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Understanding Mitochondrial Eve

(Advance note: Is there a reader who has video editing skills who might want to put together a short and amusing BSG-based parody video that I have conceived of? Contact me if so. Or if you have a complete collection of BSG videos or DVDs and are willing to find some scenes you could also help.)

Because it turns out that Moore decided to turn the entire plot around the idea of Hera being Mitochondrial Eve (MTE), I thought it would be good to add some information on just what this means, and why it was not a great choice as a plot fulcrum. The desire to have Hera be MTE placed the story 150,000 years in the past, and mandated a number of its less consistent elements.

First of all, people should understand that MTE is not “Eve” (mother of us all) nor is she the most recent common ancestor of us all — that person lived far more recently. MTE is an artifact of some simple genetic math. She is the most recent common ancestor calculated only through female lines. I pointed to this article about MTE before and it’s worth a read to understand why she exists and why she is less important than imagined. There is a “Y Adam” too, who is the most recent common ancestor calculated only through male lines, who also lived much more recently.

Everybody is descended from MTE, but everybody is descended from almost everybody who lived at that time. She is actually not particularly special. In fact, we are all descended from almost everybody who lived about 15,000 years ago. It would be more recent than that if human genetic lines had not been isolated for long periods in Australia, the Americas and a few other islands. Be clear: Just about everybody who lived then was ancestor to all of us. The only ones we are not descended from will be those whose lines died out quickly — they left no children or grandchildren. Once you start have several great-grandchildren, your contribution to the world’s genetic pool is all but assured, barring extraordinary events. Go a few more generations and your line is very hard to kill off, especially if it spreads a bit.

So while Moore wanted humans to be descended from Cylons, in fact that could have taken place 15,000 years ago. And if, as shown in the show, populations were distributed to the different continents, it could have been done even 4,000 years ago, or any point in between such as 40,000 years ago for the “great leap forward” when technology and agriculture and language started to really flourish. Australians isolated themselves from the rest of humanity about 50,000 years ago, however, when seas were low they got more recent influxes keeping them related to the rest of the world.

The next thing to understand is mitochondrial DNA (MTDNA) itself. MTDNA is special, in that it is only inherited from your mother. It does not change due to sexual DNA mixing like the rest of our (non-Y) DNA. Instead, it stays mostly the same. However, from time to time, it gets a small mutation that is not sufficient to kill the organism. So while our MTDNA is identical with our siblings, mother and our cousins who share a female descent line, it is very slightly different from other humans. The less related they are to you (ie. the further back your common great^n grandmother is,) the more slight differences there will be. It is precisely because we know the rate of mutation and can look at how different the MTDNA of different humans is that we can calculate when MTE lived, approximately. Estimates get debated, and you will see rates quoted at one mutation per 300-600 generations down to one every 40 generations.

But we don’t just share our MTDNA with our fellow humans. We share it with all the complex life on Earth. We share it with mushrooms and of course with chimps and monkeys and lemurs and mice. We’ve all got the same basic MTDNA, derived from an ancestor long ago that did a symbiotic “deal” with some bacteria to incorporate their energy processing engine into our cells.

Just as you have the same MTDNA, with minor changes, as an Australian aboriginal, you have the same MTDNA as a chimp, but with more of the small mutations. That’s because your common ancestor lived perhaps 50,000 years ago with the Australian, and 6 million years ago with the chimp. There will be 120x as many small differences. They are quite small so that does not turn out to be that much.

And thus we see the problem with how BSG used this process. On the BSG Earth, the chimps and the early humans would have shared fairly common MTDNA. But then Hera’s MTDNA replaced the human MTDNA. On BSG Earth, human MTDNA now bears no common ancestry with the chimps, or all the other animals of the Earth. This is quite different from the real Earth.

Now, we might consider that in fact they are not different because the God of Galactica (Gog) did a little intelligent design on both the evolved Earth life, and Cylons so that they would have the same MTDNA. Or rather that the difference between them would match what the genetic mutation clock says it should match based on the time back to the common ancestor proto-ape. And yes, when you invoke miracles, you can pull off anything, I suppose. But if so, it means the two sets of MTDNA were largely identical. In which case what’s the point? What is the meaning of Hera’s MTDNA supplanting that of the native humans if the two are identical to begin with?

If they are not identical, if there is a real, meaningful difference between Hera’s MTDNA and that of the rest of Earth-life, then this would have been big, big news in BSG-Earth’s National Geographic a decade or two ago. That’s because geneticists would have published stunning papers revealing that human MTDNA was not the same as that of apes or any other Earth life. This would have turned the scientific world upside-down. It would have have been strongly touted as hard evidence by the creationists. It would no longer be the world we live in.

This is also true for the rest of Hera’s Cylon DNA. Unless that DNA is impossible to tell from ours, or unless none of what she had was passed through to us, our genetic sequencing projects would have revealed the fact that we aren’t related to other Earth life, in some subtle way. We would have proteins and genes not found in any other Earth-life. But we don’t. We share all our genes with our animal cousins. For those, like Moore, hoping to draw a connection from the alternate history BSG Earth and ours, you must either say that none of the Cylon DNA made it through, or that it was so identical as to not make a difference.

Of course, there is an answer, discussed on this blog before. If the story had been written to say that the Kobolians came about through the abduction of primitive humans from this Earth some 6,000 or more years before the events of the show, then of course everybody is still related as they should be. The colonials, in this version, are just a lost branch of Earth-kind come home. But alas, this was never suggested in the show.

The miracle of aliens who can breed with Earth-life is indeed impossible without divine action. But it’s more than that. If there was a miracle, it created creatures so alike that in the end they contributed nothing new to the genetic code of humanity on Earth. In other words, a somewhat pointless miracle.

Now you may think that this is a somewhat subtle point, not known to the ordinary viewer who has no grounding in genetics. But this is no minor issue in the show, it is the single element upon which the entire ending, and indeed the entire plot was chosen to turn. Such a fulcrum deserves special attention. And it is also a sign of just how hard it is to try to write an alternate history which you can claim could be our real history. Something is going to catch you up. This is why many viewers never expected Moore to do something so foolhardy as to date the show in the past.