Be introduced to the First Universal Common Ancestor ( FUCA ) : the great-grandmother of LUCA ( Last Universal Common Ancestor )

The existence of a common ancestor to all living organisms in Earth is a necessary corollary of Darwin idea of common ancestry. The Last Universal Common Ancestor (LUCA) has been normally considered as the ancestor of cellular organisms that originated the three domains of life: Bacteria, Archaea and Eukarya. Recent studies about the nature of LUCA indicate that this first organism should present hundreds of genes and a complex metabolism. Trying to bring another of Darwin ideas into the origins of life discussion, we went back into the prebiotic chemistry trying to understand how LUCA could be originated Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 4 June 2018 doi:10.20944/preprints201806.0035.v1 © 2018 by the author(s). Distributed under a Creative Commons CC BY license. 2 under gradualist assumptions. Along this line of reasoning, it became clear to us that the definition of another ancestral should be of particular relevance to the understanding about the emergence of biological systems. Together with the view of biology as a language for chemical translation, on which proteins are encoded into nucleic acids polymers, we glimpse a point in the deep past on which this Translation mechanism could have taken place. Thus, we propose the emergence of this process shared by all biological systems as a point of interest and propose the existence of this non-cellular entity named FUCA, as the First Universal Common Ancestor. FUCA was born in the very instant on which RNA-world replicators started to be capable to catalyze the bonding of amino acids into oligopeptides. FUCA has been considered mature when the translation system apparatus has been assembled together with the establishment of a primeval, possibly error-prone genetic code. This is FUCA, the greatgrandmother of LUCA. Running title Meet FUCA: the ancestor of LUCA

Miller's experiment updated the simulation of primeval Earth conditions and confirmed the production of amino acids using only water (H2O), methane (CH4), ammonia (NH3), and hydrogen (H2) as input (Miller, 1953). Thus, a point of view about the initial organization of the biological systems started as the studies of prebiotic chemistry provided an innovative way to think about the Origins of life.
Then, by the early years of 1980s, the discovery of catalytic properties of RNA molecules introduced another element through which the initial organization of biological systems could be understood (Kruger et al, 1982, Guerrier-Takada et al. 1983). For the first time, these findings made possible the proposition of hypotheses capable to describe biological entities that did not need cellular structures. This idea culminated with the proposition of a molecular-based RNA world on which selfreplicative and catalytic molecules of nucleic acids could interact and be target of natural selection, pushing forward the path into the origins of life (Gilbert, 1986).

The lineage of LUCA
It has been shortly before the proposal of the RNA world theory, in the late 1970s, that the American microbiologist Carl Woese started to produce sequences for a specific RNA molecule known as 16S rRNA, a constituent of the small ribosomal subunit (Woese and Fox, 1977a

LUCA and the viruses
The questions about the deep lineage of LUCA are complex to approach, once non-cellular living organisms do not exist in current days. The sole non-cellular biological systems that exist today are viruses. Viruses are frequently considered as non-living organisms because they are not free-living entities, as they need to highjack cells to be able to manifest their metabolism and reproduce. Besides, viruses have been excluded from deep trees of life by the simple reason that they do not have ribosomes; and without 16S rRNA molecules they cannot figure in those trees.
However, nowadays there is an emerging view that virus-like biological systems may have played important roles in precellular living systems . Besides, the evolutionary history of viruses seems to be polyphyletic as there is evidence that some groups were formed by the further simplification of cellular organisms. Thus, virus should be understood much more as a strategy of life rather than a monophyletic group that share the same evolutionary origin (Nasir and Caetano-Anolles, 2015).
Recent researches with giant mimivirus seem to indicate polyphyly, although there is still much controversy in those grounds (Harish et al, 2016;Forterre and Gaia, 2016). By the way, it is difficult to use the word "life" and "living" to represent viruses and it seems reasonable to consider that virus-like biological systems may have existed long before lipid cellular barriers were coopted to be the basis of life in Earth.

Life versus biological systems
The suggestion of an alternative view to the origins of life requires that we take a special attention to the concepts we are Although viruses do not present cells, they do present something that is strikingly important to any biological system: a wellestablished genetic code. That code is actually the key to provide a chemical translation and guarantee that their nucleic acid information will produce proteins that will allow their metabolism and reproduction. Viruses may not be cellular, viruses may not be living, but viruses do speak the language of biology. They do present complex proteins encoded in their genomes and they do have a plan of existence chemically written in their nucleic acids (being DNA or RNA).
Though they do not have ways to execute their plans by their own means, it is possible to consider a world made of non-cellular, virus-like entities that might deliver their codes into a Translation system operating outside them. Thus, it is not difficult to imagine pre-cellular virus-like particles capable to exist and to attach into other systems that might translate their information and allow their reproduction.
Even Carl Woese knew that pre-cellular entities were needed to explain the origin of life in Earth before LUCA. He defined the concept of progenotes as being protocells that probably presented (i) error-prone genetic codes, (ii) high mutation rates and (iii) high exchange of genetic material (Woese and Fox, 1977b). These entities were necessary to accumulate genetic material from different biochemical pathways to be able to form larger genomes.
Thus, we propose the usage of the term Biological System as an alternative for life. This decision has proven to be fertile and clarifying in many circumstances, as we shall see. However, to make a better use of this term, we need to clearly define it. For us, a biological system is such a system on which its molecular nature is centered in the controlled interaction between polymers of nucleotides (nucleic acids) and polymers of amino acids (peptides and/or proteins).
In general, the biological system can be considered a system based in a process of chemical translation, on which information stored in some chemical polymer can be translated information in another chemical polymer.
This view puts the process of protein synthesis in the center of what we consider Biology to be. All Biology is based on a process of chemical encoding, a system of chemical language translation on which a polymer become another by following specific grammatical rules. Thus, biological systems came to be in the very moment on which those rules emerged.

Origins versus emergence
Another important conceptual issue that must be better understood into this new view of what are biological systems is the difference between the terms origin and emergence. The term origin often refers to the first rise of something that has never been before. It suggests the occurrence of a very singular, special event that, for the first time, brings something from non-existence to existence. It is a jump, a rising, an appearance.
On the other hand, the concept of emergence as we use it here should be understood as a more continuous path on which something come to be. It is not a drastic appearance as the concept of origin denotes, but a more subtle process on which a system can be built slowly and persistently over time.
Also, inhering the concept of emergence from chaos theory, we aim to signify a process on which "the whole is greater than the sum of the parts". This is clearly what happened at the emergence of biological systems, on which a polymer of nucleic acids starts to produce peptides initially by nearly random attachments until the emergence of the genetic code.

Biological systems are chemical translators
Although it is possible to wonder the existence of non-cellular biological systems, it is almost impossible to think in a biological entity harboring a metabolism without the Translation apparatus for protein synthesis. Having (i) proteins encoded in a three-letter genetic code made of nucleic acids and a (ii) metabolism primarily coordinated by proteins and enzymes seems to be a feature that unequivocally defines a biological system. nowadays compose the genomes of most organisms can be seen much more like a high-security media to store molecular information that emerged later and allowed an important stabilization for both heredity and the control of metabolism.
Although its importance has been inestimable, it was probably absent at the scenario on which the biological systems emerged.
The existence of viruses harboring RNA genomes, even in single strand, can be seen as evidence that DNA is not essential to biological systems. Being a secure media to store molecular information, it has been placed in a safe place in the nucleus of eukaryotic cells, though it seems to have no role in the beginnings.
The emergence of a chemical translation process on which RNAlike molecules convert their information into peptides seems to be the main characteristic shared by any biological system and therefore seem to have evolved from a common origin at the First Universal Common Ancestor (FUCA). In that sense, the birth of biological systems are not at the emergence of the code, but at the rise of some sort of ribozyme that was capable to bind together amino acids. FUCA is therefore a process that started with no code, but her point of maturation happened when the code has been completely established. Thus, to understand the initial steps in the emergence of FUCA we need to study deeply the anatomy of ribosomes.

The anatomy of ribosomes
Ribosomes of any biological system present two subunits. The smaller one binds the messenger RNA while the larger presents three sites for the binding of transfer RNAs bringing specific amino acids to the system.
The exact ribosomal site on which amino acids are bound together is considered as its catalytic center. This catalytic center has been named as Peptidyl Transferase Center, or simply PTC.
The PTC is part of the 23S RNA of bacteria and it is known to catalyze the binding of two separate amino acids into a dipeptide using an Adenine as the most important catalytic center. In this synthesis by dehydration, a molecule of water is jumped out while the C-terminal of an amino acid binds to the N-terminal of another, starting with the polymerization. Other amino acids can be further added to this dipeptide, raising the number of amino acids that can be bound together and allowing the formation of oligopeptides.
In our view, the appearance of a nucleic acid molecule in the early Earth that was capable to bind together two amino acids, somehow started a process of chemical symbiosis on which the binding of this nucleic acid to the peptides produced by itself allowed the system to both (i) stabilize under a self-referential perspective and (ii) aggregate complexity in layers, probably through the mechanism known as accretion.

FUCA is born at the proto-PTC
Here we propose that the biological systems emerged in the very moment on which a macromolecule of nucleic acid containing dozens of nucleotides were capable to fold in the 3D space and catalyze the junction of two amino acids into a dipeptide. Thus, FUCA was born when a proto-PTC emerged for the first time, allowing already existing self-replicating RNA-like polymers to produce random di-and oligopeptides. These random peptides produced possibly bound back to the RNA polymers and allowed a higher stabilization of the system that got more robust and was further bound to other stabilizing molecules.
Chaos theory advocates might say that a strange attractor should have been formed at that point. Astrophysics uses the term accretion to explain how planets and other bodies may be formed by the aggregation of material due to gravity. Here, we also use the idea of accretion to explain how the ribosome has been assembled from the proto-PTC into a higher-level system.
Although the peptides were bound randomly, the process itself cannot be seen as random because there should have been very simple and specific types of amino acids existing in the primordial pool of molecules. These early amino acids were most likely Glycine, Serine, Alanine and others (Miller, 1953, Paker et al. 2011. Their bound together attracted other amino acids and have possibly formed a first layer of peptides that bound to the proto-PTC and stabilized its interaction to the proto-tRNAs. After subsequent layers of complexification, these processes would evolve to the creation of the larger ribosomal subunit with its tRNA sites A, P and E. Many works have already tried to understand how this subunit has been clearly formed; some considering that PTC were at the beginning (Petrov et al, 2015) and others presenting evidences that the ribosome structure started elsewhere (Caetano-Anolles, 2015). It is consensual, however, that the process of ribosome assembly took a long time until it could became functional and efficient in its task of amino acid binding.

The maturation of FUCA
The As a molecule, FUCA has no gender and sex would only arise in biology billions of years later. We decided to describe FUCA as a woman, the great-grandmother of LUCA (referring as "it" but sometimes as "she") to homage the bravery of women in our patriarchal societies. As we understand the birth of FUCA as a (i) process of chemical symbiosis and as a (ii) revelation about the importance of symbiotic processes to the emergence of most fundamental biological process, the feminine allusion is also a tribute to the memory of Lynn Margulis. The idea of a great-grandmother is important for the understanding that FUCA also emerged much earlier than LUCA.
The emergence of a proto-PTC has been a contingent moment of enlighten to the creativity of the universe and to the union of macromolecules. At that time, molecules could only collaborate by interacting but never before by building other collaborator molecules. When these ancient nucleic acids were capable to domesticate the abundant amino acids and interact with them achieving more stabilization than each one by itself, a new age has risen and FUCA has been born.
The molecular nature of the universe has discovered that they This system further specialized to be the very language of biology, the language of chemical interchange that further evolved until the formation of the complete ribosome, the genetic code and the maturation of the molecular translational process.

Conclusions
We however has probably taken much time but we consider her to be matured at the moment on which the system known nowadays as Translation has been completely developed together with a functional genetic code responsible to translate the information present in a nucleic acid into a peptide under an organized form.

Conflicts of Interest:
The authors declare no conflict of interest.