The Mother of all Humans... (?)

I read an article about a study which was attempting to age the "Mitochondrial Eve" by using Mitochondrial DNA.  The article said that mitochondria, the tiny organelles that serve as energy factories inside all human cells, have their own genome.  Besides containing 37 genes that rarely change, they contain a "hypervariable" region, which changes fast enough to provide a molecular clock calibrated to times comparable to the age of modern humanity.  Since each person's mitochondrial genome is inherited from his or her mother, all mitochondrial lineages are maternal.  By studying the changes in mitochondria they are able to discern some basic evolutionary time periods. The next part of the study is harder for me to understand, they took blood samples from different donors and determined relatedness based on their blood samples, which somehow allowed the scientists to age the mitochondrial DNA.  It was interesting to read about mitochondrial DNA and that it is only passed down through the mother, and using such processes scientists have traced and aged the mitochondrial DNA, estimating that DNA originated 200,000 years ago.

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