All about (genetic) Eve: the quest to find the female founder of our human species
Image Courtesy of Mike Kayser (Wellcome Collection)
"Click here to listen to the full podcast episode"
To start with, let’s find out all about Mitochondrial Eve. And to do that, we need to do a little bit of digging into mitochondria and their DNA.
Mitochondria were first spotted in 1857 by Swiss physiologist and anatomist Albert von Kolliker, who noticed strange granules in muscle cells, although he didn’t give them a name. German pathologist Richard Altman became obsessed with these little things, calling them “bioblasts” (or life germs), before they were renamed “mitochondria” by his fellow countryman and microscope enthusiast Carl Benda.
Mitochondria are effectively the power stations of cells - tiny organelles that generate energy by breaking down the food we eat and turning it into adenosine triphosphate (ATP), a kind of portable chemical ‘energy currency’ that cells use to power all their biochemical reactions. So they’re pretty important, and also - although I say so myself - pretty cool.
As we covered back in episode 23 of our very first series, Mergers and Acquisitions, we now know that the mitochondria in living cells are the result of endosymbiosis - an energy-generating bacterium getting swallowed up by another cell, somewhere around 1.45 billion years ago. As a result of this unusual origin, mitochondria actually retain a small amount of their own DNA - a mere 16,500 or so DNA ‘letters’ or base pairs, compared with the 3 billion base pairs that make up the main human genome, known as the nuclear genome (because it’s found in the cell nucleus). But though it’s small, the mitochondrial genome is essential for life, and gets copied out every time the mitochondria - and the cells that they live in - multiply.
Now here’s where it gets really cool. In the 1970s, scientists discovered that the way we inherit nuclear and mitochondrial DNA is slightly different. During fertilisation, when mummy and daddy love each other very much, nuclear DNA from sperm combines with nuclear DNA in the egg - half a genome from each - resulting in a mixture of both parents’ genes. By contrast, although sperm are packed with mitochondria that provide them with the energy to swim where they’re needed, they are abandoned outside the egg and only the nuclear DNA goes in.
So, this means that only the mitochondria that were packed in the resulting fertilised egg go on to be part of the cells of the developing embryo. This effectively means that every one of us only inherits our mother’s mitochondria - and mitochondrial DNA. And if you’re someone who becomes pregnant, then your baby will also inherit them too. And on and on down the generations. Because of this, mitochondrial DNA (mtDNA) can be used for tracing the genetic maternal line way back in time, to find the mother of all mothers. At least, in theory.
In 1987, scientists looking at the patterns of alterations in mitochondrial DNA from 147 people around the world estimated that they all originally evolved from one common female ancestor who lived roughly 150,000 -200,000 years ago in Africa. To put it simply, all people who are alive today are genetically connected to this one woman. They rather misleadingly named her mitochondrial Eve, after the biblical Eve.
This nickname has caused no small amount of confusion, as it implies that all humans are descended solely from this single woman. Rather, there were many men and women who contributed their genes to our species. Mitochondrial Eve is just the last most recent female ancestor to have survived. Because the human population would have been very small at various points such an early stage in our evolutionary history, and many people died along the way without having children, at some point only the offspring from this one individual female survived and went on to have female children of her own, who then went on to have female children of their own, and so on and so on.
The finding that we can trace our origins all the way back to one female founder also doesn’t mean that every human alive today still has exactly the same mitochondrial DNA. Just as the DNA in our nuclear genomes has changed over time as we pick up alterations and mutations as our species has expanded, the DNA in our mitochondria has done the same since the time that Mitochondrial Eve walked the Earth. But because mitochondrial DNA doesn’t get mixed up along the way when eggs and sperm are made, unlike the nuclear DNA, it changes at a much slower rate.
Human geneticists refer to what’s known as haplogroups, each with their own name, to represent branch points on this mitochondrial family tree where natural genetic mutations have occurred to alter the genetic code in these little powerhouses. Essentially, mitochondrial haplogroups are groups of people that share similar mitochondrial DNA, although they all eventually trace back to the same common ancestor on the maternal line.
Bryan Sykes, author of the semi-fictional book “The Seven Daughters of Eve”, popularised the idea that Europeans alive today can be split into seven mitochondrial haplogroups, which he calls “clan mothers”, which all descended from the original Mitochondrial Eve.
But it’s not quite as simple as Sykes first suggested. As research progresses, geneticists and Sykes himself have now added more haplogroups for European mitochondrial lineages, as well as many more for global lineages, in populations both large and small. A recent version of the global mtDNA tree comprises over 5400 haplogroups and sub haplogroups.
While terms like Mitochondrial Eve and ‘clan mothers’ are appealing and catchy, it’s important to remember that these individuals are nothing special - they’re just lucky. There is no God and Garden of Eden stuff, just the random influence of birth, breeding and death in small founder populations, like the one that gave rise to modern humans. And the concept of a mitochondrial Eve isn’t unique to humans either - for example, researchers have tracked down a sperm whale ‘Eve’ who lived some time around 10 to 80,000 years ago - millions of years after her species evolved. So, while all sperm whales currently living in the world are descended from her, she wasn’t just a single whale swimming around in the sea on her own - she was one of many, but one who won the genetic lottery, and ended up with an unbroken line of female descendants.
As a wonderfully clear article in Smithsonian Magazine by Joshua Rapp Learn puts it,
“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.”
It’s also important to remember the fact highlighted by Clara Takahashi in her Obscure Dinosaur Facts blog about Mitochondrial Eve - in a sexually reproducing population every daughter has a mother, but not every mother has a daughter. The human Eve was the one that got lucky, out of many women living back in the early days of our species. And if you could go back even further there would be a Mitochondrial Eve of all hominids, and back and back and back and back until that very first symbiotic event. Mind. Blown.
