Mitochondrial Eve represents one of the most poorly understood scientific ideas of the 20th century. In its time it has been feted and condemned, frequently by the same parties at different times. The premise of the idea is that we can all be traced back to a single woman living in Africa approximately 200,000 years ago.
What are Mitochondria?
Mitochondria are the powerhouse of the cell. Technically 'organelles', mitochondria are essentially symbiotes1 – they consume the sugars that our bodies have converted from food, and in return produce electricity with which to power the cell. But why is it considered separate from the cell? This is an important distinction. The reason why mitochondria are considered separate is because they have their own DNA – DNA which is unaffected by other genetic exchanges.
Why is Mitochondrial DNA important?
Well, put it this way. You are a genetic mix of your parents, each of which contributed half of your genetic material. They, of course, have gone through the same process and share an equal split of their parent's DNA. This means if you stop this back-tracking process with your grandparents, you are already a genetic mix of six distinct individuals who may have come from different regions of the planet.
But one factor remains constant – the mitochondrial DNA hasn't altered at all – it remains intact through the female line. Male sperm contains only enough mitochondria to power the sperm to the surface of the egg – it does not enter the egg. The egg, however, contains mitochondria that have been passed from mother to daughter for countless generations. The only way for mitochondrial DNA to alter is by natural mutations, which occur very slowly when compared with the almost frantic gene mixing we and our parents take part in.
How Does This Relate to an 'Eve' Concept?
Because the rate of mitochondrial genetic mutation is slow, it can be used as a clock to turn back time to a period before the mutations had crept in. When mitochondrial DNA from certain populations in Africa are sampled, they can be compared with European mitochondrial DNA. The mutation difference between the two populations can then be compared, and a 'clock' can be produced, enabling the rate of mutation in mitochondria to be established. This produces a time-scale which indicates when modern Europeans first left Africa.
The genetic survey that produced the whole Mitochondrial Eve scenario didn't just sample Africans and Europeans – it sampled genes from people all over the planet. When mitochondrial DNA was compared, the survey discovered a startling result. Fundamental similarities in mitochondrial DNA in living humans suggested that we all contain genetic material from a single woman who was living in Africa around 200,000 years ago.
That's Ridiculous – How Could a Single Being Populate a Planet?
And this is where the confusion sets in. A single organism can't populate a planet (arguments about amoeba aside). The evidence didn't suggest a single woman living in isolation from members of her own species. What it suggested was a genetic bottleneck – a period in human history when the population was so small that the genetic expressions of a single woman could have an impact on all humans living on the planet today.
She didn't live alone – she would have lived within a community. She didn't just pump babies out, either. There is no reason to suppose that she had more than one female child. But there is reason to suppose that whatever female children she had, they contained specific advantages for survival over the rest of the population.
But How Would the Population Become So Depleted?
Truth is, nobody knows the causes of the population crash. It could have been due to environmental pressures or a raging plague. The other important thing to consider is why did Eve survive and prosper where so many others died? Isolation is a tempting route to go down. She missed what-ever happened to everyone because she and her people where somehow isolated from the general population. Perhaps her people lived in a geographically remote valley, emerging when the threat had passed. Perhaps her people had access to food when starvation was rife – it is almost certain we will never know the cause for the depletion of the general population. A more fruitful line of inquiry is to question which traits made her progeny so successful.
Why Was She So Successful?
The reasons are all around you. What makes us so successful? An ability to share ideas, to help one another in dire circumstances, a certain creative flair to overcome everyday problems. Or perhaps she introduced the ability to slaughter those who came between us and required resources. We, as her children, display all of these traits. It could have been something as simple as wanderlust – a yearning to see what lay over the horizon. They were perhaps more fertile, were more agile, more resistant to disease, or could throw missiles more accurately than anyone else around at that time. If you want to find out, then next time you're on a bus, or train, or walking down the street, look around you – look at the behaviour of your extended family.
Is Any of This True?
Well, yes and no. To get a completely accurate result the tests would have to be performed on every single person living on the planet today. The dates are in dispute, but the date is perhaps the least importmant point. Broadly speaking, populations do pass through bottlenecks. Eve had many ancestors – it helps if you think about her as an hourglass – she was the pinch in the glass through which our genes ran. There had been many more Eves before her, she is just our most recent common ancestor. There will probably be more population bottlenecks and more Mitochondrial Eves in the future.