It’s an encouraging thought, perhaps, to realize that in the great scheme of things, I am fairly closely related to Albert Einstein, Lord Robert Winston and Sammy Davis Jr. However, I am probably also fairly closely related to Genghis Khan, George W. Bush and even Richard Dawkins.

And so, my friends, are you.

This amusing story, the result of work by the New England Historic Genealogical Society, shows that Barack Obama and George W. Bush are tenth cousins, once removed, and that Mr Obama is also a distant cousin of Brad Pitt. On the other hand, Hillary Clinton is related to Brad Pitt’s current squeeze, Angelina Jolie.

Mr Obama is also related to Dick Cheney, Gerald Ford, Lyndon Johnson and Winston Churchill. Mrs Clinton’s pedigree is glitzier, including such names as Alanis Morisette, Madonna and Camilla Parker-Bowles. If Brangelina decided on an extended family get-together, says the report, “it could provide the perfect opportunity for the two Democratic presidential rivals to get together”.

Great stuff. But should it be a surprise? After all, we have (or had) two parents, four grandparents, eight great-grandparents and so on, so the number of the avoteinu v’imoteinu grows exponentially as we go back through the doroteinu. Because the population size is finite, it isn’t long before our family trees start to show up the same people, and, eventually, a common ancestor of us all (Scheming Serpents and Trees Of Good and Evil are available only as optional extras).

When did this common ancestor live? Simple models (which assume that mating is random) really are very simple indeed – the time back to a common ancestor is, approximately, the logarithm (to base 2) of the population size: the current human population of around six billion, then, had a common ancestor between 20 and 33 generations ago – assuming a leisurely generation time of 25 years, that’s no more than 825 years ago. The common ancestor of us all lived at the time of the Crusades. But there’s more – this person, whoever it was, didn’t occupy the world alone, and going back beyond that time, one finds an increasing number of the population are universal ancestors, so were one to go back in time, it gets more and more likely that the person who greets you as you materialize next to the Hanging Gardens of Babylon will be your direct ancestor.

This model, though, really is too simple to be credible, because mating isn’t random. Historically, people tend to choose their mates from within their social or ethnic group, and who don’t live continents apart. In 2004, Douglas Rohde and colleagues modelled the ancestry of extant humans by taking geography and migration into account. They pushed the time of the most recent common ancestor (MRCA) back just a few thousand years, and showed, in addition, that “among all individuals living more than just a few thousand years earlier than the MRCA, each present-day human has exactly the same set of genealogical ancestors.” In a commentary on this paper, Jotun Hein notes that modelling the details of migration and isolation will always be difficult, and will push the date of the MRCA still further backwards, especially when one takes long-isolated populations into account. The native population of Tasmania, for example, was isolated from the rest of humanity for many thousands of years, so the MRCA must have lived before that isolation took place.

Hein also notes that tracing genealogy is not quite the same thing as tracing the history of genetic material, and, discussing this, raises further exciting questions, such as how far back would one have to go to find a single couple who are the lone ancestors of everybody? And how much could be known about humanity’s pedigree if we knew the genome of everybody?