Mitochondrial DNA

The small circular chromosome carried inside mitochondria, the cell's energy compartments, and inherited only from the mother rather than from both parents.

Most of a person's DNA sits in the cell nucleus and comes half from each parent. Mitochondrial DNA does not. The US National Human Genome Research Institute defines it as the circular chromosome found inside the mitochondria, the organelles in the cell's cytoplasm that handle energy production and other metabolic work. Mitochondria are unusual among organelles in having any DNA of their own, and offspring inherit their mitochondria, and with them their mitochondrial DNA, from their mother.

That single line of inheritance is what makes mitochondrial DNA both useful and limited in the study of ancient remains. It can survive and be read where nothing else can: in the Harbin cranium, attempts on a tooth and on the petrous bone produced no DNA at all, and mitochondrial DNA recovered from calcified dental plaque was the only genetic material the study obtained.

But because it traces one maternal thread rather than the whole of a family history, a result resting on mitochondrial DNA alone can place a fossil close to a population without settling how its full ancestry relates to other groups. That is why the authors of the Harbin work say nuclear DNA from fossils across East and South East Asia is still needed.

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