From the RNA world to the DNA-protein world: clues to the origin and early evolution of life in the ribosome.

ABSTRACT

“What is life” is a more diffi cult question to answer than “What is a living being”. According to the classical defi nition, a living being is an entity capable of carrying out autosynthesis, autocatalysis and excitability. Autosynthesis means the capability of synthesising its own material from external materials, which involves nutrition, metabolism and growth; autocatalysis implies the capability of producing new beings (reproduction), and excitability is the capacity to respond to stimuli in such a way that the response is more than a passive refl ection of the stimulus. Although these three functions can be recognized in all living beings, the defi nition of life is a more diffi cult question, because some of these features – as well as others considered to be characteristic of life, such as self-organization – are also found in non-living materials like crystals and inert dissipative structures. In fact, what distinguishes life from inert material is its ability to evolve by natural selection. Therefore, a possible defi nition of life is that it is the totality of all the properties of a chemical transformation system that make it possible for the system to undergo Darwinian evolution by natural selection. (We say ‘chemical transformation system’ because the only forms of life known to us are based on chemistry. In principle, any transformation system would do so long as the above properties are satisfi ed.) Evolution by natural selection is, thus, the main and most characteristic feature of life. Therefore the conditions necessary for it are those essential for life. In order to evolve for by natural selection, an entity must possess three features information, metabolism and a membrane. Metabolism is a facet of the chemical machinery of autosynthesis. It includes the capacity to produce all cellular structures and a source of storable energy (usually ATP) from external sources. Metabolism is determined by enzymes (catalytic molecules) whose main feature must be to catalyse reactions specifi cally – and also at a rate higher than the chemical background noise, so as to be able to favour specifi c chemical reactions. Information involves the capacity to safeguard the means through which the system can be perpetuated. Because this requires the ability to replicate, it is also the basis of autocatalysis and provides the necessary basis for cumulative evolution. Finally, natural selection demands that the system must become ‘selfi sh’, as it must safeguard its achieved improvements in the catalytic material, all the while being in competition with other similar entities that can take advantage of it in the sense of increasing their representation in the population relative to it. A selective membrane that determines its individuality is necessary in order to ensure that both metabolism and information can take place without compromising the integrity of the system (Meléndez-Hevia et al. 2008). The emergence of life was diffi cult, and it is obvious that life could not appear with the high complexity seen in present living beings – even the simplest ones that may imply around 2,000 different specifi c structural and catalytic molecules. Life had to start with the minimum possible necessary material, but enough to fulfi l the three basic requirements information, metabolism and a membrane. If it were possible to demonstrate that a single entity could account for at least two of these three features, the a priori possibilities for life getting a start would improve and the problem of the origin of life would be simplifi ed. An interesting feature of life, which allows the reconstruction of its history, is that the materials used in any stage are generally preserved. That is, the emergence of new materials does not destroy the old ones; at least some of the earlier materials remain in the later structures. This feature is the logical consequence of life’s continuity. Otherwise, life would have had to start again many times. Therefore, it is expected that traces of many steps in the evolution of life remain at present as they were near the beginning, even if they are very ancient.