Introduction

There are three routes to the discovery of the history of life. First, since modern biology and biochemistry are the result of genetic and phenotypic divergence, descriptions of past life can be derived from comparative biology and biochemistry; this route extrapolates backwards from the secure present. Second, since all biology is of necessity constrained by the laws of chemistry and physics, descriptions of past life can be derived by attempting to understand what living systems may have been a priori possible, rather than what living systems have a posteriori evolved; this route looks forward from an imagined past (for a perspective, see James and Ellington 1995). Finally, since evolutionary changes in form and function (either molecular or organismal) have frequently been shown to be based on preexisting form and function, the biochemistry of ancient and modern organisms should be linked by a continuous (but not necessarily gradual) path. This route is an amalgam of the first two, and it attempts to connect the present and the past. For example, if this last proposition is true, it is unlikely that complex assemblages such as ribosomes arose fully formed, but rather they were likely derived from molecules of similar form or function that preceded them. By demonstrating that molecules with properties similar to the ribosome could have existed, we provide credence for this route; the more intermediates that can be demonstrated or extrapolated, the more sure is the link between present and past.