Are Molecular Alphabets Universal Enabling Factors for the Evolution of Complex Life?

Abstract

Terrestrial biosystems depend on macromolecules, and this feature is often considered as a likely universal aspect of life. While opinions differ regarding the importance of small-molecule systems in abiogenesis, escalating biological functional demands are linked with increasing complexity in key molecules participating in biosystem operations, and many such requirements cannot be efficiently mediated by relatively small compounds. It has long been recognized that known life is associated with the evolution of two distinct molecular alphabets (nucleic acid and protein), specific sequence combinations of which serve as informational and functional polymers. In contrast, much less detailed focus has been directed towards the potential universal need for molecular alphabets in constituting complex chemically-based life, and the implications of such a requirement. To analyze this, emphasis here is placed on the generalizable replicative and functional characteristics of molecular alphabets and their concatenates. A primary replicative alphabet based on the simplest possible molecular complementarity can potentially enable evolutionary processes to occur, including the encoding of secondarily functional alphabets. Very large uniquely specified (‘non-alphabetic’) molecules cannot feasibly underlie systems capable of the replicative and evolutionary properties which characterize complex biosystems. Transitions in the molecular evolution of alphabets can be related to progressive bridging of barriers which enable higher levels of biosystem organization. It is thus highly probable that molecular alphabets are an obligatory requirement for complex chemically-based life anywhere in the universe. In turn, reference to molecular alphabets should be usefully applied in current definitions of life.

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Notes

  1. 1.

    Secondary structural effects (ultimately dictated by primary sequences) within concatenates may potentially alter properties of alphabet residues, as shown by profound deviations in pKa values for specific nucleobases in certain ribozymes (Wilcox et al. 2011).

  2. 2.

    The enzyme 4-oxalocrotonate tautomerase is encoded as only 62 residues, but the active form consists of a homopentamer of this subunit (Chen et al. 1992).

  3. 3.

    For the RNA alphabet, for example, the core set involves synthesis of the four NTPs.

  4. 4.

    If the two alphabets were mutually orthogonal, they would at least initially compete for resources and replicative predominance, favoring their re-segregation into separate biosystems, or preventing their co-operation in the first place. Orthogonal systems could in principle develop independent biological trajectories, as proposed for ‘shadow biosphere’ models (Davies et al. 2009).

  5. 5.

    Within the solar system, interplanetary exchange of materials could hypothetically transfer biologically relevant samples capable of blurring true life independence (Nicholson 2009). On a much broader cosmic scale, proponents of panspermia (Wickramasinghe 2004) suggest that widespread dispersal of basic biologicsl ‘seeds’ has initiated life on Earth and (by inference) the same process is likely to be responsible for life elsewhere. Since neither factual evidence nor compelling scientific argument for this exists, it will not be considered further here.

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Acknowledgments

I thank Matt Lawler for many valuable comments on the manuscript, and Jim Kurnick for provision of materials.

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Dunn, I.S. Are Molecular Alphabets Universal Enabling Factors for the Evolution of Complex Life?. Orig Life Evol Biosph 43, 445–464 (2013). https://doi.org/10.1007/s11084-014-9354-9

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Keywords

  • Molecular alphabets
  • Life definitions
  • Complementarity
  • Complexity