Mitochondrial DNA and Human Evolution

What is the Mitochondrion?

The mitochondrion is a cell organelle found in most eukaryotic cells. These mitochondria fulfill the function of “cellular power plant”, as they are responsible for the production of adenosine triphosphate (ATP), the cell’s primary source of energy. Some cells contain only one mitochondrion, whereas others may contain hundreds or even thousands of these tiny factories.

What makes these mitochondria quite special, is that, although most of the cell’s DNA is stored in the nucleus, they carry their own DNA (mtDNA). This opens up some interesting perspectives. The one discussed here, is the link between mitochondrial DNA and human evolution.

Why Mitochondrial DNA?

One could ask: why specifically mitochondrial DNA? If the nucleus contains most of the cell’s DNA, than why not nuclear DNA? The most important reasons why mitochondrial DNA is used are:

• High copy number: Whereas most cells only have one nucleus (and thus one copy of nuclear DNA), there are several cell types that contain a lot of mitochondria, carrying many copies of mtDNA, making it easy to obtain for analysis.
• Maternal inheritance: Human mtDNA is known for its strictly maternal inheritance. This enables researchers to trace lineages back in time, with focus on the maternal ancestry of a population.
• Lack of recombination: Maternal and paternal mtDNA do not recombine (although there is one known case where this occurred in 0.7% of the muscle cells), keeping the mtDNA sequence “clean”.
• Mutation rate: the mutation rate of mtDNA is fairly high, but reasonably fixed, making it quite useful to trace DNA changes through time. Certainly for human evolution, as we’ve not been around that long.

Mitochondrial Eve

These characteristics of mtDNA make it particularly well-suited to trace back the human lineage to its most recent common matrilineal ancestor, often referred to as “mitochondrial Eve”. This mitochondrial Eve can be traced back to Eastern Africa, about 200,000 years ago. The results place a population of a few thousand individuals in current Tanzania and close-by regions at the base of the lineage that gave rise to modern man.

This provides strong support to the theory that modern man originated in Eastern Africa quite recently and from there, spread over the globe, replacing “older” human populations, such as the Neanderthals. It also means a serious setback to the multiregional model that states that modern man arose from single, continuous species spread across the globe, in the Pleisocene, around 2.5 million years ago.

At present, partially due to mtDNA research, the out of Africa model of human evolution is the dominant theory explaining human evolution and distribution.

References

• Cann, R.L.; Stoneking, M. & Wilson, A.C. (1987). Mitochondrial DNA and human evolution. Nature 325, pp. 31 – 36.
• Dawkins, R. (2004). The ancestor’s tale: a pilgrimage to the dawn of evolution, Boston: Houghton Mifflin
• Maynard Smith, J. (1999) Evolutionary Genetics. Second Edition. Oxford University Press.
• Pakendorf, B. and Stoneking, M. (2005). Mitochondrial DNA and Human Evolution. Annu. Rev. Genomics Hum. Genet. 6, pp. 165 – 183.
• Pierce, B.A. (2002) Genetics: A Conceptual Approach. First edition. W.H.Freeman Publishing.