This Has All Happened Before, and It Will All Happen Again: Mitochondrial Eve Comes of Age

(WARNING: THE FOLLOWING ARTICLE CONTAINS MAJOR BATTLESTAR GALACTICA SPOILERS. DO NOT PROCEED IF YOU ARE STILL GETTING THROUGH THE DVDs.)

In late spring, 2009, while watching the series finale of Battlestar Galactica, I learned something new: I was part cylon.  I sat on my couch, box of Kleenex in hand, as the humans and their newfound cylon allies settled on a planet that looked very much like Earth, and where small groups of primitive human-like people hunted the Great Plains. I saw a young girl named Hera—the first half-human/half-cylon child ever to be born—walking into the distant blades of grass.

In the episode’s final moments, we travel forward in time to present day. A newsstand in present-day New York City displays an article about a scientific discovery: the fossil remains of our species’ oldest maternal ancestor.  As I sat spellbound, the narrators implied that these remains were Hera herself, and that she and the rest of the BSG crew didn’t just arrive on an Earth-like planet; they actually arrived on Earth itself. The primitive human-like people I had seen were an earlier version of us—Homo sapiens. The narrators continued by hinting that, as Hera and the others settled in to their new homes on Earth, they ‘mingled with the natives’, so to speak.  And, 200,000 years later, we all carry a bit of Hera’s DNA —Hera’s half-cylon DNA—in us.

Aside from the fact that the finale of Battlestar Galactica was totally and completely awesome, that it was based on actual scientific evidence was, as a scientist, immensely satisfying. The writers, in developing their story, had clearly come across a 1987 scientific paper published in Nature entitled “Mitochondrial DNA and human evolution.” In it, scientists had used a novel technique to trace the origins of humanity not by digging in the ground, but by digging through our DNA. In so doing, they changed the study of human evolution forever.

In the mid 1980s, a small team of scientists at Berkeley, led by graduate student Rebecca Cann, wanted to use DNA to better understand the evolution of our species. However, using DNA in this manner can be tricky business. Nuclear DNA—the 23 pairs of DNA in the nucleus of each of our cells, the material which helps to determine the color of your eyes and the size of your feet—is not passed down from generation to generation in an orderly fashion. With each generation, the DNA from each parent is mixed, removing almost any clear and signal trace of history.

There are a few exceptions, however, and one of them is the mitochondrial DNA, or mtDNA. Unlike nuclear DNA, mtDNA lives outside the nucleus, and is passed down intact, from a mother to her children—a little bit like the female equivalent of a surname. Cann and her colleagues shifted their focus to this type of DNA.

First, they sampled tissues from 145 people hailing from major populations all over the world. They then compared genetic differences, called polymorphisms, between each person’s mtDNA. Finally, they used these polymorphisms to construct a complex ‘family tree’ linking these 145 pieces of mtDNA to a common ancestor. But unlike your own family tree, which likely traces your ancestors back just a few generations, this one went back thousands.

Think of the maternal line on your family tree. If you start at you, and then follow the lines up to your mother, your mother’s mother, and so on, eventually (after about 8,000 generations), you’ll find your oldest maternal ancestor. And not only would she be your oldest maternal ancestor, but she’d be the oldest maternal ancestor of everyone alive today. Cann calculated that this ancestral woman likely lived about 200,000 years ago, and that she was probably African.

The conclusions from this paper were important for two reasons. First, it was the first time mtDNA had been used in such a way to trace the origins of Homo sapiens. And second, it lent support to the idea that we evolved in Africa, an hypothesis that was quickly gaining ground among anthropologists.

Unsurprisingly, this paper created a lot of headlines. Much to the dismay of the authors, the media dubbed this ancestral woman “Mitochondrial Eve,” and her likeness even made it to the cover of Newsweek. Over the next few decades, Mitochondrial Eve grew in popularity until she became something of a cultural icon—which is likely how she came to make an appearance in Battlestar Galactica.

The scientific community, while excited at the prospects of this paper, was less receptive at first to the paper’s conclusions. Some, like population biologist Alan Templeton, decried the paper as inferring far too much from such a tiny segment of our DNA. Others, including anthropologist Milford Wolpoff, continued to argue that the present-day make up of humans was not the result of a recent migration out of Africa, but of millions of years of gene flow across continents.

Yet subsequent research over the next several years showed that, even if the paper’s methods were rudimentary, its basic conclusions were well supported by the evidence. Hundreds of papers have now revealed that all humans alive today can, in fact, trace their maternal ancestry (their mother’s mother’s mother, and so on) via mtDNA back to a single woman.

As DNA analysis techniques grew more complex, so did our questions about our own origins. Research shifted from studying just one segment of our DNA to studying many simultaneously, and the use of computer simulations allowed us to test hypotheses with even more precision. Today, millions of people around the world have tested their own DNA, tracing their ancestry back to their ancestral homeland. And, with the sequencing of the human genome, it seems that there are no secrets hidden in our DNA that we cannot discover.

Of course, one mystery remains. Scientists have yet to find that elusive cylon DNA that apparently exists in us all. But I’m sure it’s only a matter of time until they do.