Origins and emergent evolution of life: the colloid microsphere hypothesis revisited

An astrobiological perspective on microbial biofilms: their importance for habitability and production of detectable and lasting biosignatures

The search for life elsewhere in the universe has remained one of the main goals of astrobiological exploration. In this quest, extreme environments on Earth have served as analogs to study the potential habitability of Mars and icy moons, which include but are not limited to hydrothermal vent systems, acid lakes, deserts, and polar ice, among others. Within the various forms that life manifests, biofilms constitute one of the most widespread phenotypes and are ubiquitous in extreme environments. Biofilms are structured communities of microorganisms enclosed in a matrix of extracellular polymeric substances (EPS) that protect against unfavorable and dynamic conditions. These concentrated structures and their associated chemistry may serve as unique and persistent signatures of life processes that may aid in their detection. Here we propose biofilms as a model system to understand the habitability of extraterrestrial systems and as sources of recognizable and persistent biosignatures for life detection. By testing these ideas in extreme analog environments on Earth, this approach could be used to guide and focus future exploration of samples encompassing the geologic record of early Earth as well as other planets and moons of our solar system.

Protocells: Milestones and Recent Advances

The origin of life is still one of humankind's great mysteries. At the transition between nonliving and living matter, protocells, initially featureless aggregates of abiotic matter, gain the structure and functions necessary to fulfill the criteria of life. Research addressing protocells as a central element in this transition is diverse and increasingly interdisciplinary. The authors review current protocell concepts and research directions, address milestones, challenges and existing hypotheses in the context of conditions on the early Earth, and provide a concise overview of current protocell research methods.

Carbon dioxide photoreduction in prebiotic environments

The reduction of carbon dioxide is one of the hottest topics due to the concern of global warming. Carbon dioxide reduction is also an essential step for life's origins as photoautotrophs arose soon after Earth formation. Both the topics are of high general interest, and possibly, there could be a fruitful cross-fertilization of the two fields. Herein, we selected and collected papers related to photoreduction of carbon dioxide using compounds easily available on the Earth and considered of prebiotic relevance. This work might be useful also to scientists interested in carbon dioxide photoreduction and/or to have an overview of the techniques available.

Cooperative formation of porous silica and peptides on the prebiotic Earth

Modern technology has perfected the synthesis of catalysts such as zeolites and mesoporous silicas using organic structure directing agents (SDA) and their industrial use to catalyze a large variety of organic reactions within their pores. We suggest that early in prebiotic evolution, synergistic interplay arose between organic species in aqueous solution and silica formed from rocks by dynamic dissolution-recrystallization. The natural organics, for example, amino acids, small peptides, and fatty acids, acted as SDA for assembly of functional porous silica structures that induced further polymerization of amino acids and peptides, as well as other organic reactions. Positive feedback between synthesis and catalysis in the silica-organic system may have accelerated the early stages of abiotic evolution by increasing the formation of polymerized species.

Prebiotic Peptides: Molecular Hubs in the Origin of Life

The fundamental roles that peptides and proteins play in today's biology makes it almost indisputable that peptides were key players in the origin of life. Insofar as it is appropriate to extrapolate back from extant biology to the prebiotic world, one must acknowledge the critical importance that interconnected molecular networks, likely with peptides as key components, would have played in life's origin. In this review, we summarize chemical processes involving peptides that could have contributed to early chemical evolution, with an emphasis on molecular interactions between peptides and other classes of organic molecules. We first summarize mechanisms by which amino acids and similar building blocks could have been produced and elaborated into proto-peptides. Next, non-covalent interactions of peptides with other peptides as well as with nucleic acids, lipids, carbohydrates, metal ions, and aromatic molecules are discussed in relation to the possible roles of such interactions in chemical evolution of structure and function. Finally, we describe research involving structural alternatives to peptides and covalent adducts between amino acids/peptides and other classes of molecules. We propose that ample future breakthroughs in origin-of-life chemistry will stem from investigations of interconnected chemical systems in which synergistic interactions between different classes of molecules emerge.

Twenty Years of "Lipid World": A Fertile Partnership with David Deamer

"The Lipid World" was published in 2001, stemming from a highly effective collaboration with David Deamer during a sabbatical year 20 years ago at the Weizmann Institute of Science in Israel. The present review paper highlights the benefits of this scientific interaction and assesses the impact of the lipid world paper on the present understanding of the possible roles of amphiphiles and their assemblies in the origin of life. The lipid world is defined as a putative stage in the progression towards life's origin, during which diverse amphiphiles or other spontaneously aggregating small molecules could have concurrently played multiple key roles, including compartment formation, the appearance of mutually catalytic networks, molecular information processing, and the rise of collective self-reproduction and compositional inheritance. This review brings back into a broader perspective some key points originally made in the lipid world paper, stressing the distinction between the widely accepted role of lipids in forming compartments and their expanded capacities as delineated above. In the light of recent advancements, we discussed the topical relevance of the lipid worldview as an alternative to broadly accepted scenarios, and the need for further experimental and computer-based validation of the feasibility and implications of the individual attributes of this point of view. Finally, we point to possible avenues for exploring transition paths from small molecule-based noncovalent structures to more complex biopolymer-containing proto-cellular systems.

Nanotube-Mediated Path to Protocell Formation

Cellular compartments are membrane-enclosed, spatially distinct microenvironments that confine and protect biochemical reactions in the biological cell. On the early Earth, the autonomous formation of compartments is thought to have led to the encapsulation of nucleotides, thereby satisfying a starting condition for the emergence of life. Recently, surfaces have come into focus as potential platforms for the self-assembly of prebiotic compartments, as significantly enhanced vesicle formation was reported in the presence of solid interfaces. The detailed mechanism of such formation at the mesoscale is still under discussion. We report here on the spontaneous transformation of solid-surface-adhered lipid deposits to unilamellar membrane compartments through a straightforward sequence of topological changes, proceeding via a network of interconnected lipid nanotubes. We show that this transformation is entirely driven by surface-free energy minimization and does not require hydrolysis of organic molecules or external stimuli such as electrical currents or mechanical agitation. The vesicular structures take up and encapsulate their external environment during formation and can subsequently separate and migrate upon exposure to hydrodynamic flow. This may link the self-directed transition from weakly organized bioamphiphile assemblies on solid surfaces to protocells with secluded internal contents.

Acytota - associated kingdom of neglected life

There is a huge variety of RNA- and DNA-containing entities that multiply within and propagate between cells across all kingdoms of life, having no cells of their own. Apart from cellular organisms, these entities (viroids, plasmids, mobile elements and viruses among others) are the only ones with distinct genetic identities but which are not included in any traditional tree of life. We suggest to introduce or, rather, revive the distinct category of acellular organisms, Acytota, as an additional, undeservedly ignored full-fledged kingdom of life. Acytota are indispensable players in cellular life and its evolution. The six traditional kingdoms (Cytota) and Acytota together complete the classification of the biological world (Biota), leaving nothing beyond.

Current Ideas about Prebiological Compartmentalization

Contemporary biological cells are highly sophisticated dynamic compartment systems which separate an internal volume from the external medium through a boundary, which controls, in complex ways, the exchange of matter and energy between the cell's interior and the environment. Since such compartmentalization is a fundamental principle of all forms of life, scenarios have been elaborated about the emergence of prebiological compartments on early Earth, in particular about their likely structural characteristics and dynamic features. Chemical systems that consist of potentially prebiological compartments and chemical reaction networks have been designed to model pre-cellular systems. These systems are often referred to as "protocells". Past and current protocell model systems are presented and compared. Since the prebiotic formation of cell-like compartments is directly linked to the prebiotic availability of compartment building blocks, a few aspects on the likely chemical inventory on the early Earth are also summarized.