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  • Print publication year: 2016
  • Online publication date: June 2016

6 - The emergence of a biosphere from geochemistry

Summary

The major divergence in approaches to biogenesis turns on the source and mechanisms of the first selectivity: whether it came from smallmolecule chemistry in a geochemical environment, or from a model of hierarchical control by macromolecules along the lines of Crick's Central Dogma. We argue that the emergence of life followed a path of least resistance along the “long arc of planetary disequilibrium,” involving the atmosphere, oceans, and dynamic mantle, described in Chapter 3. The major temporal stages followed the architectural layers of biochemistry described in Chapter 4. The first carbon fixation was mineral hosted. Feedbacks, initially via cofactors and later via oligomer catalysis, lifted core metabolism “off the rocks.” The emerging identity of the biosphere reflected the growth of autonomy as much as of chemical invention. Passage to an oligomer phase corresponding to the “RNA World” was a complex and heterogeneous transition, which transpired, and froze into place, in an already ordered organosynthetic context. We propose that cellularization occurred relatively late, and relied on functions of oligomers established in a mineral-hosted environment. The emergence of ribosomal translation originated in two parallel worlds of iron-RNA condensation-catalysis and template-directed ligation, which came together to form the first translation apparatus from mRNA to peptides. The refinement of translation fidelity, together with more precise RNA or DNA replication, ushered in the era of vertical descent along lines first appreciated by Carl Woese. Even in the era of evolution of effectively modern cells, many of the major transitions have been determined by biogeochemical reorganizations.

From universals to a path of biogenesis

In this chapter we suggest a sequence of stages in the emergence of life, based on the biological universals and the connection of life to the geochemical world, reviewed in the previous five chapters. We will propose that the hierarchy in extant life records a temporal sequence of stages, and that core biosynthetic pathways of living systems today reflect the pathways by which the same components first entered the incipient biosphere and dictated its form.

The emergence of a biosphere was an extended, and we believe a multistage and heterogeneous, sequence of transformations. In the early stages of this sequence, the processes that would become core biochemistry took shape but were not yet separate from a geochemical background.

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