No, a Mitochondrial ‘Eve’ Is Not the First Female in a Species
The latest story about a sperm whale “Eve” shows how people misunderstand the evolutionary term. Fear not: We can clarify
Recently, researchers made waves with the announcement that they had found the mother of all sperm whales: that is, the female from whom all modern sperm whales are descended. "Sperm Whales Have an ‘Eve,’” ran the news headlines, capitalizing on the biblical idea of the “first woman.” The idea of a mitochondrial "Eve” is not new; researchers often use it to refer to the first female genetic ancestor of a species. The problem is, most people misunderstand exactly what the term means, thinking it means the very first female in a species.
In fact, the sperm whale “Eve” arose far later than the first sperm whale. By examining 1,600 sperm whales across the world, researchers surmised that this magnificent matriarch arose sometime in the range of 10,000-80,000 years ago. Meanwhile, the fossil record of sperm whales places them back millions of years.
“We’re not saying that there was just one female swimming around the ocean at that time and suddenly all these sperm whales arose from her,” Alana Alexander, a postdoctoral researcher at the Biodiversity Institute at the University of Kansas and lead author of the study, tells Smithsonian.com. Instead, this so-called Eve "was one of many females, but she happened to be the only one who passed down the mitochondrial DNA in an unbroken female to female way.”
For those who are new to the debate: a “mitochondrial Eve” refers to mitochondrial DNA, the unique genetic code that is passed down from female to female. Mitochondrial DNA, which is responsible for energy metabolism, is cordoned off from the rest of our DNA, sitting off on its own in its own container. Researchers believe that at some point early in our evolution, the single cell organism that is our oldest ancestor engulfed another bacteria, and eventually the two slowly developed a symbiotic relationship that continues to this day. That bacteria became the first mitochondria.
If you want to go all the way back to the actual biological Eve, you would have to seek out the ancestor of our mitochondria, says Roger Bull, a senior research assistant in the molecular biodiversity lab at the Canadian Museum of Nature. “If ever there was an organism that should be deemed the mitochondrial Eve, it is that unassuming little single cell that engulfed a littler single cell to make quite the unbeatable biological team: the ancestor of dinosaurs, trees, butterflies, (even) Donald Trump,” Bull says in an email.
In nearly all multicellular organisms, mitochondrial DNA is passed down the matrilineal line from generation to generation. This fact is extremely handy for researchers, who can use these DNA biomarkers to trace back the matrilineal history of a species. It’s also cool for you, if you’re a lady: It means that the mitochondria in every cell in your body can be traced back your great-great-great-you-get-the-picture grandmother. Alas, while mothers also pass their mitochondrial DNA to males, those sons cannot pass this DNA to their own children.
In the 1980s, when the concept of a mitochondrial Eve began proliferating in popular media, it instantly caused a degree of hair-pulling among researchers. “The use of the term was a misinterpretation, given that the research was about the most recent common mitochondrial ancestor of all living humans … not about the first human woman ever,” says Bull.
In reality, a mitochondrial Eve is not the first female of a species, but merely the most recent female historically from which all living animals of a species can trace their ancestry. Think of her like the peak of a genealogical pyramid, in which all ancestors of a species meet. While everyone below is descended from her, that doesn’t mean that there is no other female above her, or that lived at the same time as her. Perhaps some of her contemporaries had no surviving children. Or they only had sons, which wouldn’t have passed on their mitochondrial DNA.
“In any generation there will be some individuals that will leave no progeny,” Marek Kimmel, a professor of statistical genetics and molecular evolution at Rice University, tells Smithsonian.com. “Their genes will be eliminated.” He added that the number of individuals passing their genes to further generations is shrinking all the time, meaning that mitochondrial Eve isn't a fixed individual over time, but could become more recent as lineages die out.
“If you reach deep enough into the past, you always find a common ancestor of everybody,” says Kimmel, who published a study in 2010 that places the mitochondrial Eve of humans back to around 100,000 to 250,000 years ago (a 2013 study estimated the age as a little more recent).
So how did this particularly successful sperm whale mother take over, genetically? Usually, female sperm whales are more sedentary, tending to stick to smaller parts of the ocean than males, says Alexander. This could mean that this Eve was kind of a traveling vagabond who went from ocean to ocean leaving her progeny—essentially the Genghis Khan of sperm whale mothers.
Or, her genetic takeover could have been just the result of a slow expansion of her progeny over many generations. “We know she must have been successful at leaving female offspring,” Alexander says. “My suspicion is that if Eve was similar to her descendants in ecology, then she wasn't roaming all of the oceans, but because both she and her daughters were successful, they eventually expanded out of which ever ocean they were originally based in.”
Sperm whales and humans aren’t the only organisms for which researchers have found a mitochondrial Eve. Inger Winkelmann, an evolutionary genetics researcher at the Natural History Museum of Denmark, and her coauthors have also estimated a rough date for the age of the sperm whale’s notorious nemesis: the giant squid. It’s harder to place an age for squid as their boneless soft tissue doesn’t fossilize well, Winkelmann says, but she and her coauthors used the mutation rate of snails—a relatively close species—to put the date of the mitochondrial squid Eve at around 35,000 years ago.
Researchers often hone in on the mitochondrial DNA because there’s a lot of it in cells, it survives well in old tissue samples and it sticks out like a sore thumb. But, theoretically, any part of DNA could be traced back. For example, in mammals, the Y-chromosome is only found in males, and parts of it could be traced back to a single ancestor as well. That means that, in the kind of evolutionary language we are talking about, there’s a male equivalent: the “Y-chromosomal Adam.”
What would be fascinating, Alexander says, would be if you had a situation where you could trace this Y-chromosome Adam to a similar time as the mitochondrial Eve, as this would suggest some quick or sudden population expansion—the kind that comes when a species moves into new territory. In the case of squid and sperm whales, it might also point to the reason for a population expansion among the progeny of the mitochondrial Eve. There could be some kind of parallel bottleneck, such as a big environmental change, that led to a similar pattern of low mitochondrial diversity.
But Alexander is cautious of pointing out a possible link. “It’s a very sexy story, but I also feel honor-bound to point out that the giant squid are only one of the many species that sperm whales eat," she says. That's too bad; it would have been a whale of a tale.