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HOME > PAST ISSUE > May-June 2009 > Article Detail

FEATURE ARTICLE

The Origin of Life

A case is made for the descent of electrons

James Trefil, Harold J. Morowitz, Eric Smith

RNA World

Figure 2. RNA World vs. Metabolism FirstClick to Enlarge ImageThe next major advance came in the early 1980s, when Thomas Cech and Sidney Altman showed that some RNA molecules can act as enzyme-like catalysts. The frozen-accident argument was then replaced by a suggestive scenario in which something like RNA was assembled by chance, and was then able to fill twin roles as both enzyme and hereditary molecule in the runup to life. The RNA systems were then acted upon by natural selection, leading to greater molecular complexity and, eventually, something like modern life. Whereas most scientists believe, on the basis of Cech and Altman’s work, that life went through an early RNA-dominated phase (dubbed “RNA World”), the “RNA First” scenario has again a quality of frozen accident. Between prebiological chemistry and RNA World, a large leap occurs, requiring that molecules appear having at least a familial resemblance to the complex molecules in the vials of Cech and Altman, for that is the assumption upon which the relevance of their findings to the origin of life depends.

Inserting RNA molecules into an RNA First scenario without explaining how they got there seems to us an inadequate foundation for an origin theory. The RNA molecule is too complex, requiring assembly first of the monomeric constituents of RNA, then assembly of strings of monomers into polymers. As a random event without a highly structured chemical context, this sequence has a forbiddingly low probability and the process lacks a plausible chemical explanation, despite considerable effort to supply one. We find it more natural to infer that by the time complex RNA was possible, life was already well on the road to complexity. We believe further that we can see the primordial chemical architecture preserved in the universal metabolic chemistry we observe today.

The compelling feature of RNA World is that a primordial molecule provided both catalytic power and the ability to propagate its chemical identity over generations. As the catalytic versatility of these primordial RNA molecules increased due to random variation and selection, metabolic complexity began to emerge. From that stage, RNA had roles in both control of metabolism and continuity across generations, as it does today. Depending on which function one prefers to emphasize, these overall models have been called “Control First” or “Genetics First.” In either case, the proliferation of metabolism depended on RNA being there first.

Adherents have come to call the other possibility “Metabolism First” (though by this they have meant many slightly different things). In our version of Metabolism First, the earliest steps toward life required neither DNA nor RNA, and may not even have involved spatial compartments like cells; the earliest reactions could have occurred in the voids of porous rock, perhaps filled with organic gels deposited as suggested in the Oparin-Haldane model. We believe this early version of metabolism consisted of a series of simple chemical reactions running without the aid of complex enzymes, via the catalytic action of networks of small molecules, perhaps aided by naturally occurring minerals. If the network generated its own constituents—if it was recursive—it could serve as the core of a self-amplifying chemical system subject to selection. We propose that such a system arose and that much of that early core remains as the universal part of modern biochemistry, the reaction sequences shared by all living beings. Further elaborations would have been added to it as cells formed and came under RNA control, and as organisms specialized as participants in more complex ecosystems.

Networks of synthetic pathways that are recursive and self-catalyzing are widely known in organic chemistry, but they are notorious for generating a mass of side products, which may disrupt the reaction system or simply dilute the reactants, preventing them from accumulating within a pathway. The important feature necessary for chemical selection in such a network, which remains to be demonstrated, is feedback-driven self-pruning of side reactions, resulting in a limited suite of pathways capable of concentrating reagents as metabolism does. The search for such self-pruning is one of the most actively pursued research fronts in Metabolism First research.








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