MRCA – Life is a Random Draw

I confess to a digression from the usual here and that I am indeed avoiding writing about that policy question I asked. But give me a break, will you! I promise that I will get to it tomorrow.

Here’s what’s on my mind — MRCA or “Most Recent Common Ancestor.” The question is how recent is the most recent common ancestor to all contemporary humans today? Beyond a certain time in the past, all of us have all our ancestors in common — going back all the way to three or four billion years. The question is restricted to our most recent common ancestor.

To grasp the basic concept, here’s how we proceed. The most recent common ancestor (MRCA) to me and my siblings are my parents. The MCRA to me, my siblings and my cousins (the children of my parents’ siblings) are my grandparents. If my siblings and my cousins are a strict superset of the set of my siblings, it means that my grandparents are the MRCA to all of us. Proceeding in this way, we can keep enlarging the set of descendants to include all of the present set of humans and identify a person who is the most recent common ancestor to all living humans today.

There must be a MRCA to all people in the world today. That fact is logically necessary and there is no need for any empirical verification. It’s what is called an apodictic statement — true because it’s a logical necessity. Given that it must be that some (unidentifiable) individual (at some unknown time) in the past is the most recent common ansestor of the set of all humans today is incontrovertible, the question here is “how long ago did the MRCA live?”

Off the top of my head I would have guessed tens of thousand of years. But the interesting thing is that the MRCA probably lived only a couple of thousand years ago. What’s amazing is that with 99 percent certainty, the MRCA of all people living today lived as recently as three or four thousand years ago. That’s what researchers have figured out.

Every one of us can trace our ancestory to include that one particular person who is the MRCA. We don’t know (and can’t know) who that was but it is logically necessary that such a person must have existed — and that too in the not too distant past, as revealed by empirical evidence.

And here’s another fact that logically follows from the nature of the biological world. Every person belongs exclusively to one of two groups, A and B. Every individual in group A will at some time in the future (and that time differs for every member of group A) have no living descendants at all, and every individual in group B will at some point in the future (and that time differs for every member of group B) be an ancestor to the entire population of the world. Groups A and B are mutually exclusive and exhaustive. You, dear reader, will one day have either zero descendants or will have the entire population of the world as your descendants — there’s no intermediate fate for you.

Just to focus on an example, since I don’t have any children, I clearly belong to group A defined above: there is a time in the future (now!) that I have zero descendants. It’s unclear at the present whether my siblings belong to group A or group B. They each have two children and so they are not currently in group A but could eventually end up in it.

Another point we have to note is that the MCRA is a moving target. The person who is the MCRA today will not be an MCRA in the future. For instance, the MCRA of a hundred years hence will be a descendant of the current MCRA.

Alright. Enough of the MCRA business. There is much more to this but for now let’s give it a rest. I wanted to explore this topic a bit as a small diversion from the usual bits that feature on this blog. If you found this interesting, let me know in the comments. Now back to our regularly scheduled programming.