Mitochondrial Eve

In the last scene of the last episode of Battlestar Galactica, Angel Six and Angel Baltar appear behind a bearded man (Ron Moore, in a goodbye cameo) at a New York newsstand, reading an issue of National Geographic magazine over his shoulder. Angel Six, in voiceover, reads, “Mitochondrial Eve is the name scientists have given to the most recent common ancestor for all human beings now living on Earth.” We’re supposed to assume they’re referring to Hera, Helo and Athena’s half-Colonial, half-Cylon daughter.

Lords of Kobol! An actual admitted scientific mistake!

The problem arises from the conflation of two very different terms: “Mitochondrial Eve,” and “most recent common ancestor.” The Most Recent Common Ancestor is, as the name suggests, the most recent common ancestor of all humans alive on this planet. Mitochondrial Eve is the most recent common ancestor of all humans along the matrilineal line. And to explain the difference—as always—we need a little background.

Biological cells come in two main types: prokaryotes and eukaryotes. For our purposes, the most important difference is that eukaryotes have a distinct nucleus and distinct organelles with their own membranes, while prokaryotes generally have neither. Mitochondria are tiny organelles embedded deep in the cytoplasm of almost every cell in your body. They have been called biological batteries or powerhouses because their chief task is to convert glucose into adenosine triphosphate (ATP), the energy unit of cellular metabolism.

Though mitochondria are embedded in your cells, they are self-contained entities, very similar to prokaryotes. For this reason, biologist Lynn Margulis suggested in 1966 that billions of years ago, primitive mitochondria actually were prokaryotes that entered into a symbioticⁱ relationship with other cells. That hypothesis was reinforced in the 1980s when researchers showed that mitochondria have their own set of DNA, different from their parent cell.

Geneticists almost immediately realized that DNA from the mitochondria (mtDNA) could help them to track evolution and heredity along the female line. Since sperm do not contribute mitochondriaⁱⁱ to the developing embryo, an analysis of mtDNA can help to track matrilineal descent through the use of specific DNA markers. And because mtDNA isn’t repaired as efficiently as nuclear DNA, it mutates approximately ten times faster.

Since mtDNA comes only from the mother, you will have that same code sequence in your DNA; if you are female, you’ll pass that code sequence to your children. If you happen to have a mutation to your mtDNA, you’ll pass that mutation, which will be shared with all of your subsequent descendants. By tracking layers of mutations backwards, geneticists can determine which populations are ancestors to which other populations.

Mitochondrial Eve is the term given to the woman who was the matrilineal most recent common ancestor for all humans living on planet Earth today. Passed down from mother to offspring, the mitochondrial DNA of every human is directly descended from hers. Although they lived thousands of years apart, Mitochondrial Eve has a male counterpart in Y-chromosomal Adam, the patrilineal most recent common ancestor. By tracking mtDNA mutations, scientists have determined that Mitochondrial Eve lived approximately 170,000 years agoⁱⁱⁱ, give or take a few tens of thousands of years. She most likely lived in East Africa^iv, when modern Homo sapiens was branching off as a species distinct from other humans.

It’s important to emphasize that Mitochondrial Eve and her contemporaries had offspring, and those offspring had other offspring. But throughout the subsequent generations, for one reason or another, the lineages of Eve’s contemporaries all died out. Of all the women alive then (and in our case, that means the entire female population of Galactica and the fleet), only one has offspring alive today. We know her as Hera Agathon^v.

This does not necessarily mean that Hera is our Most Recent Common Ancestor (MRCA). Hera populated today’s Earth solely through her daughters and daughters’ daughters. The MRCA is the person who, while no doubt descended from Hera, populated today’s Earth via their daughters and/or sons. By adding males to the mix, the MRCA almost certainly cannot be the same as Mitochondrial Eve. In fact, most researchers today feel that the MRCA lived only about five thousand years ago, 145,000 years after Hera.

Does such a recent MRCA imply that the human race was once nearly wiped out, where it had to bring itself back with a small number of survivors after almost going extinct? Not necessarily. If cousins mate with each other, as has been known to happen in tightly knit tribal societies, then the number of ancestors each person could have would be constrained. In some societies, even more recent MRCAs are possible.

There was a real population bottleneck in our history. It took place seventy-five thousand years ago and was called the Toba catastrophe.

Somewhere in our deep past, a giant volcanic eruption—most probably of Mt. Toba on the island of Sumatra—created the volcanic version of a nuclear winter. The supercolossal explosion was the equivalent of one trillion tons of TNT, and sent volcanic debris more than twenty-five miles into the stratosphere. The resulting ash cloud covered much of the world, causing temperatures to drop as much as 5 degrees and possibly triggering an ice age. The number of humans, already relatively small, dwindled to approximately fifteen thousand, spread throughout Africa and southwest Asia. Yet those fifteen thousand managed to regroup and repopulate Africa within a few tens of thousands of years, and to move out into the rest of the world thirty thousand years later.

Population biologists talk of something called a minimum viable population, which is the smallest number of individuals that can survive “in the wild.” For terrestrial vertebrates, that number is around four thousand. Of course, more individuals are always better for the species, as long as the food holds out, because they bring more genetic diversity into the population. As long as there are at least four thousand souls in a single population group, then Hera’s children should have survived.

i. Sym = together, bio = life; a symbiotic relationship is one in which at least two different organisms live together, usually (but not always) providing a benefit to both creatures. For example, we provide cats with shelter and a steady food supply; cats provide us with rodent control and aloof companionship.

ii. Specifically, the sperm’s mitochondria are marked for elimination by the egg’s cytoplasmic destruction machinery. Talk about being a ballbuster.

iii. Incidentally, the use of the term “Eve” is not meant to indicate that she was the only human female of the time, simply that she was mother to us all.

iv. The article that Angel Six reads over Ron Moore’s shoulder purports to say Tanzania.

v. Or do we? Angel Baltar gleefully said, “Along with her Cylon mother . . . and human father!” It’s totally reasonable to think that once the fleet settled on Earth, other Cylon-Colonial pairs had offspring in the same generation as Hera, perhaps even Six and Baltar. Imagine if their child was Mitochondrial Eve!

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Authors: Patrick Di Justo

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