomes – two copies of each of the 22 autosomal chromosomes, and two sex-determining chromosomes (women have two copies of the X-chromosome, and men have one X-chromosome and one Y-chromosome). When we reproduce, we each give our children 23 chromosomes – one of the two copies of our autosomal chromosomes, and one of our two sex-determining chromosomes.
We also have mitochondria in our cells. These are little organelles that are a key part of the process that lets us use oxygen to make energy. Mitochondria have their own DNA, and reproduce on their own within our cells. With very, very rare exceptions, we get all of our mitochondria from the egg, which means that we only inherit mitochondrial DNA from our mothers. (So, guys, you can feel free, the next time your wife complains about our unequal contribution to childbirth, to point out that we contribute less than half of the DNA, so it’s only fair that we also do less than half of the work. If you do this, have your next of kin tell me how it went.)
That’s the genetic basis, and the easy part. Now we get to how genes are passed on, and here’s where it gets complex.
What it comes down to is this: we get half of our chromosomes from each parent, but that doesn’t mean each of our grandparents contributed 25%, or that each of our great-grandparents chipped in 12.5%. If you go back far enough, you will find that the majority of your biological ancestors have left you with absolutely nothing in the way of DNA. That’s where this gets really fun – at least for the mathematical population biology geeks.
It’s possible to think of each one of your genes – each single copy – as if it was an independent organism. A tiny little thing that has a parent and that can have offspring. That’s a fiction, of course, but a useful one. If we use that as our model, we can trace the lineage of a single gene instead of a single person. It’s a really convenient way of doing things (partly because the math gets a little easier).
If we treat genes as individual organisms, we can create a “gene genealogy” – a gene family tree – for individual genes. Here’s an example I just made. Since I’m more or less completely unskilled when it comes to developing complex simulations with a computer, I raided one of my kids’ board games. I started with five individuals, and rolled a die for each. On a 1 or a 2, the individual produced no offspring. On a three or a four, it had a single offspring. On a five or a six, it produced two kids. Here’s the result:
As you can see, after four generations, all of the individuals shared a single ancestor for that gene. But not all genes are going to have the same gene genealogies. In fact, hardly any will. Let’s look at another example, using exactly the same rules:
In this scenario, not all individuals shared a single ancestor for this gene after four generations, and it’s clear that this gene is going to have a very different genealogy from the first. In fact, this time it took a lot longer before all of the offspring in a generation shared a common ancestor for this gene:
It took much longer to arrive at a point when all of the members of a generation shared a common ancestor, but when that finally happened, the most recent common ancestor wasn’t part of the initial generation – it was a member of generation 5.
It is possible for two very different gene genealogies to exist within a single population. Remember, we get one copy of each gene from each of our parents, and they get one copy from each of theirs. That means that if your maternal grandmother contributed the DNA that determines your blood type (for example), then your maternal grandfather didn’t play any role in your blood type.
This diagram is a bit more complex than the last one, but it’s the best I could do (at least at present). It represents a hypothetical gene genealogy for your blood type. You are the yellow triangle. Biological descent is shown with lines, and descent for the one gene – let’s call it “eyebrow thickness” – is shown with the colored arrows:
Now, let’s do the same thing, but look at a different trait – let’s call this one “nose length”:
If you look at the two gene genealogies superimposed on each other – I’m going to save your eyes, and let you do that in your imagination – you will see that:
- Your maternal grandmother is your biological ancestor and your “eyebrow thickness” ancestor, but not your “nose length” ancestor.
- Your maternal grandfather is your biological ancestor and your “nose length” ancestor, but not your “eyebrow thickness” ancestor.
- Your paternal grandmother is your biological ancestor, your “nose length” ancestor, and your “eyebrow thickess” ancestor.
- Your paternal grandfather is your biological ancestor, but not your “nose length” ancestor or your “eyebrow length” ancestor.
- Your maternal grandfather does have biological descendants, but he does not have “eyebrow thickness” descendants.
As you can see, different genes can have different genealogies even within a single biological genealogy. That’s why it’s not only possible but also unsurprising that the mitochondrial gene genealogy that traced back to “mitochondrial eve” and the Y-chromosome genealogy that traced back to “Y-chromosome Adam” don’t lead back to individuals who were married to each other, or even lived within 50,000 years of each other. It’s just part of the glorious complexity that you get to see when you look at the way genes get passed through populations.
The real reason that you hear about “mitochondrial Eve” and “Y-chromosome Adam” instead of “nose length Edith” and “eyebrow thickness Archie” is because of the way that those particular markers – the mitochondria and the Y-chromosome – are passed from generation to generation. Unlike most of our genes, these have very, very simple genealogies. Mitochondria are passed on from mother to daughter, and the mitochondrial lines end whenever you have a woman who has only sons (or no kids at all, of course). Y-chromosomes are passed on from father to son, and the Y-chromosome lines end whenever you have a man who has only daughters (or, again, no kids at all). For this, and other reasons, they are much easier to work with for these sorts of studies. And, of course, you have the added bonus of knowing that the last common Y-chromosome ancestor was a man, so you know you can get away with calling him “Adam” – after all, he’s been dead for so long that we’d never know that he was really named “Steve.”