It is generally assumed that life had a single origin — or, at least, that all extant life descended from a 'universal common ancestor' (UCA) — although this view has been called into question by evidence for extensive horizontal gene transfer. Here, the UCA view is framed as a formal hypothesis and tested (crucially, without assuming that genetic similarity reflects genetic kinship). The UCA view triumphs: a single origin of life is overwhelmingly more likely than any competing hypothesis.
