No, said Aziraphale, it's ineffable.

Anonymous asked:

Seen that post about how humans are descended from two people "says scientists"? Even though they have no sourced studies and the studies don't even say that.

I haven’t but the issues is that… we cannot prove that due to how the genetics there work.

Long term descendancy tracking like that does usually rely on mitrochondrial DNA (mtDNA) or the Y-Chromosome. mtDNA is somewhat easier to get because every cell contains a lot of mitochondria and thus its DNA, and the genome there is pretty short, meaning that even in very old material, it survives better.

That means that generally speaking, if you wanna test really old bones, the maternal line is gonna be easier to track.

Now you can g about it like this:

first you do comprehensive mtDNA analysis of extant people, and with that i MEAN comprehensive. If you want to understand the migration pattern of humanity as a whole, and therefore who descended from whom, when, you need a broad broad global sample collection. In some of these populations, you can will already be able to see if they’re ‘younger’ or ‘older’ due to how many mtDNA mutations are unique to that population and how many can be mapped to other populations.

Say you have group A. They have ten mutations, and all but one of them are shared with the surrounding groups. That means that A hasn’t had a lot of genetic drift happening since they split from the larger group.

Group B also has ten mutations but of those ten, 3 can be found in all humans, EVER, and the rest if unique to them. That means two things. One, the group has been split off a long time ago to acquire that many unique mutations, and two, since the three shared mutations are global, they must be related to the last common female ancestor of all other extant humans.

Now you might note that I said ‘female ancestor’ and that’s because mtDNA inherits only along the female line. That does of course lead you to a problem. If a woman only has sons, her mtDNA line effectively ends with her despite the fact that she clearly kept contributing DNA (in the form of her sons) to the gene pool. But on the mtDNA record, her line is gone.

so if you take all the people alive now and start tracking their mtDNA down and map it out and follow it back, you’re inevitably going to arrive at a single woman who is the last female ancestor that all humans alive today share. That doesn’t mean that she’s the first human, not even close. It just means that she’s the last person WE CAN TRACK this way. There were presummably many many many others at the same time. But somewhere down the line they had sons and their contribution vanished.

another point to consider is that mtDNA acquires mutations in a pretty stable, but slow rate. That means that you probably share mtDNA that is identical, or nearly so, with all your close female relatives that aren’t married in. Your sister, mother, aunts, cousins (if descended from your mother’s siblings), and grandmother probably all share the same mtDNA.

Meaning that even if you track the mtDNA down to a single point, chances are you haven’t actually reached an individual. No, you’ve reached a family cluster of closely related females and any of them could be the ‘source’ of the common mtDNA. But cannot know which one.

The same game can be played with tracking the paternal line through Y-chromosomes. The moment a man only has daughters, his line vanishes from the record. He still contributed DNA, potentially even A LOT of DNA, but we can no longer tell.

and that leads to a funny little effect. If you track down the last common male ancestor of all male humans, and the last common female ancestor of all female humans…. they don’t have to live at the same time. Because due to how the lines are tracked, it’s entirely possible for one of them to be traceable for longer than the other. That doesn’t mean that one of them didn’t exist before that point, it just means that we don’t have extant ‘relatives’ anymore because at some point the even older family only had daughters, or only had sons, and that blocks our tracking methods, if those methods even allow us to pin down a single individual to begin with.

And like I said, at least for mtDNA, we can’t.

See, that’s what the app is perfect for.

TSK: I was really looking forward to working on that jellybean tbh