Born 9,000 years apart, a pair of mitochondriacs

Your or my possible relationship to Cheddar Man (or to just about anybody else alive at the time) is merely a matter of statistics, writes Colin Tudge. Adrian Targett's is virtually certain.

For the mitochondrial DNA in Mr Targett's body cells is almost precisely the same as that found in the pulp in Cheddar Man's teeth, and that makes their relationship almost as copper-bottomed (though not quite) as a DNA fingerprint as used in modern courts of law.

Mitochondrial DNA is very peculiar and uniquely revealing stuff. DNA is, of course, the material of which genes are made; and genes are borne in chromosomes, which reside in the cell nucleus. But the nucleus of each cell is surrounded by material called "cytoplasm" and contains a range of specialised structures known as "organelles". Mitochondria, which process sugars and supply energy in the cells of both plants and animals, are organelles.

Mitochondria almost certainly began as bacteria, which came to live as virtual parasites in the cytoplasm of plant and animal ancestors, and still retain some original bacterial DNA. So each of our cells now contains two lots of DNA: about 97 per cent nuclear and about three per cent mitochondrial.

Mitochondrial DNA is odd in several ways. It mutates fairly frequently, so if two creatures have very similar mitochondrial DNA they are probably closely related. Furthermore, in contrast to nuclear DNA, mitochondrial DNA has no bearing on sex.

Nuclear DNA is built for sex. Eggs and sperm each contain half the DNA of each parent. The whole point of sex is to endow the offspring with a blend of maternal and paternal features. But mitochondrial DNA has no ability to re-combine. On top of that, in most creatures, when the sperm combines with the egg it leaves all its mitochondria behind: only the nucleus of the sperm, with the nuclear DNA, contributes to the new embryo. The cytoplasm of the new embryo is provided almost entirely by the mother - which means that all the embryo's mitochondrial DNA also comes only from the mother.

The genealogical implications are wonderful. Any one nuclear gene could have been spread around the world a dozen times in 9,000 years, passed among the village maids by the droit de seigneur of brutish barons, taken overseas by press-ganged sailors and brought home by immigrants. Sex really is the great unifier.

But mitochondrial DNA keeps itself to itself; shuttled intact from generation to generation exclusively from mother to daughter. Those who fear for their mitochondrial DNA must have daughters. To produce only sons is to die without mitochondrial issue.