The discovery of mitochondrial DNA has advanced the progession of understanding human evolution. Here is a website with a detailed description of the mysteries mDNA has uncovered.
http://www.talkorigins.org/faqs/homs/mtDNA.html Recent studies show that humans underwent a dramatic change about 40,000 years ago. Scientists theorize that this is due to human advancement in behavior and population expansion (Stoneking, 1994).
Mitochondrial DNA and the molecular clock, Douglas Wallace
Here is the link to a brief yet very informative interview about how mDNA helps determine the evolution of human beings. Mitochondrial DNA can also be used to find the "Mitochondrial Eve."
Image 1: Location of mDNA
Image 2: Comparison of nuclear verses mitochondrial DNA
Stoneking, M. 1994. Mitchondrial DNA and human evolution. Journal of Bioenergetics and Biomembranes, 26,3,251-259.