I'm trying to picture what was actually DONE in this study, and it's driving me crazy. I want to be able to TRACK all the thinking that went into arriving at the conclusion.
Firstly, let me be clear that I am not a biologist. So what I say below is probably wrong is some details.
Where are our biologists today? I have seen several answers. But they seem to be answering a different question.
Here is my understanding.
If we take two siblings, and compare their mitochondrial DNA, then there is a good chance it is the same. After all, both got if from the same mother.
If we take two cousins twice removed (or whatever the term is), and compare the mDNA, then we would expect slight differences. Both were inherited from the same female ancestor. But there could have been some mutation in between, perhaps a change in one or two base pairs.
What the scientists are doing, is taking mDNA from two people, and comparing. Assuming a common ancestor, the number of differences is a measure of the number of mutations in the mDNA since the time of the common ancestor.
Now it happens that, at least in some parts of the mDNA, the mutations occur at a relatively stable rate. Thus the number of base pair differences between two individuals is measure of the amount of time since their common ancestor.
Apparently, when you do these comparisons, you never get anything further back than around 200,000 years. It will be often shorter, due to a more recent common ancestor. If the longest is 200,000 years, and if you only get this long by including african mDNA, then that suggests a common ancestor in Africa at around 200,000 years ago.
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