The Genetic and Metabolic Views of the Origin of Life
In all contemporary organisms, a distinction can be made between genotype and phenotype. The degree to which early organisms had 'genotypes' and 'phenotypes' is an important issue facing those wishing to understand the nature of the first organisms. Opinion on this matter has tended to be divided between those emphasising the rôle of the genotype and those emphasising the rôle of the phenotype. These views can be characterised as follows:-
The Genetic View is that template replicators came first. Those holding this view tend to empasise the importance of accurate transfer of hereditary information for the process of evolution. The Genetic View emphasises the rôle of the genotype in early organisms. Often these organisms are seen as 'naked genes'; typically replicating molecules such as DNA or RNA, but which do not initially synthesise proteins or control a metabolism in the way that contemporary DNA / RNA do. Such organisms could be regarded as having a genotype, but no separate phenotype.
By contrast, the Metabolic View holds that metabolism came first. The first organisms would have been metabolic systems, which would have been capable of reproducting in some way, but would not have been template replicators.
Most proponents of the Genetic View have envisaged the first replicators as RNA-like molecules. As indicated in the section on transfer of hereditary information, the Error Threshold represents a difficulty for this view, since in order to enable hereditary information to be transferred adequately, it places a very low limit on the size of RNA-like replicators. Eigen's Hypercycle theory is an attempt to overcome this difficulty.
The Metabolic View has been criticised on the basis that it is difficult to see how hereditary information can be transferred from 'parent' to 'child'. Theories of Autocatalytic Sets and Compositional Inheritance attempt to overcome this difficulty.