http://www.madsci.org/posts/archives/oct2000/971201680.Ev.r.html
Posted By: Andrew Karam, Staff, Radiation Safety / Geological Sciences, University of Rochester
Actually, there are a lot of thoughts about mitochondria lately and what we can learn from them. But, before going into that, I should back up a bit and go over some of the basics.
Mitochondria are commonly called the "powerhouses of the cell". They help to generate energy for the cells. What is less commonly appreciated is that there is a very high probability that they originated as independent cells a few billion years ago and merged with other small cells in a symbiotic relationship. This is evidenced by the fact that they continue to have their own DNA, and this DNA is distinct from that of the parent cell. Chloroplasts are thought to have originated in the same way, as are other organelles and (maybe) even the cell's nucleus.
In most complex animals, the female reproductive cells have mitochondria and the male cells don't. This means that you can figure out mother-child relationships by comparing mitrochondrial DNA. By looking for similarities in the mitochondrial DNA of many people from all the different races, researchers several years ago postulated that the data indicated a common female ancester for all these people. Since that announcement, many scientists have cast doubt on this "Eve hypothesis", and it is now thought to be an oversimplification.
In addition, mitochondrial DNA is sometimes used as a molecular "clock" to help gauge rates of evolution. The theory is that mutations accumulate at a constant rate, so counting the numbers of these genetic differences between, say, a lizard and a hummingbird will tell us how long ago the DNA looked the same and, from that, when these two lineages diverged. This technique is used frequently, but it has some problems, not the least of which is that different DNA sections and different organisms seem to mutate at different rates, making the "clock" a bit unstable.
You can find further information on molecular clocks by doing a search under those terms. A number of researchers are doing work in this area. Papers in the scientific literature by Doolittle, Wray, and Runnegar are among the most informative. Lynn Margulis has written an interesting book on cell symbiosis, called "Microcosmos". Happy reading!