In the first part of the finale, Admiral Adama leads a small human and cylon crew (including all the show's principal characters, natch') on a mission aboard the failing Galactica to rescue Hera Agathon - the only known human-cylon hybrid - from her cylon captors. It is only through a series of highly improbable events that our heroes escape - ending up on our very own Earth.
It's not really "our" Earth though, it's the Earth of 150,000 years ago. The few tens of thousands of survivors from the defeat of the 12 colonies land in the grasslands of East Africa, just about the time that Homo sapiens emerged as a separate species. And there are indeed already inhabitants there. We don't learn much about them, except that they carry long spears, travel in groups, and likely have no language. And in what appears to be a coincidence of supernatural proportions, their DNA is similar enough to that of the humans (or perhaps "humans") from the fleet so that they are genetically compatible. The improbability of populations evolving so similarly on planets that are millions of light years apart is even commented upon by Doc Cottle, so it's no careless mistake by the writers. That the refugee and Earth populations will be able blend together seems to be part of the divine plan that is influencing the characters' fates1.
That plan involves more than just humans, though. Young Hera Agathon's father Helo is human, and her mother Sharon is a humanoid cylon. The humanoid cylon models had tried unsuccessfully to reproduce for many years, and their experiments attempting to breed human-cylon hybrids also ended in failure. It's not clear to them - or us - why that is the case. They are anatomically identical to humans down to the "cellular level", and presumably pass standard fertility and genetic tests (otherwise why would they think reproduction was possible at all). With all of their resurrection ships destroyed the need to find a solution becomes urgent to the remaining humanoid cylons. They see Hera as the key to their own survival.
I always thought their hope in Hera holding the secret to cylon reproduction was probably unfounded. Would she be any more fertile than a mule? It turns out that I was totally wrong.
In the closing scenes to the finale we see the divine (or otherwise supernatural) versions of Baltar and Number Six in present-day New York City. They look at a magazine in a science article that claims that scientists had discovered the fossilized bones of "Mitochondrial Eve" and they note that she had a cylon mother and human father. Sounds like Hera!
But who exactly is Mitochondrial Eve? I always thought the name was a bit unfortunate, because the Biblican Adam and Eve story has so much cultural baggage. Just to be clear she was not the first female human, and she lived roughly 100,000 years before the similarly poorly-named "Y-Chromosome Adam". To understand what geneticists mean by the term, the first thing you have to know is how mitochondria are inherited. Mitochondria are energy generating organelles found inside our cells. Importantly, they carry their own DNA. Unlike the chromosomal DNA which we inherit equally from both parents, our mitochondria are inherited only from our mothers. That means that geneticists can use the sequence of our mitochondrial DNA to follow maternal inheritance2. They followed this back for many many generations, until they arrived at the most recent woman who is the ancestor of all presently living humans. She is "Mitochondrial Eve" and she lived roughly 150,000 - 200,000 years ago.
If you are having trouble imagining how that works, Krishna Kunchithapadam has a pretty good explanation at Talk.Origins:
Consider all the humans alive today on Earth. Put them into a set S.Well, it makes it clearer to me, anyway. She's not our most recent common ancestor or even our only common ancestor. She's not the only woman who was living at the time. She may not be the our common ancestor for chromosome 1 or 7 or 20.
Next, consider the set of all those women who were the mothers of the people in the set S. Call this set S'. A few observations about this new set S'. It consists of only women (while set S consists of both men and women)---this is because we chose to follow only the mother-of relationship in going from set S to set S'. Also note that not every member of set S' needs to be in set S---set S consists of all people living today, while some of the mothers of living people could have died, they would be in set S' but not in set S. Third, the size of set S' is never larger than the size of set S. Why? This is because of the simple fact that each of us has only one mother. It is however overwhelmingly more likely that the size of set S' is much smaller than that of set S---this is because each woman usually has more than one child.
Repeat the process of following the mother-of relationship with set S' to generate a new set S''. This set will consist of only women, and will be no larger (and very likely smaller) than set S'.
Continue this process. There will come a point when the set will consist of smaller and smaller number of women, until we finally come to a single woman who is related to all members in our original set via the transitive-closure of the mother-of relation.
And because the way mitochondrial inheritance works, in the Battlestar Galactica universe our mitochondria - passed down to us from Hera who inherited them from her number eight model mother - are entirely cylon.
That makes the series' suggestion that humanity is caught in an ever repeating cycle of technological development that ultimately leads to our destruction that much more circular because we are are inherently a part of that very technology.
We are them.
Read the original Mitochondrial Eve-related publication: Cann RL et al. "Mitochondrial DNA and human evolution" Nature 325, 31 - 36 (01 January 1987); doi:10.1038/325031a0 (free pdf or text version)
1. I think the scenario presented in the finale brings up a lot of issues of ethics and race that are beyond the scope of my post. One question that I have Racefail to thank: Is it problematic that the technologically advanced and mostly white members of the fleet are shown arriving on Earth and either breeding with or supplanting the native African population so that they became our ancestors? I think so, but I haven't been able to put my thoughts clearly into words. I'm hoping others will post on that subject.
2. In the 1980s, geneticist Mary-Claire King used mitochondrial genetics help identify children who had been taken away from their imprisoned mothers during Argentina's so-called "dirty-war". Since then she's worked with human rights organizations to help identify missing personas all over the world. It's not science fiction, but her work is an important demonstration of using science to support human rights.
Tags:Battlestar Galactica, genetics, evolution, mitochondria