Conceptual challenges for the emergence of the biological system: Cell theory and self-replication

Decoding viruses: An alternative perspective on their history, origins and role in nature

This article provides an alternative perspective on viruses, exploring their origins, ecology, and evolution. Viruses are recognized as the most prevalent biological entities on Earth, permeating nearly all environments and forming the virosphere-a significant biological layer. They play a crucial role in regulating bacterial populations within ecosystems and holobionts, influencing microbial communities and nutrient recycling. Viruses are also key drivers of molecular evolution, actively participating in the maintenance and regulation of ecosystems and cellular organisms. Many eukaryotic genomes contain genomic elements with viral origins, which contribute to organismal equilibrium and fitness. Viruses are involved in the generation of species-specific orphan genes, facilitating adaptation and the development of unique traits in biological lineages. They have been implicated in the formation of vital structures like the eukaryotic nucleus and the mammalian placenta. The presence of virus-specific genes absent in cellular organisms suggests that viruses may pre-date cellular life. Like progenotes, viruses are ribonucleoprotein entities with simpler capsid architectures compared to proteolipidic membranes. This article presents a comprehensive scenario describing major transitions in prebiotic evolution and proposes that viruses emerged prior to the Last Universal Common Ancestor (LUCA) during the progenote era. However, it is important to note that viruses do not form a monophyletic clade, and many viral taxonomic groups originated more recently as reductions of cellular structures. Thus, viral architecture should be seen as an ancient and evolutionarily stable strategy adopted by biological systems. The goal of this article is to reshape perceptions of viruses, highlighting their multifaceted significance in the complex tapestry of life and fostering a deeper understanding of their origins, ecological impact, and evolutionary dynamics.

Origin of life: Drawing the big picture

Trying to provide a broad overview about the origin of life in Earth, the most significant transitions of life before cells are listed and discussed. The current approach emphasizes the symbiotic relationships that emerged with life. We propose a rational, stepwise scenario for the origin of life that starts with the origin of the first biomolecules and steps forward until the origins of the first cells. Along this path, we aim to provide a brief, though comprehensive theoretical model that will consider the following steps: (i) how nucleotides and other biomolecules could be made prebiotically in specific prebiotic refuges; (ii) how the first molecules of RNAs were formed; (iii) how the proto-peptidyl transferase center was built by the concatenation of proto-tRNAs; (iv) how the ribosome and the genetic code could be structured; (v) how progenotes could live and reproduce as "naked" ribonucleoprotein molecules; (vi) how peptides started to bind molecules in the prebiotic soup allowing biochemical pathways to evolve from those bindings; (vii) how genomes got bigger by the symbiotic relationship of progenotes and lateral transference of genetic material; (viii) how the progenote LUCA has been formed by assembling most biochemical routes; (ix) how the first virion capsids probably emerged and evolved; (x) how phospholipid membranes emerged probably twice by the evolution of lipid-binding proteins; (xi) how DNA synthesis have been formed in parallel in Bacteria and Archaea; and, finally, (xii) how DNA-based cells of Bacteria and Archaeabacteria have been constituted. The picture provided is conjectural and present epistemological gaps. Future research will help to advance into the elucidation of gaps and confirmation/refutation of current statements.

RNP-world: The ultimate essence of life is a ribonucleoprotein process

The fundamental essence of life is based on process of interaction between nucleic acids and proteins. In a prebiotic world, amino acids, peptides, ions, and other metabolites acted in protobiotic routes at the same time on which RNAs performed catalysis and self-replication. Nevertheless, it was only when nucleic acids and peptides started to interact together in an organized process that life emerged. First, the ignition was sparked with the formation of a Peptidyl Transferase Center (PTC), possibly by concatenation of proto-tRNAs. This molecule that would become the catalytic site of ribosomes started a process of self-organization that gave origin to a protoorganism named FUCA, a ribonucleic ribosomal-like apparatus capable to polymerize amino acids. In that sense, we review hypotheses about the origin and early evolution of the genetic code. Next, populations of open biological systems named progenotes were capable of accumulating and exchanging genetic material, producing the first genomes. Progenotes then evolved in two paths: some presented their own ribosomes and others used available ribosomes in the medium to translate their encoded information. At some point, two different types of organisms emerged from populations of progenotes: the ribosome-encoding organisms (cells) and the capsid-encoding organisms (viruses).

The Theory of Chemical Symbiosis: A Margulian View for the Emergence of Biological Systems (Origin of Life)

The theory of chemical symbiosis (TCS) suggests that biological systems started with the collaboration of two polymeric molecules existing in early Earth: nucleic acids and peptides. Chemical symbiosis emerged when RNA-like nucleic acid polymers happened to fold into 3D structures capable to bind amino acids together, forming a proto peptidyl-transferase center. This folding catalyzed the formation of quasi-random small peptides, some of them capable to bind this ribozyme structure back and starting to form an initial layer that would produce the larger subunit of the ribosome by accretion. TCS suggests that there is no chicken-and-egg problem into the emergence of biological systems as RNAs and peptides were of equal importance to the origin of life. Life has initially emerged when these two macromolecules started to interact in molecular symbiosis. Further, we suggest that life evolved into progenotes and cells due to the emergence of new layers of symbiosis. Mutualism is the strongest force in biology, capable to create novelties by emergent principles; on which the whole is bigger than the sum of the parts. TCS aims to apply the Margulian view of biology into the origins of life field.

Is it possible that cells have had more than one origin?

Cells occupy a prominent place in the history of life in Earth. The central role of cellular organization can be understood by the fact that "cellular life" is often used as a synonym for life itself. Thus, most characteristics used to define cell overlap with those ones used to define life. However, innovative scenarios for the origin of life are bringing alternative views to describe how cells may have evolved from the open biological systems named progenotes. Here, using a logical and conceptual analysis, we re-evaluate the characteristics used to infer a single origin for cells. We argue that some evidences used to support cell monophyly, such as the presence of elements from the translation mechanism together with the universality of the genetic code, actually indicate a unique origin for all "biological systems", a term used to define not only cells, but also viruses and progenotes. Besides, we present evidence that at least two biochemical pathways as important as (i) DNA replication and (ii) lipid biosynthesis are not homologous between Bacteria and Archaea. The identities observed between the proteins involved in those pathways along representatives of these two ancestral domains of life are too low to indicate common genic ancestry. Altogether these facts can be seen as an indication that cellular organization has possibly evolved two or more times and that LUCA (the Last Universal Common Ancestor) may not have existed as a cellular entity. Thus, we aim to consider the possibility that different strategies acquired by biological systems to exist, such as viral, bacterial and archaeal were most likely originated independently from the evolution of different progenote populations.

Molecules to Microbes

How did life begin on Earth? And is there life elsewhere in the Cosmos? Challenging questions, indeed. The series of conferences established by NoR CEL in 2013 addresses these very questions. This paper comprises a summary report of oral presentations that were delivered by NoR CEL’s network members during the 2018 Athens conference and, as such, disseminates the latest research which they have put forward. More in depth material can be found by consulting the contributors referenced papers. Overall, the outcome of this conspectus on the conference demonstrates a case for the existence of “probable chemistry” during the prebiotic epoch.

Molecules to Microbes

How did life begin on Earth? And is there life elsewhere in the Cosmos? Challenging questions, indeed. The series of conferences established by NoR CEL in 2013, addresses these very same questions. The basis for this paper is the summary report of oral presentations that were delivered by NoR CEL’s network members during the 2018 Athens conference and, as such, disseminates the latest research which they have put forward. More in depth material can be found by consulting the contributors referenced papers. Overall, the outcome of this conspectus on the conference demonstrates a case for the existence of “probable chemistry” during the prebiotic epoch.

Molecules to Microbes

How did life begin on Earth? And is there life elsewhere in the Cosmos? Challenging questions, indeed. The series of conferences established by NoR CEL in 2013, addresses these very same questions. The basis for this paper is the summary report of oral presentations that were delivered by NoR CEL’s network members during the 2018 Athens conference and, as such, disseminates the latest research which they have put forward. More in depth material can be found by consulting the contributors referenced papers. Overall, the outcome of this conspectus on the conference demonstrates a case for the existence of “probable chemistry” during the prebiotic epoch.