Origin of Life: RNA World versus Autocatalytic Anabolism
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Origin of Life: RNA World versus Autocatalytic Anabolism GÜNTER WÄCHTERSHÄUSER
Introduction The deep past of the earth is unobservable. Therefore, the problem of the origin of life, the emergence of the first evolvable entity, which is the primordial ancestor of all extant organisms, can only be solved by a theory. Theories on the origin of life are scientific rather than myth, if they have empirical significance: empirical biological significance by providing evolutionary explanations for extant facts of biology and/or empirical chemical significance by predicting unknown but testable chemical reactions. Preferably, they also should have geological significance by being compatible with geological theories on the early history of the earth, which themselves, however, need to be scrutinized for their power to explain and predict facts of geology and chemistry. These requirements follow from Popper’s methodology of science (Popper, 1959; Popper, 1963; Popper, 1972). They constitute the challenge of the field. Only two theories on the origin of life are detailed enough for an evaluation by the above criteria: 1) The RNA world theory (Joyce, 1989; Gesteland et al., 1999) assumes that the first organism was a “living” RNA-like molecule replicating in a prebiotic broth of activated nucleotides. 2) The autocatalytic anabolist theory (Wächtershäuser, 1988; Wächtershäuser, 1990; Wächtershäuser, 1992; Wächtershäuser, 1997; Wächtershäuser, 2000) assumes that life began on minerals with an anabolic metabolism of synthetic, autocatalytic carbon fixation cycles. These two theories will be discussed here in a comparative manner. For a detailed list of references and for a description of further studies, the readers should consult the available comprehensive reviews (Bengtson, 1994; Brack, 1998; deDuve, 1991; Fry, 2000; Lahav, 1999; Smith and Szathmary, 1995; Zubay, 2000). Given the huge body of literature, the specific
references given here cannot be anything but incomplete; the emphasis is placed on references that are extraordinarily relevant for the present discussion, or very recent, or effectively forgotten.
Nutrients and Energy All organisms require nutrients and energy for growth and reproduction. In the RNA world theory, activated nucleotides are required as nutrients. Because these nucleotides are quite complex, the theory is forced to assume a rather protracted “prebiotic chemistry” for generating a prebiotic pool of activated nucleotides. There is some experimental support, most notably the formation of adenine from HCN (Oró, 1960) and the formation of pyrimidines from cyanoacetylene (Ferris et al., 1968). However, the various reactions reported require different and mutually incompatible reaction conditions (Shapiro, 1986). A further difficulty of this approach resides in high dilution and in the destructive force of hydrolysis, notably in a hot prebiotic broth. As a solution of this problem an adsorption to minerals,
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