10/10/2019

Mitochondrial Eve and Y-chromosomal Adam

Filed under: — Bravus @ 10:12 pm

Recent discoveries in genetics have led to the publication of some interesting work that has suggested that all currently living human beings can be traced back to a single human female on the order of 120,000 to 150,000 years ago. In the 1980s this individual was dubbed ‘Mitochondrial Eve’. Similarly, all modern humans can have their ancestry traced back to a common male ancestor, dubbed ‘Y-chromosomal Adam’, who lived a similar time span, or perhaps 20-30,000 years longer, ago.

While this is not 6,000 years, or even the fewer-than-20,000 often accepted by creationists, misconceptions about these concepts led to considerable excitement in creationist circles. Many assumed that these two individuals were married to each other, and were the single married couple of humans from whom all modern human beings descended.

It mightn’t be 6,000, but it wasn’t millions, and with their related misconceptions about dating, the dates could be set aside. The key was the ability to link it to the Genesis story of a First Couple, and to claim that humans have not evolved, but have always and only descended from humans.

These are misconceptions, though, and the purpose of this post is to very briefly explain why. Obviously it’s a short and simple explanation: there are more detailed ones out there, and the Wikipedia explanations of both are good and detailed.

First, in both cases, saying that all of us can trace our lineage back to an individual does not mean that that individual was the only person alive at that time!

We all have a vast number of ancestors. There’s a conundrum here. I have two parents, 4 grandparents, 8 great-grandparents, 16 great-great-grandparents and so on. It’s a sequence going up in powers of 2: I’m 20, my parents 21, grandparents 22, great-grandparents 23 and so on.

If we assume a human generation is about 20 years, that’s 5 generations a century, 50 a millennium. Perhaps 50 x 150 = 7,500 in 150,000 years. But the thing is, 27500 is 5.3 x 102257. There are only about 7.5 x 109 people on Earth right now, and that’s the most there’ve ever been: certainly not that other outlandish number.

The solution is that some of our ancestors were the same people. Quite a lot of them, as it happens. That is, given slightly older and younger child-bearing and other things, my 6-greats-grandfather on my father’s side might also be my 7-greats-grandfather on my mother’s side… or even take on more roles across generations.

And, of course, we don’t simply multiply the number of my ancestors by the number of people in the world to found out how many ancestors there were in the world, because some of my ancestors are also the ancestors of other people. Most trivially, my parents are also the parents of my siblings. My dad in particular is from large families going back generations, so some of my great-grandparents are ancestors of an enormous number of people, not just of me.

Mitochondrial Eve and Y-chromosomal Adam are simply the two individuals – probably living in Africa, but almost certainly not close together, in either space or time – who in each case are the most recent ancestor shared by all modern living humans when traced back using particular genetic techniques. And they are probably not unique.There may be multiple people fulfilling that criterion, although conceptual only one individual can be the most recent.

It’s also important to note that, in both cases, these are theoretical concepts, not actual individual people with names and addresses who have been identified.

So, to take ‘Eve’ first, mitochondria are the little ‘energy factories’ in our cells. They have different DNA in them than the DNA in the remainder of our cells. (They may well have arisen as bacteria that were symbiotic with other cells and then were incorporated, but that’s a whole separate fascinating story.)

Mitochondrial DNA is matrilineal (passed down via our mothers), and we can look at shared characteristics and their changes over time to calculate a ‘clock’ back in time until we have a common female ancestor. This is what is meant by the time back to the ‘Mitochondrial Eve’ theoretical concept.

Being able to make this calculation is an important and very interesting development in our understanding of genetics, but it does not mean that there was only a single human woman, 120,000 to 150,000 years ago, who was mother to us all.

The story for ‘Adam’ is very similar, but Y chromosomes tend to be passed down patrilinearly, from fathers to sons. The tracking back is essentially by a similar process of genetic reconstruction.

Exciting, very interesting science… but, when properly understood, no particular comfort to those who want all human beings to have descended from a single divinely created couple in Eden.

For ease of navigation I will include links to each of the other posts in this series at the bottom of each post.

Why I think it’s important to understand evolution
Cosmogenesis, abiogenesis and evolution
Evolution and entropy
Facts, Theories and Laws
Radiocarbon dating
Radiometric dating and deep time
Four Forces of the Universe
Probability and evolution
Species and ‘baramin’, macro- and micro-evolution
Transitional fossils
Complexity – irreducible and otherwise

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