Most people have very limited horizons when they think about their ancestors. It’s hard to feel a direct personal connection with anyone more remote than a great-grandparent. Eyes glaze over when you try to tell people of earlier generations, and one good reason is that the numbers inflate so rapidly, to the point of disbelief. How can you possibly have almost 33,000 direct ancestors just five centuries back? (The answer, of course, is that you can’t: think cousin marriage. Then think of something else.)
But when you lift your eyes to the geological timescale things start to get really peculiar. A simple, striking, scientific fact is that every single life-form so far examined shares the same ancestor. You, me, Kim Jong-Un, bacteria, jellyfish, dinosaurs, mushrooms and slime mould all descend from a single, original, living being. It has even acquired its own acronym: LUCA, short for Last Universal Common Ancestor. Current theory posits it as a small, single-cell organism, estimated to have lived some 3.5 to 3.8 billion years ago.
One of the implications is that the beginning of life on earth seems to have been a unique occurrence in the 4.2 billion-year history of the planet. Understandably, this makes many scientists squeamish – such an event is so vanishingly unlikely it begins to look like evidence for some kind of outside intervention, and legions of microbiologists are busy positing alternatives – unfound alien lines, multiple lines that were outcompeted by ours, cross-species sharing of genetic material. But the strongest evidence is still for a single, unique origin, as Darwin put it in The Origin of Species, “some one primordial form, into which life was first breathed”.
Hence my favourite of the many excuses for unsuccessful genealogical research: “We’re all brothers anyway, man.” Or at least 500th cousins.