MINERALS AND THE ORIGIN OF LIFE - HYPOTHESES AND EXPERIMENTS IN HETEROGENEOUS CHEMISTRY

It is generally accepted now that the emergence of living entities was a gradual process which took place on the Earth's surface some 3.8-3. 9 billions years ago. This process is called 'molecular evolution', an d the 'time window' during which it is hypothesized to have taken plac e is estimated between a few million to 200 million years. The possibl e role of minerals in this process is an important feature of many, bu t not all, of the molecular evolution hypotheses suggested so far. The hypothesized involvement of minerals in the molecular evolution proce ss encompasses a variety of reactions, from mere adsorption to templat e-directed synthesis. These include synthesis of amino acids and sugar -like molecules, stereoselective adsorption of organic molecules, cata lytic condensation of peptides and oligonucleotides, and energy source for the primordial fixation of carbon dioxide. The prebiotic terrestr ial environments hypothesized to have served as arenas for these proce sses range from the sea bottom to the clouds, including deep sea hydro thermal springs, bubble-aerosol-droplets at the sea surface, water dro plets in the atmosphere, lagoons and puddles. The possible role of min erals in the molecular evolution process was critically reviewed in th e present work, using as the main guideline the 'principle of biologic al continuity'. According to this principle, 'for any postulated stage in biogenesis there must be a continuous path backward to the prebiot ic state of the Earth and forward to modern organism' [1]. It is concl uded that there is a basic incompatibility between living forms on the one hand, and minerals on the other hand, with regard to information processing (template-directed synthesis of peptides and oligonucleotid es). Guided by the principle of continuity, this discrepancy is extend ed also to the prebiotic era; it may be bridged, however, by the 'adso rbed-temptate' model, in which minerals serve as supports for biochemi cal templates. Regarding a possible continuity between the elemental c omposition of the two prebiotic systems, it seems that the only (specu lative) candidates for such relationships are catalytic metal clusters , and their corresponding prebiotically-plausible minerals. The best k nown pair of this kind is that of the iron-sulfur clusters found in ma ny proteins, and pyrite (FeS2), which was suggested recently as a cent ral constituent of a molecular evolution scenario [2]. According to mo st suggestions advanced so far for the possible involvement of mineral s in molecular evolution processes, the role of minerals is limited to environmental parameters. These include adsorption, catalysis, shield ing from high energy irradiation, and energy supply for organic reacti ons. Nevertheless, even in this capacity, the role of minerals is pres umably critical to the process of molecular evolution. Two of the most interesting and challenging research directions regarding the possibl e prebiotic role of minerals in molecular evolution are: 1. The possib le relationships between bio-metal clusters and prebiotically-plausibl e minerals; and 2. Primitive template-directed syntheses according to the 'adsorbed-template' model.