Human genetics got a real boost with the discovery that mitochondria have their own DNA. Mitochondria are tiny organelles that live in the cytoplasm of cells, the fluid-filled space between the cell nucleus and the outer membrane. There are thousands of mitochondria in each cell, and each one has its own small circle of DNA, a reminder of their distant bacterial ancestry.
What makes mitochondrial DNA (or mDNA for short) so special and so useful? First is its unique inheritance pattern. Human eggs are full of mitochondria, while sperm have only a hundred or so, just enough to power it while it swims towards the egg. After fertilisation, when the sperm penetrates the egg, these few male mitochondria are immediately destroyed. This means that, while we all receive our nuclear DNA, with the exception of the X and Y sex chromosomes, from both parents, we get all of our mDNA from our mothers. She got it from her mother, who got it from hers – and so on back in time.
Mitochondrial DNA is most useful in connecting the maternal lines of living people in different parts of the world.
The other handy thing about mDNA is that it changes much more rapidly than nuclear DNA, about 20 times as fast, because mitochondria lack an efficient proof-reading system to check for errors when DNA is copied. The high mutation rate means that there is plenty of variation in the sequence of mDNA between people, and variation is the lifeblood of genetics.
For geneticists and historians, mDNA is a natural for tracing maternal genealogies but the real excitement is in tracing much deeper connections. It is so abundant in cells that traces can still be found in human remains many thousands of years old, like Oetzi the Iceman. In 1994 my research team showed that Oetzi had exactly the same mDNA as many people alive today. Its retrieval from ancient bone has been instrumental in many famous cases of historic significance, such as confirming the identity of the remains of the last Tsar and his family.
But I think mDNA is most useful in connecting the maternal lines of living people in different parts of the world, and here it has been used to solve many oustanding riddles. For example, we used mDNA to show that all Polynesians could traced their ancestry back to southeast Asia and not to the Americas, as the late Thor Heyerdahl famously claimed. Others have used mDNA to prove that the ancestors of native Americans really did cross from Siberia about 13 000 years ago, putting an end to such unlikely claims as they were the descended from a lost tribe from the Middle East.
Tracing the early history of the human colonisation of Europe beginning about 45 000 years ago has been another success story for mDNA, proving that most Europeans trace their ancestry to hunter-gatherers who arrived during the last Ice Age, rather than farmers coming from the Middle East. And also that the Neanderthals have left no trace in the European gene pool and almost certainly became extinct.
Among native Europeans, almost everybody can trace their maternal genealogy, using mDNA, to one of only seven women, their ancient clan mother. To give them an identity I have given these women names: Ursula, Xenia, Helena, Velda, Tara, Katrine and Jasmine. The women lived between 10 000 and 45 000 years ago, six of the seven were hunter-gatherers, the seventh, Jasmine, was an early farmer. These seven women are also related to each other, and these connections can also be followed by mDNA. They join up with the clan mothers from other parts of the world and ultimately coalesce in one woman – mitochondrial Eve, who lived in Africa about 150 000 years ago. Wherever we live on the planet, we are all her descendants.
Bryan Sykes is Professor of Human Genetics at the Weatherall Institute of Molecular Medicine, University of Oxford.