ABSTRACT
The unobservability of the past has the inescapable consequence that the problem of the origin and early evolution of life can only be solved by theory. Alternative theories on the origin of life compete to explain the facts of the extant biosphere. Therefore, selection among such theories should be carried out on the basis of their explanatory power (Popper, 1963): their power to explain many observable facts with few evolutionary assumptions. In constructing a theory of early evolution, explanatory power can best be achieved by the postulation of common evolutionary precursor functions for disparate extant successor functions.
