She's the mother of us all - literally!
I received several questions that can be answered here - who was "Mitochondrial Eve"? If she's the mother of all of us, who's the dad? What is a population bottleneck, and what does it have to do with this genetic mother person anyway?
First, let's explore the science of mitochondrial inheritance.
Recall from class that DNA can be found in two separate organelles in the cell: the nucleus and the mitochondria. Mitochondria are the cell's powerhouses - they produce energy that the cell uses to perform all of its functions. To produce energy, the mitochondrion must go by its own set of DNA instructions. This set of DNA, called mtDNA for short, never mixes with nuclear DNA, which instructs the cell to make proteins and other stuff. Why not? They are essentially two different "animals", two separate life entities. The mitochondria are basically little symbiotic organisms living within our cells - our cells give them a nice, protected place to stay, and the mitochondria provide energy.
Ok, so we have "alien" life forms in our cells, but what does that mean for inheritance? Does it work the same way for mtDNA as it does for nuclear DNA?
No.
You inherited half your nuclear DNA from mom, and half from dad. But your mtDNA all came from mom! How? Remember that eggs are much bigger than sperm. They have to be, because they have to take on all cellular functions after the sperm inject their DNA package. Sperm do have their own mitochondria (it takes energy to swim, after all!), but these mitochondria don't make it into the zygote. So the mitochondria and the DNA in them gets passed though mothers- you have mtDNA from your mother, she got hers from her mother, and so forth.
mtDNA - the Genetic Clock Darlings
Geneticists have a special place in their hearts for mtDNA because it is not under the same selective pressures as nuclear DNA, so it probably accumulates mutations at a constant rate that aren't subject to many selective forces. Plus, while each cell contains only one nucleus, a cell may have many mitochondria, so there are many times more copies of mtDNA per cell than nuclear DNA, which makes it easy to find and extract from a sample, even one thousands of years old! mtDNA has only a fraction of the genes that nuclear DNA has, so it's genome was easily mapped. Put all of this together, and you have the ingredients for a potentially very good molecular "clock" - get a close estimation of the mutation rate, look at mtDNA from people around the world, then work backwards.
Did we all really descend from one woman in Africa 200,000 years ago?
Seems like it. How is this possible? Actually, with a little bit of math and some logical thinking, it's not too hard to see. Remember set theory from middle school math? Imagine that the mothers of everyone on the planet today constitutes a set. Now imagine their mothers - a set that cannot be larger than the original, but must either equal the original or be less than the original set. Since many mothers have more than one kid, set two is smaller than set one. Keep going in this fashion, and we can eventually come up with a set with only one member in it - that mother is so-called Mitochondrial Eve. Her mtDNA was left in her descendants, and those daughters that survived long enough passed it on to their kids, and those daughters to their daughters, and so on. For a more involved discussion, look here.
The Bottleneck - A natural disaster, not a beer container!
This does not imply that the entire human population of the earth 200,000 ya consisted of one pregnant lady! It simply means that the other women around at the time did not leave enough daughters, or their daughters did not leave as many daughters, and (rinse, wash, repeat). What it does imply is that the human population probably experienced a bottleneck - some kind of natural happenstance that reduced the number of members of the population quickly and randomly. Think volcanic disaster or some other more or less random event. A population bottleneck reduces the number of individuals, and reduces genetic variation - but the reason why some alleles survive in the population and not others is simply due to random chance alone - not selective advantage.
Is there a mitochondrial "Adam"?
Not as such. Since mtDNA comes from eggs, we're talking ladies only in this line. But it is possible to look at the inheritance of the Y chromosome with respect to accumulation of mutations in a similar way. Indeed, researchers have already done this (aren't they clever?) and found a Y-Chromosome "Adam", if you will. What is really interesting is that the ancestor of all male humans lived only about 59,000 years ago - much more recently than Mitochondrial Eve!
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