The universal ancestor lived in a thermophilic or hyperthermophilic environment

Galtier et al. (Science 1999, 283, 220-221) exploit the correlation between the optimal growth temperature in prokaryotes and the G+C content of rRNAs and establish that the last universal common ancestor (LUCA) lived in a mesophilic environment. This result was achieved by estimating the G+C content of the ancestral sequences of the rRNAs of the LUCA through use of a complex Markov model. I have re-analysed their alignments of the rDNAs with maximum parsimony and I have found that their result is not robust and is, in all likelihood, incorrect. In particular, the rRNA ancestral sequences reconstructed with maximum parsimony from these rDNA alignments as well as those reconstructed after eliminating all the sites that turn out to be ambiguous to the parsimony algorithm and to a site-by-site inspection of these alignments, are such as to suggest that the LUCA lived in a thermophilic or hyperthermophilic environment. This finding is also supported by some tRNA ancestral sequences. The main conclusion of this analysis is that if the LUCA was a progenote then the origin of life might have taken place at a high temperature.     

The Universal Ancestor and the Ancestor of Bacteria Were Hyperthermophiles

The definition of the node of the last universal common ancestor (LUCA) is justified in a topology of the unrooted universal tree. This definition allows previous analyses based on paralogous proteins to be extended to orthologous ones. In particular, the use of a thermophily index (based on the amino acids propensity to enter the [hyper] thermophile proteins more frequently) and its correlation with the optimal growth temperature of the various organisms allow inferences to be made on the habitat in which the LUCA lived. The reconstruction of ancestral sequences by means of the maximum likelihood method and their attribution to the set of mesophilic or hyperthermophilic sequences have led to the following conclusions: the LUCA was a hyperthermophile organism, as were the ancestors of the Archaea and Bacteria domains, while the ancestor of the Eukarya domain was a mesophile. These conclusions are independent of the presence of hyperthermophile bacteria in the sample of sequences used in the analysis and are therefore independent of whether or not these are the first lines of divergence in the Bacteria domain, as observed in the topology of the universal tree of ribosomal RNA. These conclusions are thus more easily understood under the hypothesis that the origin of life took place at a high temperature.     

The universal ancestor was a thermophile or a hyperthermophile.

By exploiting the correlation between the optimal growth temperature of organisms and a thermophily index based on the propensity of amino acids to enter thermophile/hyperthermophile proteins, an analysis is conducted in order to establish whether the last universal common ancestor (LUCA) was a mesophile or a (hyper)thermophile. This objective is reached by using maximum parsimony and maximum likelihood to reconstruct the ancestral sequences of the LUCA for two pairs of sets of paralogous protein sequences by means of the phylogenetic tree topology derived from the small subunit ribosomal RNA, even if this is rooted in all three possible ways. The thermophily index of all the reconstructed ancestral sequences of the LUCA belongs to the set of the thermophile/hyperthermophile sequences, thus supporting the hypotheses that see the LUCA as a thermophile or a hyperthermophile.     

The universal ancestor was a thermophile or a hyperthermophile: tests and further evidence

The existence of a correlation between the optimal growth temperature of various organisms and a thermophily index (based on the propensity of amino acids to enter more frequently into the proteins of thermophiles/hyperthermophiles) allows inferences to be made on the mesophilic or thermophilic nature of the last universal common ancestor (LUCA). By reconstructing the ancestral sequences of the various ancestors using methods based on maximum likelihood and maximum parsimony, these sequences can be attributed to the mesophiles or (hyper)thermophiles and the following conclusions can be drawn. (1) There is no evidence that the LUCA might have been a mesophile and observations seem to imply that the LUCA was a thermophile or a hyperthermophile; (2) The ancestors of the Archaea and Bacteria domains seem to be (hyper)thermophiles while that of the Eukarya domain turns out to be a mesophile. These conclusions are independent of both (i) where the root is located on the topology of the universal tree (based on that of the small subunit ribosomal RNA) and (ii) the presence of hyperthermophile bacteria near the node of the Bacteria domain ancestor. These conclusions are easier to interpret in the light of the hypotheses that see the origin of life taking place at a high temperature.     

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.     

The molecular signal for the adaptation to cold temperature during early life on Earth

Several lines of evidence such as the basal location of thermophilic lineages in large-scale phylogenetic trees and the ancestral sequence reconstruction of single enzymes or large protein concatenations support the conclusion that the ancestors of the bacterial and archaeal domains were thermophilic organisms which were adapted to hot environments during the early stages of the Earth. A parsimonious reasoning would therefore suggest that the last universal common ancestor (LUCA) was also thermophilic. Various authors have used branch-wise non-homogeneous evolutionary models that better capture the variation of molecular compositions among lineages to accurately reconstruct the ancestral G + C contents of ribosomal RNAs and the ancestral amino acid composition of highly conserved proteins. They confirmed the thermophilic nature of the ancestors of Bacteria and Archaea but concluded that LUCA, their last common ancestor, was a mesophilic organism having a moderate optimal growth temperature. In this letter, we investigate the unknown nature of the phylogenetic signal that informs ancestral sequence reconstruction to support this non-parsimonious scenario. We find that rate variation across sites of molecular sequences provides information at different time scales by recording the oldest adaptation to temperature in slow-evolving regions and subsequent adaptations in fast-evolving ones.     

The evolution of the genetic code took place in an anaerobic environment

We have compared orthologous proteins from an aerobic organism, Cytophaga hutchinsonii, and from an obligate anaerobe, Bacteroides thetaiotaomicron. This comparison allows us to define the oxyphobic ranks of amino acids, i.e. a scale of the relative sensitivity to oxygen of the amino acid residues. The oxyphobic index (OI), which can be simply obtained from the amino acids' oxyphobic ranks, can be associated to any protein and therefore to the genetic code, if the number of synonymous codons attributed to the amino acids in the code is assumed to be the frequency with which the amino acids appeared in ancestral proteins. Sampling of the OI variable from the proteins of obligate anaerobes and aerobes has established that the OI value of the genetic code is not significantly different from the mean OI value of anaerobe proteins, while it is different from that of aerobe proteins. This observation would seem to suggest that the terminal phases of the evolution of genetic code organization took place in an anaerobic environment. This result is discussed in the framework of hypotheses suggested to explain the timing of the evolutionary appearance of the aerobic metabolism.     

The very early evolution of protein translocation across membranes

In this study, we used a computational approach to investigate the early evolutionary history of a system of proteins that, together, embed and translocate other proteins across cell membranes. Cell membranes comprise the basis for cellularity, which is an ancient, fundamental organizing principle shared by all organisms and a key innovation in the evolution of life on Earth. Two related requirements for cellularity are that organisms are able to both embed proteins into membranes and translocate proteins across membranes. One system that accomplishes these tasks is the signal recognition particle (SRP) system, in which the core protein components are the paralogs, FtsY and Ffh. Complementary to the SRP system is the Sec translocation channel, in which the primary channel-forming protein is SecY. We performed phylogenetic analyses that strongly supported prior inferences that FtsY, Ffh, and SecY were all present by the time of the last universal common ancestor of life, the LUCA, and that the ancestor of FtsY and Ffh existed before the LUCA. Further, we combined ancestral sequence reconstruction and protein structure and function prediction to show that the LUCA had an SRP system and Sec translocation channel that were similar to those of extant organisms. We also show that the ancestor of Ffh and FtsY that predated the LUCA was more similar to FtsY than Ffh but could still have comprised a rudimentary protein translocation system on its own. Duplication of the ancestor of FtsY and Ffh facilitated the specialization of FtsY as a membrane bound receptor and Ffh as a cytoplasmic protein that could bind nascent proteins with specific membrane-targeting signal sequences. Finally, we analyzed amino acid frequencies in our ancestral sequence reconstructions to infer that the ancestral Ffh/FtsY protein likely arose prior to or just after the completion of the canonical genetic code. Taken together, our results offer a window into the very early evolutionary history of cellularity.     

Globin-coupled sensors, protoglobins, and the last universal common ancestor

The strategy for detecting oxygen, carbon monoxide, nitric oxide, and sulfides is predominantly through heme-based sensors utilizing either a globin domain or a PAS domain. Whereas PAS domains bind various cofactors, globins bind only heme. Globin-coupled sensors (GCSs) were first described as regulators of the aerotactic responses in Bacillus subtilis and Halobacterium salinarum. GCSs were also identified in diverse microorganisms that appear to have roles in regulating gene expression. Functional and evolutionary analyses of the GCSs, their protoglobin ancestor, and their relationship to the last universal common ancestor (LUCA) are discussed in the context of globin-based signal transduction.     

The origin of DNA genomes and DNA replication proteins

In recent years, it has became clear that most proteins involved in cellular DNA precursor synthesis or DNA replication have been 'invented' more than once, indicating that the transition from RNA to DNA genomes was more complex than previously thought. Several authors have suggested that DNA viruses, which often encode their own version of these proteins, played an important role in this process. The nature of the genome of the last universal cellular ancestor (LUCA) -- that is, RNA or DNA, prokaryotic-like or eukaryotic-like -- remains in dispute. A hyperthermophilic LUCA would have suggested a circular, double-stranded DNA genome; however, recent data favor a mesophilic or moderately thermophilic LUCA.     

Combinations of Ancestral Modules in Proteins

Twenty-seven protein sequence elements, six to nine amino acids long, were extracted from 15 phylogenetically diverse complete prokaryotic proteomes. The elements are present in all of these proteomes, with at least one copy each (omnipresent elements), and have presumably been conserved since the last universal common ancestor (LUCA). All these omnipresent elements are identified in crystallized protein structures as parts of highly conserved closed loops, 25–30 residues long, thus representing the closed-loop modules discovered in 2000 by Berezovsky et al. The omnipresent peptides make up seven distinct groups, of which the largest groups, Aleph and Beth, contain 18 and four elements, respectively, which are related but different, while five other groups are represented by only one element each. The LUCA modules appear with one or several copies per protein molecule in a variety of combinations depending on the functional identity of the corresponding protein. The functional involvement of individual LUCA modules is outlined on the basis of known protein annotations. Analyses of all the related sequences in a large, formatted protein sequence space suggest that many, if not all, of the 27 omnipresent elements have a common sequence origin. This sequence space network analysis may lead to elucidation of the earliest stages of protein evolution.     

DNA Repair Systems in Archaea: Mementos from the Last Universal Common Ancestor?

DNA repair in the Archaea is relevant to the consideration of genome maintenance and replication fidelity in the last universal common ancestor (LUCA) from two perspectives. First, these prokaryotes embody a mix of bacterial and eukaryal molecular features. Second, DNA repair proteins would have been essential in LUCA to maintain genome integrity, regardless of the environmental temperature. Yet we know very little of the basic molecular mechanisms of DNA damage and repair in the Archaea in general. Many studies on DNA repair in archaea have been conducted with hyperthermophiles because of the additional stress imposed on their macromolecules by high temperatures. In addition, of the six complete archaeal genome sequences published so far, five are thermophilic archaea. We have recently shown that the hyperthermophile Pyrococcus furiosus has an extraordinarily high capacity for repair of radiation-induced double-strand breaks and we have identified and sequenced several genes involved in DNA repair in P. furiosus. At the sequence level, only a few genes share homology with known bacterial repair genes. For instance, our phylogenetic analysis indicates that archaeal recombinases occur in two paralogous gene families, one of which is very deeply branched, and both recombinases are more closely related to the eukaryotic RAD51 and Dmc1 gene families than to the Escherichia coli recA gene. We have also identified a gene encoding a repair endo/exonuclease in the genomes of several Archaea. The archaeal sequences are highly homologous to those of the eukaryotic Rad2 family and they cluster with genes of the FEN-1 subfamily, which are known to be involved in DNA replication and repair in eukaryotes. We argue that there is a commonality of mechanisms and protein sequences, shared between prokaryotes and eukaryotes for several modes of DNA repair, reflecting diversification from a minimal set of genes thought to represent the genome of the LUCA.     

Why the Lipid Divide? Membrane Proteins as Drivers of the Split between the Lipids of the Three Domains of Life

Recent results from engineered and natural samples show that the starkly different lipids of archaea and bacteria can form stable hybrid membranes. But if the two types can mix, why don't they? That is, why do most bacteria and all eukaryotes have only typically bacterial lipids, and archaea archaeal lipids? It is suggested here that the reason may lie on the other main component of cellular membranes: membrane proteins, and their close adaptation to the lipids. Archaeal lipids in modern bacteria could suggest that the last universal common ancestor (LUCA) had both lipid types. However, this would imply a rather elaborate evolutionary scenario, while negating simpler alternatives. In light of widespread horizontal gene transfer across the prokaryotic domains, hybrid membranes reveal that the lipid divide did not just occur once at the divergence of archaea and bacteria from LUCA. Instead, it continues to occur actively to this day. Also see the video abstract here https://youtu.be/TdKjxoDAtsg .     

Timing and evolution of the most recent common ancestor of the Korean clade HIV subtype B based on <Emphasis Type="Italic">Nef</Emphasis> and <Emphasis Type="Italic">Vif</Emphasis> sequences

Molecular phylogenetic studies of the HIV-1 isolated from Koreans have suggested the presence of the so-called “Korean clade”, which can be defined as a cluster free of foreign isolates. The Korean clade accounts for more than 60% of Korean isolates and exerts characteristic amino acid sequences. Thus, it is merited to estimate when this Korean clade first emerged in order to understand the evolutionary pattern of the Korean clade. We analyzed and reconstructed the most recent common ancestor (MRCA) sequences from nef (n=229) and vif (n=179) Korean clade sequences. Linear regression analyses of sequence divergence estimates were plotted against sampling years to infer the year in which there was zero divergence from the MRCA sequences. MRCA sequences suggested the Korean clade was first emerged around 1984, before the first detection of HIV-1 in Korea in 1985. Further studies on synonymous and nonsynonymous substitution rates suggested positive selection event for the Korean clade, while other subtype B had undergone negative to neutral evolution.     

The non-monophyletic origin of the tRNA molecule and the origin of genes only after the evolutionary stage of the last universal common ancestor (LUCA)

A model has been proposed suggesting that the tRNA molecule must have originated by direct duplication of an RNA hairpin structure [Di Giulio, M., 1992. On the origin of the transfer RNA molecule. J. Theor. Biol. 159, 199-214]. A non-monophyletic origin of this molecule has also been theorized [Di Giulio, M., 1999. The non-monophyletic origin of tRNA molecule. J. Theor. Biol. 197, 403-414]. In other words, the tRNA genes evolved only after the evolutionary stage of the last universal common ancestor (LUCA) through the assembly of two minigenes codifying for different RNA hairpin structures, which is what the exon theory of genes suggests when it is applied to the model of tRNA origin. Recent observations strongly corroborate this theorization because it has been found that some tRNA genes are completely separate in two minigenes codifying for the 5' and 3' halves of this molecule [Randau, L., et al., 2005a. Nanoarchaeum equitans creates functional tRNAs from separate genes for their 5'- and 3'-halves. Nature 433, 537-541]. In this paper it is shown that these tRNA genes codifying for the 5' and 3' halves of this molecule are the ancestral form from which the tRNA genes continuously codifying for the complete tRNA molecule are thought to have evolved. This, together with the very existence of completely separate tRNA genes codifying for their 5' and 3' halves, proves a non-monophyletic origin for tRNA genes, as a monophyletic origin would exclude the existence of these genes which have, on the contrary, been observed. Here the polyphyletic origin of genes codifying for proteins is also suggested and discussed. Moreover, a hypothesis is advanced to suggest that the LUCA might have had a fragmented genome made up of RNA and the possibility that 'Paleokaryotes' may exist is outlined. Finally, the characteristic of the indivisibility of homology that these polyphyletic origins seem to remove at the sequence level is discussed.     

On the origin of genomes and cells within inorganic compartments

Building on the model of Russell and Hall for the emergence of life at a warm submarine hydrothermal vent, we suggest that, within a hydrothermally formed system of contiguous iron-sulfide (FeS) compartments, populations of virus-like RNA molecules, which eventually encoded one or a few proteins each, became the agents of both variation and selection. The initial darwinian selection was for molecular self-replication. Combinatorial sorting of genetic elements among compartments would have resulted in preferred proliferation and selection of increasingly complex molecular ensembles--those compartment contents that achieved replication advantages. The last universal common ancestor (LUCA) we propose was not free-living but an inorganically housed assemblage of expressed and replicable genetic elements. The evolution of the enzymatic systems for (i) DNA replication; and (ii) membrane and cell wall biosynthesis, enabled independent escape of the first archaebacterial and eubacterial cells from their hydrothermal hatchery, within which the LUCA itself remained confined.     

The evolutionary relationships between the two bacteria Escherichia coli and Haemophilus influenzae and their putative last common ancestor

We have tried to approach the nature of the last common ancestor to Haemophilus influenzae and Escherichia coli and to determine how each bacterium could have diverged from this putative organism. The approach used was exhaustive analysis of the homologous proteins coded by genes present in these bacteria, using as criteria for sequence relatedness an alignment of at least 80 amino acid residues and a PAM distance (number of accepted point mutations per 100 residues separating two sequences) below 250. Evolutionarily significant similarities were found between 1,345 H. influenzae proteins (85% of the total genome) and 3,058 E. coli. proteins (75% of the total genome), many of them belonging to families of various sizes (from 666 doublets to 35 large groups of more than 10 members). Nearly all the genes found by this approach to be duplicated in both bacteria were already duplicated in their last common ancestor. This was deduced from (1) the comparison of the respective distributions of evolutionary distances between orthologs (genes separated only by speciation events) and paralogs (genes duplicated in the same genome) and (2) the analysis of the phylogenetic trees reconstructed for each family of paralogs containing at least two members belonging to each bacterium. The distributions of the different categories of homologs show a significant loss of paralogous genes in H. influenzae (reduction proportional to the genome size), of many sequences which are still present in one copy in E. coli, and of some entire gene families. Phylogenetic trees also confirmed this recent loss of paralogous genes in H. influenzae. Thus, the genome size of the last common ancestor of these two bacteria would have been close to that of present-day E. coli, and the evolution of H. influenzae toward a parasitic life led to an important decrease in its genome size by some mechanism of streamlining. During this recent evolution, the memory of the gene order present in the last common ancestor has been blurred, but a few short conserved chromosomal fragments can still be detected in present-day E. coli and H. influenzae.      

The nature of the last universal ancestor and the root of the tree of life, still open questions.

The nature of the last universal ancestor to all extent cellular organisms and the rooting of the universal tree of life are fundamental questions which can now be addressed by molecular evolutionists. Several scenarios have been proposed during the last years, based on the phylogenies of ribosomal RNA and of duplicated proteins, which suggest that the last universal ancestor was either an RNA progenote or an hyperthermophilic prokaryote. We discuss these hypotheses in the light of new data on the evolution of DNA metabolizing enzymes and of contradictions between different protein phylogenies. We conclude that the last universal ancestor was a member of the DNA world already containing several DNA polymerases and DNA topoisomerases. Furthermore, we criticize current data which suggest that the rooting of the universal tree of life is located in the eubacterial branch and we conclude that both rooting the universal tree and the nature of the last universal ancestor are still open questions.     

Monophyly of class I aminoacyl tRNA synthetase,USPA, ETFP,photolyase, and PP-ATPase nucleotide-binding domains: implications for protein evolution in the RNA

Protein sequence and structure comparisons show that the catalytic domains of Class I aminoacyl-tRNA synthetases, a related family of nucleotidyltransferases involved primarily in coenzyme biosynthesis, nucleotide-binding domains related to the UspA protein (USPA domains), photolyases, electron transport flavoproteins, and PP-loop-containing ATPases together comprise a distinct class of alpha/beta domains designated the HUP domain after HIGH-signature proteins, UspA, and PP-ATPase. Several lines of evidence are presented to support the monophyly of the HUP domains, to the exclusion of other three-layered alpha/beta folds with the generic "Rossmann-like" topology. Cladistic analysis, with patterns of structural and sequence similarity used as discrete characters, identified three major evolutionary lineages within the HUP domain class: the PP-ATPases; the HIGH superfamily, which includes class I aaRS and related nucleotidyltransferases containing the HIGH signature in their nucleotide-binding loop; and a previously unrecognized USPA-like group, which includes USPA domains, electron transport flavoproteins, and photolyases. Examination of the patterns of phyletic distribution of distinct families within these three major lineages suggests that the Last Universal Common Ancestor of all modern life forms encoded 15-18 distinct alpha/beta ATPases and nucleotide-binding proteins of the HUP class. This points to an extensive radiation of HUP domains before the last universal common ancestor (LUCA), during which the multiple class I aminoacyl-tRNA synthetases emerged only at a late stage. Thus, substantial evolutionary diversification of protein domains occurred well before the modern version of the protein-dependent translation machinery was established, i.e., still in the RNA world.     

Did the last common ancestor have a biological membrane?

All theories about the origin and evolution of membrane bound cells necessarily have to cope with the nature of the last common ancestor of cellular life. One of the most important aspect of this ancestor, whether it had a closed biological membrane or not, has recently been intensely debated. Having a consensus about it would be an important step towards an eventual (though probably still remote) synthesis of the best elements of the current multitude of cell evolution models. Here I analyse the structural and functional conservation of the few universally distributed proteins that were undoubtedly present in the last common ancestor and that carry out membrane-associated functions. These include the SecY subunit of the protein-conducting channel, the signal recognition particle, the signal recognition particle receptor, the signal peptidase, and the proton ATPase. The conserved structural and functional aspects of these proteins indicate that the last common ancestor was associated with a hydrophobic layer with two hydrophilic sides (an inside and an outside) that had a full-fledged and asymmetric protein insertion and translocation machinery and served as a permeability barrier for protons and other small molecules. It is difficult to escape the conclusion that the last common ancestor had a closed biological membrane from which all cellular membranes evolved.