Origin Of Life: Issues

Transfer of Hereditary Information, the Error Threshold, and Eigen's Paradox

Inheritance Requires Information Transfer

Evolution is essentially a process of trial and error. Within a population of organisms, variations can occur between the characteristics of different organisms. When a variation occurs that is beneficial to an organism in some way, it will tend to have more offspring than organisms without the variation. The variation is inherited by the organism's offspring, which will also tend to have more offspring themselves. As a result of this process of natural selection, the variation can soon become prevalent in the population.

In order for this process to work, it is essential that child organisms should inherit their parents' characteristics. Otherwise any beneficial characteristic 'discovered' by the process of trial and error would be lost when the organism in which the characteristic manifested itself died.

Inheritance of characteristics from one generation to the next involves information transfer; a parent must pass information about its characteristics to its children in order for those characteristics to be inherited. The information received by the child must be accurate, otherwise characteristics will not be properly inherited, and the characteristics of offspring will 'drift away' from those of their parents, with the effect that beneficial characteristics will be lost.

The Error Threshold

In contemporary organisms, most of the information transferred between generations in relation to inherited characteristics is carried by DNA and RNA molecules.

These organisms use sophisticated error correction mechanisms involving multiple enzymes to ensure that information is transferred accurately when replication occurs. The effectiveness of these correction mechanisms is such that in the E. coli bacterium, the mutation rate is in the region of 5 x 10-10 mutations per base per replication. That is to say, when E. coli replicates, the probability that a particular base mutates is about one in 2 billion. Since the size of the E. coli genome is about 4.7 x 106 base pairs, the chance that even a single base mutates when an E. coli replicates is very low.

Under the Genetic View of the origin of life, it is thought that early lifeforms were template replicators. These replicators would have been 'naked genes', meaning that they did not affect their environment or control a metabolism in the way that modern genes do. RNA has often been thought of as a candidate material from which naked genes could have been constructed.

RNA based naked genes would not have had the sopisticated error correction mechanisms available to contemporary organisms. It has been estimated that without error correcting mechanisms the mutation rate for an RNA based naked gene could not be less than 0.01 mutations per base per replication . For a genome consisting of N bases, the probability that there are no errors when replication takes place is:-

(1 - mutation rate)N

This means that in the case of a 100 base (RNA) naked gene, the likelihood that a replication event occurs with no errors is 0.37. For an RNA naked gene with 300 bases, the corresponding figure is 0.049. The significance of this is that without sophisticated error correction mechanisms, the size of an RNA based naked gene could not be substantially more than 100 base pairs. For larger (RNA) naked genes the effect of mutations would be that any beneficial base sequences 'discovered' by evolution would soon be 'forgotten'.

The only way to counter this 'forgetting' effect would be for the naked gene to produce a very large number of offspring, and for natural selection to weed out the majority of the mutations before they could replicate, so that the small number of correctly replicated offspring would not be swamped. Given that these naked genes are envisaged as simple replicators, rather than the carriers of information specifying and controlling a metabolism on which their survival depends, it is not plausible that a single mutation could make a large enough difference to the ability of a naked gene to replicate.

We must therefore conclude that the size of a naked gene is limited by its mutation rate. This limit is known as the Error Threshold; in the case of RNA the error threshold is about 100 bases.

The Error Threshold was first highlighted by Manfred Eigen and Peter Schuster in the 1970s. They constructed detailed models of evolution among replicating molecules such as DNA or RNA which were competing with one another and subject to fixed mutation rates per nucleotide base.

Eigen's Paradox

The error correction mechanisms that exist in contemporary organisms are extremely complex. These organisms include as part of their genomes the instructions for construction of the error correction machinery. Any such set of instructions, even for a very simple error correction mechanism, would require far more than 100 base pairs. We are thus faced with the following situation:- We therefore have what appears to be a chicken and egg problem. A genome cannot be larger than 100 bases without an error correction mechanism, but in order to specify an error correction mechanism it must be substantially larger than 100 bases. This is Eigen's Paradox.