Mitochondrial DNA are small loops of DNA found within organelles in the cell, mitochondria. Mitochondria serve as the "power plants" of the cell and are believed to be the descendants of ancient bacteria that participated so symbiotically with ancient cells that they became integrated into them as organelles. The vast majority of DNA in all animals is found in the nucleus, and is known as nuclear DNA, while mitochondrial DNA is the only DNA located outside of the nucleus.
In contrast to the DNA in the nucleus, which contains about 20,000 protein-coding genes in over 3 billion base pairs, the mitochondrial DNA is relatively small, consisting of only 13 protein-coding genes in 15,000-17,000 base pairs. Unlike the nuclear DNA, which consists of linear DNA, mitochondrial DNA is found in the loop format, just like the DNA found in bacteria. The genes in the mitochondria express proteins that help build the mitochondria, though over billions of years of evolution, the nuclear DNA has taken up much of the roles of mitochondrial DNA in constructing the mitochondria.
Unlike nuclear DNA, which undergoes recombination from generation to generation due to sex between a male and a female, mitochondrial DNA only undergoes recombination with DNA from the same organelle, greatly limiting genetic change. Therefore, the only factor that introduces genetic changes is mutation, rather than mutation plus recombination, as is the case with nuclear DNA. This makes mitochondrial DNA into a highly useful genetic marker that can be used to compare different lineages, including different human lineages.
Analysis of human mitochondrial DNA has proven crucial to determining the ancestry of various human groups and the early migration patterns of humans around the world. These studies have supported the Out-of-Africa theory, the theory that asserts that the human species had its origin in Africa about 250,000 years ago, then spread across the world. The main competing theory is the multiregional hypothesis, which supposes that different human races around the world evolved independently from hominid precursors. Due to evidence from mitochondrial DNA analysis, this hypothesis has been essentially ruled out.