Integrative Perspectives: In Quest of a Coherent Framework for Origins of Life on Earth

Abstract

By taking an overarching approach, this chapter connects various leads from bottom-up and top-down considerations about origins-of-life research. As no firmly accepted standard model for the emergence and early evolution of life on Earth has yet been established, rethinking of old problems and reevaluation from first principles is given precedence over unreflected repetition of widely held assumptions. In brief, a chain of bottom-up inference focuses on tentative transitions from geochemical pore space reactor conditions in upper sedimentary layers, exposed to sunlight and wet/drying cycles. Photo-active metal sulfides, catalytic minerals, organic heterocyclic cofactors, and short prebiotic peptides are assumed to precede the emergence of oligonucleotides. Oligo-ribonucleotides, in turn, speeded up the generation of more and longer stochastic peptides, later on to be replaced by coded protein synthesis. Up to the level of a peptide/protein-assisted RNA world scenario, a primarily photoautotrophic molecular ecosystem is assumed to develop through a range of sessile precellular stages. Conceptionally, early protocells would resemble certain plasmodial/syncytial organisms, rather than free-living microbial cells. The generation and “escape” of genuine cells as quasi-autonomous and propagative entities had to await the assembly of DNA-based integral genomes. Such microbial cells of prokaryotic life style diverged in several rounds. Remnants of the communal precellular systems, however, did not die out completely, but organized themselves as more sluggishly evolving cell-like organisms of a different kind. Specializing in recycling of particulate organic matter, these larger organisms retained and perfected much higher degrees of subcellular compartmentalization and cytoskeletal infrastructure, which directly gave rise to the proto-eukaryotic lineage.