The very early evolution of biological complexity.

All extant life is descended from a common ancestor, which, despite being very ancient, appears to have been a complex cellular organism. A new study by Moody et al. shows that this ancestor was not only a complex cell, but also lived within a microbial ecology likely inhabited by other complex cells.

Electron transport chains as a window into the earliest stages of evolution.

The origin and early evolution of life is generally studied under two different paradigms: bottom up and top down. Prebiotic chemistry and early Earth geochemistry allow researchers to explore possible origin of life scenarios. But for these "bottom-up" approaches, even successful experiments only amount to a proof of principle. On the other hand, "top-down" research on early evolutionary history is able to provide a historical account about ancient organisms, but is unable to investigate stages that occurred during and just after the origin of life. Here, we consider ancient electron transport chains (ETCs) as a potential bridge between early evolutionary history and a protocellular stage that preceded it. Current phylogenetic evidence suggests that ancestors of several extant ETC components were present at least as late as the last universal common ancestor of life. In addition, recent experiments have shown that some aspects of modern ETCs can be replicated by minerals, protocells, or organic cofactors in the absence of biological proteins. Here, we discuss the diversity of ETCs and other forms of chemiosmotic energy conservation, describe current work on the early evolution of membrane bioenergetics, and advocate for several lines of research to enhance this understanding by pairing top-down and bottom-up approaches.

Scaling laws in enzyme function reveal a new kind of biochemical universality.

All life on Earth is unified by its use of a shared set of component chemical compounds and reactions, providing a detailed model for universal biochemistry. However, this notion of universality is specific to known biochemistry and does not allow quantitative predictions about examples not yet observed. Here, we introduce a more generalizable concept of biochemical universality that is more akin to the kind of universality found in physics. Using annotated genomic datasets including an ensemble of 11,955 metagenomes, 1,282 archaea, 11,759 bacteria, and 200 eukaryotic taxa, we show how enzyme functions form universality classes with common scaling behavior in their relative abundances across the datasets. We verify that these scaling laws are not explained by the presence of compounds, reactions, and enzyme functions shared across known examples of life. We demonstrate how these scaling laws can be used as a tool for inferring properties of ancient life by comparing their predictions with a consensus model for the last universal common ancestor (LUCA). We also illustrate how network analyses shed light on the functional principles underlying the observed scaling behaviors. Together, our results establish the existence of a new kind of biochemical universality, independent of the details of life on Earth's component chemistry, with implications for guiding our search for missing biochemical diversity on Earth or for biochemistries that might deviate from the exact chemical makeup of life as we know it, such as at the origins of life, in alien environments, or in the design of synthetic life.

Introduction to the Special Issue on Early Evolution and the Last Common Ancestor.

The early evolution of life spans an extensive period preceding the emergence of the first eukaryotic cell. This epoch, which transpired from 4.5 to 2.5 billion years ago, marked the advent of many fundamental cellular attributes and witnessed the existence of the Last Common Ancestor (LCA) of all life forms. Uncovering and reconstructing this elusive LCA's characteristics and genetic makeup represents a formidable challenge and a pivotal pursuit in early evolution. While most scientific accounts concur that the LCA resembles contemporary prokaryotes, its precise definition, genome composition, metabolic capabilities, and ecological niche remain subjects of contentious debate.

Evolution of Cellular Organization Along the First Branches of the Tree of Life.

Current evidence suggests that some form of cellular organization arose well before the time of the last universal common ancestor (LUCA). Standard phylogenetic analyses have shown that several protein families associated with membrane translocation, membrane transport, and membrane bioenergetics were very likely present in the proteome of the LUCA. Despite these cellular systems emerging prior to the LUCA, extant archaea, bacteria, and eukaryotes have significant differences in cellular infrastructure and the molecular functions that support it, leading some researchers to argue that true cellularity did not evolve until after the LUCA. Here, we use recently reconstructed minimal proteomes of the LUCA as well as the last archaeal common ancestor (LACA) and the last bacterial common ancestor (LBCA) to characterize the evolution of cellular systems along the first branches of the tree of life. We find that a broad set of functions associated with cellular organization were already present by the time of the LUCA. The functional repertoires of the LACA and LBCA related to cellular organization nearly doubled along each branch following the divergence of the LUCA. These evolutionary trends created the foundation for similarities and differences in cellular organization between the taxonomic domains that are still observed today.

Informatic Capabilities of Translation and Its Implications for the Origins of Life.

The ability to encode and convert heritable information into molecular function is a defining feature of life as we know it. The conversion of information into molecular function is performed by the translation process, in which triplets of nucleotides in a nucleic acid polymer (mRNA) encode specific amino acids in a protein polymer that folds into a three-dimensional structure. The folded protein then performs one or more molecular activities, often as one part of a complex and coordinated physiological network. Prebiotic systems, lacking the ability to explicitly translate information between genotype and phenotype, would have depended upon either chemosynthetic pathways to generate its components-constraining its complexity and evolvability- or on the ambivalence of RNA as both carrier of information and of catalytic functions-a possibility which is still supported by a very limited set of catalytic RNAs. Thus, the emergence of translation during early evolutionary history may have allowed life to unmoor from the setting of its origin. The origin of translation machinery also represents an entirely novel and distinct threshold of behavior for which there is no abiotic counterpart-it could be the only known example of computing that emerged naturally at the chemical level. Here we describe translation machinery's decoding system as the basis of cellular translation's information-processing capabilities, and the four operation types that find parallels in computer systems engineering that this biological machinery exhibits.

Very early evolution from the perspective of microbial ecology.

The universal ancestor at the root of the species tree of life depicts a population of organisms with a surprising degree of complexity, posessing genomes and translation systems much like that of microbial life today. As the first life forms were most likely to have been simple replicators, considerable evolutionary change must have taken place prior to the last universal common ancestor. It is often assumed that the lack of earlier branches on the tree of life is due to a prevalence of random horizontal gene transfer that obscured the delineations between lineages and hindered their divergence. Therefore, principles of microbial evolution and ecology may give us some insight into these early stages in the history of life. Here, we synthesize the current understanding of organismal and genome evolution from the perspective of microbial ecology and apply these evolutionary principles to the earliest stages of life on Earth. We focus especially on broad evolutionary modes pertaining to horizontal gene transfer, pangenome structure, and microbial mat communities.

Nucleobases in Meteorites to Nucleobases in RNA and DNA?

Nucleic acids likely played a foundational role in the origin of life. However, the prebiotic chemistry of nucleoside and nucleotide synthesis has proved challenging on a number of fronts. The recent discovery of both pyrimidine and purine nucleobases in carbonaceous chondrite meteorites has garnered much attention from both the popular press and the scientific community. Here, we discuss these findings in the context of nucleoside/nucleotide prebiotic chemistry. We consider that the main challenge of prebiotic nucleoside synthesis, that of nucleosidic bond formation, is not addressed by the identification nucleobases in meteorites. We further discuss issues of selection that arise from the observation that such meteorites contain both canonical and non-canonical nucleobases. In sum, we argue that, despite the major analytical achievement of identifying and characterizing nucleobases in meteorites, this observation does little to advance our understanding of the prebiotic chemistry that could have led to the first genetic molecules that gave rise to us.

Cofactors are Remnants of Life's Origin and Early Evolution.

The RNA World is one of the most widely accepted hypotheses explaining the origin of the genetic system used by all organisms today. It proposes that the tripartite system of DNA, RNA, and proteins was preceded by one consisting solely of RNA, which both stored genetic information and performed the molecular functions encoded by that genetic information. Current research into a potential RNA World revolves around the catalytic properties of RNA-based enzymes, or ribozymes. Well before the discovery of ribozymes, Harold White proposed that evidence for a precursor RNA world could be found within modern proteins in the form of coenzymes, the majority of which contain nucleobases or nucleoside moieties, such as Coenzyme A and S-adenosyl methionine, or are themselves nucleotides, such as ATP and NADH (a dinucleotide). These coenzymes, White suggested, had been the catalytic active sites of ancient ribozymes, which transitioned to their current forms after the surrounding ribozyme scaffolds had been replaced by protein apoenzymes during the evolution of translation. Since its proposal four decades ago, this groundbreaking hypothesis has garnered support from several different research disciplines and motivated similar hypotheses about other classes of cofactors, most notably iron-sulfur cluster cofactors as remnants of the geochemical setting of the origin of life. Evidence from prebiotic geochemistry, ribozyme biochemistry, and evolutionary biology, increasingly supports these hypotheses. Certain coenzymes and cofactors may bridge modern biology with the past and can thus provide insights into the elusive and poorly-recorded period of the origin and early evolution of life.

The Coevolution of Cellularity and Metabolism Following the Origin of Life.

The emergence of cellular organisms occurred sometime between the origin of life and the evolution of the last universal common ancestor and represents one of the major transitions in evolutionary history. Here we describe a series of artificial life simulations that reveal a close relationship between the evolution of cellularity, the evolution of metabolism, and the richness of the environment. When environments are rich in processing energy, a resource that the digital organisms require to both process their genomes and replicate, populations evolve toward a state of non-cellularity. But when processing energy is not readily available in the environment and organisms must produce their own processing energy from food puzzles, populations always evolve both a proficient metabolism and a high level of cellular impermeability. Even between these two environmental extremes, the population-averaged values of cellular impermeability and metabolic proficiency exhibit a very strong correlation with one another. Further investigations show that non-cellularity is selectively advantageous when environmental processing energy is abundant because it allows organisms to access the available energy, while cellularity is selectively advantageous when environmental processing energy is scarce because it affords organisms the genetic fidelity required to incrementally evolve efficient metabolisms. The selection pressures favoring either non-cellularity or cellularity can be reversed when the environment transitions from one of abundant processing energy to one of scarce processing energy. These results have important implications for when and why cellular organisms evolved following the origin of life.

Reactivity of Metabolic Intermediates and Cofactor Stability under Model Early Earth Conditions.

Understanding the emergence of metabolic pathways is key to unraveling the factors that promoted the origin of life. One popular view is that protein cofactors acted as catalysts prior to the evolution of the protein enzymes with which they are now associated. We investigated the stability of acetyl coenzyme A (Acetyl Co-A, the group transfer cofactor in citric acid synthesis in the TCA cycle) under early Earth conditions, as well as whether Acetyl Co-A or its small molecule analogs thioacetate or acetate can catalyze the transfer of an acetyl group onto oxaloacetate in the absence of the citrate synthase enzyme. Several different temperatures, pH ranges, and compositions of aqueous environments were tested to simulate the Earth's early ocean and its possible components; the effect of these variables on oxaloacetate and cofactor chemistry were assessed under ambient and anoxic conditions. The cofactors tested are chemically stable under early Earth conditions, but none of the three compounds (Acetyl Co-A, thioacetate, or acetate) promoted synthesis of citric acid from oxaloacetate under the conditions tested. Oxaloacetate reacted with itself and/or decomposed to form a sequence of other products under ambient conditions, and under anoxic conditions was more stable; under ambient conditions the specific chemical pathways observed depended on the environmental conditions such as pH and presence/absence of bicarbonate or salt ions in early Earth ocean simulants. This work demonstrates the stability of these metabolic intermediates under anoxic conditions. However, even though free cofactors may be stable in a geological environmental setting, an enzyme or other mechanism to promote reaction specificity would likely be necessary for at least this particular reaction to proceed.

The Oxytricha trifallax macronuclear genome: a complex eukaryotic genome with 16,000 tiny chromosomes.

The macronuclear genome of the ciliate Oxytricha trifallax displays an extreme and unique eukaryotic genome architecture with extensive genomic variation. During sexual genome development, the expressed, somatic macronuclear genome is whittled down to the genic portion of a small fraction (∼5%) of its precursor "silent" germline micronuclear genome by a process of "unscrambling" and fragmentation. The tiny macronuclear "nanochromosomes" typically encode single, protein-coding genes (a small portion, 10%, encode 2-8 genes), have minimal noncoding regions, and are differentially amplified to an average of ∼2,000 copies. We report the high-quality genome assembly of ∼16,000 complete nanochromosomes (∼50 Mb haploid genome size) that vary from 469 bp to 66 kb long (mean ∼3.2 kb) and encode ∼18,500 genes. Alternative DNA fragmentation processes ∼10% of the nanochromosomes into multiple isoforms that usually encode complete genes. Nucleotide diversity in the macronucleus is very high (SNP heterozygosity is ∼4.0%), suggesting that Oxytricha trifallax may have one of the largest known effective population sizes of eukaryotes. Comparison to other ciliates with nonscrambled genomes and long macronuclear chromosomes (on the order of 100 kb) suggests several candidate proteins that could be involved in genome rearrangement, including domesticated MULE and IS1595-like DDE transposases. The assembly of the highly fragmented Oxytricha macronuclear genome is the first completed genome with such an unusual architecture. This genome sequence provides tantalizing glimpses into novel molecular biology and evolution. For example, Oxytricha maintains tens of millions of telomeres per cell and has also evolved an intriguing expansion of telomere end-binding proteins. In conjunction with the micronuclear genome in progress, the O. trifallax macronuclear genome will provide an invaluable resource for investigating programmed genome rearrangements, complementing studies of rearrangements arising during evolution and disease.

A consensus view of the proteome of the last universal common ancestor.

The availability of genomic and proteomic data from across the tree of life has made it possible to infer features of the genome and proteome of the last universal common ancestor (LUCA). A number of studies have done so, all using a unique set of methods and bioinformatics databases. Here, we compare predictions across eight such studies and measure both their agreement with one another and with the consensus predictions among them. We find that some LUCA genome studies show a strong agreement with the consensus predictions of the others, but that no individual study shares a high or even moderate degree of similarity with any other individual study. From these observations, we conclude that the consensus among studies provides a more accurate depiction of the core proteome of the LUCA and its functional repertoire. The set of consensus LUCA protein family predictions between all of these studies portrays a LUCA genome that, at minimum, encoded functions related to protein synthesis, amino acid metabolism, nucleotide metabolism, and the use of common, nucleotide-derived organic cofactors.

Testing Abiotic Reduction of NAD+ Directly Mediated by Iron/Sulfur Minerals.

Life emerged in a geochemical context, possibly in the midst of mineral substrates. However, it is not known to what extent minerals and dissolved inorganic ions could have facilitated the evolution of biochemical reactions. Herein, we have experimentally shown that iron sulfide minerals can act as electron transfer agents for the reduction of the ubiquitous biological protein cofactor nicotinamide adenine dinucleotide (NAD+) under anaerobic prebiotic conditions, observing the NAD+/NADH redox transition by using ultraviolet-visible spectroscopy and 1H nuclear magnetic resonance. This reaction was mediated with iron sulfide minerals, which were likely abundant on early Earth in seafloor and hydrothermal settings; and the NAD+/NADH redox reaction occurred in the absence of UV light, peptide ligand(s), or dissolved mediators. To better understand this reaction, thermodynamic modeling was also performed. The ability of an iron sulfide mineral to transfer electrons to a biochemical cofactor that is found in every living cell demonstrates how geologic materials could have played a direct role in the evolution of certain cofactor-driven metabolic pathways.

Pseudomonas aeruginosa uses a cyclic-di-GMP-regulated adhesin to reinforce the biofilm extracellular matrix.

Pseudomonas aeruginosa, the principal pathogen of cystic fibrosis patients, forms antibiotic-resistant biofilms promoting chronic colonization of the airways. The extracellular (EPS) matrix is a crucial component of biofilms that provides the community multiple benefits. Recent work suggests that the secondary messenger, cyclic-di-GMP, promotes biofilm formation. An analysis of factors specifically expressed in P. aeruginosa under conditions of elevated c-di-GMP, revealed functions involved in the production and maintenance of the biofilm extracellular matrix. We have characterized one of these components, encoded by the PA4625 gene, as a putative adhesin and designated it cdrA. CdrA shares structural similarities to extracellular adhesins that belong to two-partner secretion systems. The cdrA gene is in a two gene operon that also encodes a putative outer membrane transporter, CdrB. The cdrA gene encodes a 220 KDa protein that is predicted to be rod-shaped protein harbouring a beta-helix structural motif. Western analysis indicates that the CdrA is produced as a 220 kDa proprotein and processed to 150 kDa before secretion into the extracellular medium. We demonstrated that cdrAB expression is minimal in liquid culture, but is elevated in biofilm cultures. CdrAB expression was found to promote biofilm formation and auto-aggregation in liquid culture. Aggregation mediated by CdrA is dependent on the Psl polysaccharide and can be disrupted by adding mannose, a key structural component of Psl. Immunoprecipitation of Psl present in culture supernatants resulted in co-immunoprecipitation of CdrA, providing additional evidence that CdrA directly binds to Psl. A mutation in cdrA caused a decrease in biofilm biomass and resulted in the formation of biofilms exhibiting decreased structural integrity. Psl-specific lectin staining suggests that CdrA either cross-links Psl polysaccharide polymers and/or tethers Psl to the cells, resulting in increased biofilm structural stability. Thus, this study identifies a key protein structural component of the P. aeruginosa EPS matrix.

Comprehensive computational analysis of Hmd enzymes and paralogs in methanogenic Archaea.

BACKGROUND: Methanogenesis is the sole means of energy production in methanogenic Archaea. H2-forming methylenetetrahydromethanopterin dehydrogenase (Hmd) catalyzes a step in the hydrogenotrophic methanogenesis pathway in class I methanogens. At least one hmd paralog has been identified in nine of the eleven complete genome sequences of class I hydrogenotrophic methanogens. The products of these paralog genes have thus far eluded any detailed functional characterization. RESULTS: Here we present a thorough computational analysis of Hmd enzymes and paralogs that includes state of the art phylogenetic inference, structure prediction, and functional site prediction techniques. We determine that the Hmd enzymes are phylogenetically distinct from Hmd paralogs but share a common overall structure. We predict that the active site of the Hmd enzyme is conserved as a functional site in Hmd paralogs and use this observation to propose possible molecular functions of the paralog that are consistent with previous experimental evidence. We also identify an uncharacterized site in the N-terminal domains of both proteins that is predicted by our methods to directly impart function. CONCLUSION: This study contributes to our understanding of the evolutionary history, structural conservation, and functional roles, of the Hmd enzymes and paralogs. The results of our phylogenetic and structural analysis constitute datasets that will aid in the future study of the Hmd protein family. Our functional site predictions generate several testable hypotheses that will guide further experimental characterization of the Hmd paralog. This work also represents a novel approach to protein function prediction in which multiple computational methods are integrated to achieve a detailed characterization of proteins that are not well understood.