Free The Journey of Man: A Genetic Odyssey by Spencer Wells Page B

allows us to trace a unique male lineage back in time, from son to father to grandfather, and so on. Taken to the extreme, it allows us to travel back in time from the DNA of any man alive today to our first male ancestor – Adam. But how does it link unrelated men to each other in regional patterns? Surely each man must trace his own unique Y-chromosome line back to Adam?
    The answer is no, but the reason is a bit complicated. It’s because we’re not as unrelated as we think. Imagine the situation for the majority of our genome – the parts that don’t come uniquely from our mother or our father. Since we inherit half of this DNA from each of our parents, the pattern of polymorphisms it contains can be used to infer paternity, since it connects us to both our mother and our father. If my DNA is shown in court to have a 50 per cent match with that of a child I’ve never met, it is likely that I will be paying for the support of that child for many years to come – the probability of a matchoccurring by chance is infinitesimally small. So polymorphisms define us, and our parents, as part of a unique genealogical branch. No other group of people on earth has exactly the same story written in its DNA.
    If we extend this further, and begin to think about our grandparents, and their grandparents, and so on, we lose some of the signal in each generation. I have a 50 per cent match with my father, but only a 25 per cent match with my grandfather, and only a 6 per cent match with his grandfather. This is because we acquire new ancestors in each generation as we go back in time, and they start to pile up pretty quickly. Each of my parents had two parents, and each of them had two parents, and so on. The geneticist Kenneth Kidd, of Yale University, has pointed out that if we double the number of ancestors in each generation (around twenty-five years), when we go back in time about 500 years each of us must have had over a million living ancestors. If we go back to the time of the Norman invasion of England, around a thousand years, our calculation tells us that we must have had over one trillion (1,000,000,000,000) ancestors – far more than the total number of people that have existed in the whole of human history. So what’s going on? Is our calculation flawed in some way?
    The answer is yes and no. The maths is certainly correct – the power of exponential growth has been known since at least the time of the Greeks, and we’re all acquainted with the real-world phenomenon of ‘breeding like rabbits’. The error in our ancestor tally stems not from a malfunctioning calculator, but from the assumption that each of the people in our genealogy is completely unrelated to the others. Clearly, people must share quite a bit of their ancestry, or we can’t make the numbers work. This would have the effect of multiplying by a number smaller than two in each generation – in fact, for most people the number is pretty close to one. And the reason for this can be found by doing a bit of poetic bird-watching.
Water, water everywhere …
    Samuel Taylor Coleridge, Romantic poet, failed classicist and drug addict, spent 1797–8 living in a small Dorset village. In between vigorous walks in the hills and long discussions with his neighbour, William Wordsworth, Coleridge found time for a fit of literary activity that was to produce his two greatest pieces of work,
Kubla Khan
and
The Rime of the Ancient Mariner.
The former, composed subconsciously while in an opium-induced dream state – how better to conjure up the ‘stately pleasure dome’ – is an extraordinary exercise in literary imagery. The latter, written during a more sober period, follows the travails of a ship in the South Seas. The mariner in the poem callously violates one of the unwritten laws of the sea by killing an albatross, and the entire crew are made to suffer the consequences, ending up becalmed in the sweltering sun, surrounded by ‘water, water everywhere,