In his landmark work On the Origins of Species, Charles Darwin proposes that all living beings on Earth share a single common ancestor: “Probably all organic beings which have ever lived on this earth have descended from some one primordial form.” This frequently quoted phrase epitomises Darwin’s belief that all living things come from one.

The hypothesis is called universal common ancestry (UCA). It is not to be confused with Darwinian evolution; it is, if you like, an extra on top of it. Life evolves to form different species; the tree of life, with its branches of domains, kingdoms, classes, families and species, is the standard way of visualising this. The tree (a bush would be a more accurate image, but has never been much used) is always shown as having a common trunk, or starting from a single point, a common ancestor of all life; but is that necessarily the case?