Current theories on the origin of life

Life's Late Digital Revolution and Why It Matters for the Study of the Origins of Life

The information contained in life exists in two forms, analog and digital. Analog information is manifest mainly in the differing concentrations of chemicals that get passed from generation to generation and can vary from cell to cell. Digital information is encoded in linear polymers such as DNA and RNA, whose side chains come in discrete chemical forms. Here, we argue that the analog form of information preceded the digital. Acceptance of this dichotomy, and this progression, can help direct future studies on how life originated and initially complexified on the primordial Earth, as well as expected trajectories for other, independent origins of complex life.

From prelife to life: how chemical kinetics become evolutionary dynamics

Life is that which evolves. Living systems are the products of evolutionary processes and can undergo further evolution. A crucial question for the origin of life is the following: when do chemical kinetics become evolutionary dynamics? In this Account, we review properties of "prelife" and discuss the transition from prelife to life. We describe prelife as a chemical system where activated monomers can copolymerize into macromolecules such as RNA. These macromolecules carry information, and their physical and chemical properties depend to a certain extent on their particular sequence of monomers. We consider prelife as a logical precursor of life, where macromolecules are formed by copolymerization, but they cannot replicate. Prelife can undergo "prevolutionary dynamics", including processes such as mutation, selection, and cooperation. Prelife selection, however, is blunt: small differences in rate constants lead to small differences in abundance. Life emerges with the ability of replication. In the resulting evolutionary dynamics, selection is sharp: small differences in rate constants can lead to large differences in abundance. We also study the competition of different "prelives" and find that there can be selection for those systems that ultimately give rise to replication. The transition from prelife to life can occur over an extended period of time. Instead of a single moment that marks the origin of life, prelife may have seeded many attempts for the origin of life. Eventually life takes over and destroys prelife.

About various definitions of life

The old question of a definition of minimal life is taken up again at the aim of providing a forum for an updated discussion. Briefly discussed are the reasons why such an attempt has previously encountered scepticism, and why such an attempt should be renewed at this stage of the inquiry on the origin of life. Then some of the definitions of life presently used are cited and briefly discussed, starting with the definition adopted by NASA as a general working definition. It is shown that this is too limited if one wishes to provide a broad encompassing definition, and some extensions of it are presented and discussed. Finally it is shown how the different definitions of life reflect the main schools of thought that presently dominate the field on the origin of life.

A theory for the origin of a self-replicating chemical system. I: Natural selection of the autogen from short, random oligomers

A theory is described for the origin of a simple chemical system named an autogen, consisting of two short oligonucleotide sequences coding for two simple catalytic peptides. If the theory is valid, under appropriate conditions the autogen would be capable of self-reproduction in a truly genetic process involving both replication and translation. Limited catalytic ability, short oligomer sequences, and low selectivities leading to sloppy information transfer processes are shown to be adequate for the origin of the autogen from random background oligomers. A series of discrete steps, each highly probable if certain minimum requirements and boundary conditions are satisfied, lead to exponential increase in population of all components in the system due to autocatalysis and hypercyclic organization. Nucleation of the components and exponential increase to macroscopic amounts could occur in times on the order of weeks. The feasibility of the theory depends on a number of factors, including the capability of simple protoenzymes to provide moderate enhancements of the accuracies of replication and translation and the likelihood of finding an environment where all of the required processes can occur simultaneously. Regardless of whether or not the specific form proposed for the autogen proves to be feasible, the theory suggests that the first self-replicating chemical systems may have been extremely simple, and that the period of time required for chemical evolution prior to Darwinian natural selection may have been far shorter than generally assumed. Due to the short time required, this theory, unlike others on the origin of genetic processes, is potentially capable of direct experimental verification. A number of prerequisites leading up to such an experiment are suggested, and some have been fulfilled. If successful, such an experiment would be the first laboratory demonstration of the spontaneous emergence by natural selection of a genetic, self-replicating, and evolving molecular system, and might represent the first step in the prebiotic environment of true Darwinian evolution toward a living cell.