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Emphasis on Y-DNA and mtDNA ancestors

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  • Emphasis on Y-DNA and mtDNA ancestors

    Much is being made of some recent research and literature about the possibility that all living humans might share an ancestor much more recent than our famous Y-chromosome progenitor (whom I call Noah). Here is an example article, written by Steve Olson:

    Basically, his claim is that over the last several thousand years (8000 years, tops), sufficient intermarriage has occurred throughout the entire world such that someone, perhaps 8000 years ago, is a common ancestor of all humans living today--not patrilineally or matrilineally, but nevertheless an ancestor, by the usual definition.

    His assumption of worldwide intermarriage is weak, because he totally ignores the "foreigner effect"--the common tendency of an isolated group to kill, or at least isolate, an outside invader instead of giving him wives and letting his children spread. A clear historical example is Japan, which successfully fought off both invasion and foreign intermarriage for thousands of years. An anthropological example may be Australia, which may have fought off invaders for tens of thousands of years, or at least isolated them to coastal areas (such that "pure" Australian aborigines may still exist in the interior of that expansive island).

    But even if one brushes aside that issue, there is a more fundamental genetic fact that Olson ignores: Humans only have 46 chromosomes (plus the mitochondrial DNA), and less than 25,000 total functional genes:

    So let us consider the implications of this for ancestry, first without gene recombination, then with it.

    Without considering gene recombination, I only have 47 inheritable units of DNA: 46 chromosomes and the mtDNA. Each generation of my ancestry, then, can only have 47 true ancestors; any others are meaningless genetically because they did not actually contribute to my DNA. Since Olson's calculations and conclusions are heavily dependent on an almost infinite multiplication of ancestors up the tree, his entire theory collapses if each living human only has 47 true ancestors per generation regardless of how far back in time one goes. Instead, it becomes obvious that to find the human race's true common ancestor, one must go back to a time when the total number of humans was roughly 47! This is, obviously, about the same time as our Y-chromosome Noah.

    Now let's take into account gene recombination. This is the occasional phenomenon of gene exchange between chromosomes--either individual genes, or large segments. In ancestry terms, this means that as I go farther and farther back in my family tree, the number of true ancestors does increase, slowly. Some ancestors contributed all or a major portion of an entire chromosome to my genetic inheritance; others contributed as little as a single gene. If we continue this reckoning back thousands of years, we might well reach some ancient point at which I have almost as many ancestors as genes--i.e., 20000 or so. Now this number is still far smaller than the exponential ancestry multiplication that Olson was assuming, but let's even say, generously, that Olson's conclusion does become true, in a certain sense: That, say 10000 years ago, a human lived who has contributed a gene to every human living today.

    Well, big deal! One gene out of 25000 is pretty meaningless. In contrast, we inherit an entire chromosome from Noah, and our entire mtDNA from Eve. Doesn't it make sense to place emphasis on the patrilineal and matrilineal lines, which avoid the recombination that reduces ancestry to as little as a single gene?

    There is one potential exception, described in this article:

    If the common ancestor contributed a mutation that offered such survival value as to spread throughout the entire human population, so that today we all have that mutation and indeed depend on it, then we could indeed honor that common ancestor as a major contributor, on a level with Noah and Eve. But I don't know of any such essential mutation that has cropped up in the last 20000 years and spread throughout the entire human race.

    This is essentially a defense of our use of Y-DNA and mtDNA to classify ancestry. Does anyone have any other thoughts?