Such studies are carried out in two ways. First, by analysing mitochondrial DNA that is passed, uniquely, through the maternal line. (It is this technique that demonstrated Garrison’s shared bloodline with African tribes). Or second, by carrying out studies of the Y chromosome — which is passed on from father to son. This reveals a person’s paternal lineage, and explains how Garrison’s White ‘roots’ were uncovered. In this way, both sides of our family trees would seem to be exposed by science. However, researchers warn that identifying a Y chromosome as being of European origin, or a piece of mitochondrial DNA as being African, by no means provides a definitive picture of someone’s ancestry.

‘If you look back a generation, you see only two ancestors — our mother and father,’ pointed out the historian Lord Renfrew. ‘If you look back two generations, you see four ancestors — your grand parents. Three generations reveals eight great-grandparents, and 10 generations exposes more than 1,000 ancestors. Each one of these individuals has contributed to your gene pool, but by studying only the Y chromosome and mitochondrial DNA, you reveal only two lineages: your father’s father 10 times removed, and your mother’s mother 10 times removed. You miss all the rest.’ In short, the current limitations of modern genetics can give us a skewed picture of our origins. You may see the plantation owner and the exploited slave, but you miss all the others who have gone to make up a person’s lineage.

On the other hand, Y chromosome and mitochondrial DNA data can still be very revealing — and not only for people of Afro-Caribbean background. For example, studies of the British Y chromosome by Mark Thomas and colleagues at University College London recently revealed a stark difference between men in England and those in Wales.

The group found that English Y chromosomes are almost identical to those from Friesland, an area of the Netherlands from which the Anglo-Saxons originated 1,500 years ago. Those of the Welsh were markedly different, from which Thomas concludes that Anglo-Saxons invaded the area now covered by England, overcoming between 50 and 100 per cent of the indigenous population, but failed to move into Wales.

This interpretation —based on blood taken from living humans — gives solidity to dusty historical analysis. It also contradicts arguments by many postwar historians who have claimed Anglo-Saxon influence on England was limited to political and commercial elites. ‘Our work shows the traditional idea of an invasion of Germanic tribes is the more likely one,’ says Thomas.

Or take the discovery of the startlingly high incidence of the A blood group among residents around Pembroke, Wales. Scientists believe this has a simple cause. Around 1108 AD, Henry I brought over many craftsmen from Flanders — which has a high incidence of the A blood group— and settled them in Pembroke. ‘In short, in the blood of Pembrokeshire people today, the tell-tale signs of their Norman past lingers on,’ says the geneticist Sir Walter Bodmer.

And then there is the work of scientists who have discovered the genetic fingerprint of Viking invaders in the blood of Orkney and Shetland islanders and the people of Cumbria. Peering further into the past, geneticist Prof Brian Sykes, founder of Oxford Ancestors, recently discovered the headmaster of a school in Cheddar had the same mitochondrial DNA type as the 9,000-year-old Cheddar Man’s skeleton found many decades earlier in the region’s caves. However, it is crucial to note that DNA showed the headmaster was not directly descended from the Cheddar Man. ‘It is merely that there is a link through one of Cheddar Man’s female ancestors perhaps thousands of years earlier, and far from Cheddar,’ says Sykes. Again, we should be careful about simplifying our ancestry.

It is fascinating stuff, though activists warn of dangers. By uncovering these webs of connectivity, we may end up confusing the concept of ethnicity— which defines people through their culture and language — with the idea of racial make-up. ‘This has two dangers,’ says Dr Helen Wallace, of Genewatch. ‘It could exacerbate racism in medicine, perhaps leading to some ethnic groups being considered as genetically unsuitable to take particular medicines, and it could lead to wrongly blaming the incidence of some diseases on genetic differences, rather than factors such as poverty.’ The point is also stressed by Prof Steve Jones, of University College, London: ‘If genetics shows us one thing, it’s that race is a social construct.’ Nevertheless, such surveys can bring joy. Consider Orlene Henry, a 40-year-old London woman who volunteered for the same survey that provided Len Garrison with his genetic ‘surprise’. She knew her parents had come from Guyana in South America, but could not find out any more. However, DNA tests showed that, like Garrison, she shared her genetic make-up with those of the Bubbi tribe, from equatorial Guinea. ‘I’ve always seen myself as a strong, proud Black woman, but now I feel even more so, given that I know who my ancestors are,’ said Henry, a mature student. ‘Why does it matter so much? I can’t explain that to you, but I do believe you are more than the product of your environment or your immediate family.’ (Guardian)