I read an article about the study that was going on to age the "Mitochondrial Eve" by using Mitochondrial DNA. The article said that there are 37 genes that rarely change in the mitochondria, and then there is also a region that varies frequently and quickly. By studying the changes in that they are able to discern some basic evolutionary time periods. The next part of the study is harder for me to understand, they take blood samples from different donors and determine relatedness based on their blood samples, which somehow allows 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 their processes they aged the mitochondrial DNA they estimate that DNA to have originated 200,000 years ago.
You can view the article here Article