Precambrian/Life origins/The Ecopoesis model

The Ecopoesis model (Félix de Sousa, R.A., 2000, 2006) for the origin of life is based on the idea that an atmosphere containing a high percentage of molecular oxygen, generated by the atmospheric photolysis of water vapour, is an essential feature of the Earth's earliest ecosphere. Ecopoesis differs radically from other models of biopoesis. Traditionally, the origin of life is equalled to the fortunate appearance of very simple cellular organisms, whose metabolic evolution would thenceforth conduct the general evolution of the environment (the oxygen-rich atmosphere being a result of this process). In ecopoesis, in contrast, the physical environment plays the leading role, not organisms. The early presence of oxygen determines the onset of a flow of electrons, which leaving the reducing components of the lithosphere, crosses the archean seas, pumped by the photolytic production of oxygen in the Earth's atmosphere. This flow is mediated by, and predominantly channelled through the redox transitions of the redox-sensitive elements in the hydrosphere. These large-scale environmental interactions cause the development of a geochemically based metabolism in a planetary protoplasm (holoplasm) setting the stage for the gradual evolution of organismal life to take place.

The wide difference of electrochemical potentials in the primordial environment would cause the appearance of the biogeochemical cycles. The primitive metabolic pathways are originated by the interaction of these cycles and their products. The buildup of order in the system arises from the energetically favourable transitions, particularly in the oxidation of organic matter, and from the physicochemical properties of the compounds involved. This planetary protometabolism is essentially congruent to today's biochemistry, including carbon and nitrogen fixation, and aerobic degradation of organic compounds (full oxidation to CO₂). Biological evolution, as a rule, would proceed through the increasing functional control of such reactions, rather than through their creation. A naked geochemical metabolism would thus evolve congruently towards our modern enzymatic processes. The carboxylating and condensing properties imparted to the aqueous medium (dubbed "hypercarbonic") by the presence of a heavy CO₂ atmosphere are particularly important, since they permit carbon fixation in the presence of reducing power, and chiral propagation. Since the direct reduction of carbon dioxide is outside the domains of aqueous chemistry, carbon fixation would depend on the carboxylation of pre-existing carbon compounds. Protobiological carbon cycles would run in the anabolic sense intercalating carboxylation reactions of organic compounds with reduction steps. Conversely, catabolic cycles would alternate decarboxylations and oxidation steps, as in modern metabolic cycles. The possibilities of non-enzymatic analogues of such reactions in the hypercarbonic medium are discussed to some detail in the book, as well as the mechanisms through which the reduction-oxidation equilibrium in the protobiological ecosphere would be tightly bound to the carboxylation-decarboxylation processes.

Carbon fixation would also require at least a modest input (primary input) of lithospheric carbide-derived hydrocarbons. Acetylene, in particular, originated by the action of water on divalent ionic carbides, could be directly converted, through hydration and carboxylation, into pyruvic and oxaloacetic acids, which are a part of the very core of biochemical pathways. The likelihood of this reaction suggests the bi-directional Krebs cycle as the main feature of the circulation of carbon in the early ecosphere. The availability of reducing power for anabolic processes (deriving mainly from lithospheric divalent iron and sulfides), would, in most oceanic domains, outbalance the total amount of oxidising power brought in by the slightly soluble atmospheric oxygen, causing the accumulation of organic compounds in the hydrosphere. Inquiry into the nature of these compounds and their formation processes may be helped by the presumption of a congruent evolution. The organismal characteristics of life are only gradually acquired and cells are fairly late-comers in this model.

Some key references:

• Yamaguchi, K.E. (2005a) Evolution of the atmospheric oxygen in the early Precambrian: An updated review of geological 'evidence'. In Frontier Research on Earth Evolution (ed. Y. Fukao), 2, 04-23.

'Yamaguchi's review shows us that, contrary to what origin of life researchers insist, the reducing atmosphere has never been a consensus among geologists. And also that this debate is nowadays hotter than ever.'

• Castresana, J., Saraste, M., (1995). Evolution of energetic metabolism: the respiration-early hypothesis. Trends Biochem. Sci. 20:443–48.

'Castresana and Saraste bring forth evidence of the antiquity of enzymes involved in respiration and demonstrate these should be present in the last common ancestor of all organisms. Much before some of them learned to split the water molecule and produce oxygen'

• Towe, K.M., (1996). Environmental oxygen conditions during the origin and early evolution of life . Advances in Space Research, Volume 18, Issue 12, 1996, Pages 7-15

[3] 'Towe, who has been dealing with the topic of Precambrian Oxygen since the 70s, discusses a great number geological and biological factors concerning early life and summarises his objections to the "reducing atmosphere models".'

• More information about the Ecopoesis Model can be found in the official site [1] from where the contents of this page were extracted by its author.