Koalas have fingerprints. Not something like fingerprints: loops and whorls and ridges so close to ours that the Adelaide anatomist who studied them in the 1990s said that even under a microscope you would struggle to tell the two apart. Our nearest common ancestor with a koala lived more than seventy million years ago and had nothing of the kind. Both lines worked it out separately, from the same problem: holding on to a branch without letting go. I find that lovely and slightly comic. The mark we press onto paper to prove we are nobody else turns out to be a design that anyone arrives at, given enough trees.