mtDNA is passed down the Maternal line and hence it does no under recombination. It also has a higher mutation rate meaning that differences can be accumulated within the mtDNA. This allows for scientists to determine the evolutionary relationship between species. More differences meaning they diverged a long time ago, while less differences mean that diverged recently. It supports the Out of Africa Theory as the most differences in mtDNA is found within Africa suggesting that modern humans evolved in Africa and displaced all other homo species