The first universal common ancestor (FUCA) is a proposed non-cellular entity that is the earliest ancestor of the last universal common ancestor (LUCA) and its descendants, including every modern cell.^[1][2] FUCA would also be the ancestor of ancient sister lineages of LUCA, none of which have modern descendants.^[2]

FUCA is thought to have been composed of progenotes, proposed ancient biological systems that would have used RNA for their genome and self-replication, in place of DNA.^[3][4][5] By comparison, LUCA would have had a complex metabolism and genome with hundreds of genes and gene families.^[1]

The now-extinct sister lineages descended from FUCA are thought to have horizontally transferred some of their genes into the genome of early descendants of LUCA.^[2]

Long before the appearance of compartmentalized biological entities like FUCA, life had already began to organize itself and emerge in a pre-cellular era known as the RNA world.^[3] The universal presence of both biological translation mechanism and genetic code in every biological systems indicates monophyly, a unique origin for all biological systems including viruses and cells.^[1][3]

FUCA would have been the first organism capable of biological translation, using RNA molecules to convert information into peptides and produce proteins.^[1] This first translation system would have been assembled together with primeval, possibly error-prone genetic code.^[1] FUCA would also be the first biological system to have genetic code.^[6]

The development of FUCA likely took a long time.^[1] FUCA was generated without genetic code, from the ribosome,^[1] itself a system evolved from the maturation of a ribonucleoprotein machinery.^[3] FUCA appeared when a proto-peptidyl transferase center started to first emerge, when RNA world replicators started to be capable to catalyze the bonding of amino acids into oligopeptides.^[1]

The first genes of FUCA were most likely encoding ribosomal, primitive tRNA-aminoacyl transferases and other proteins that helped to stabilize and maintain biological translation.^[6] These random peptides produced possibly bound back to the single strand nucleic acid polymers and allowed a higher stabilization of the system that got more robust and was further bound to other stabilizing molecules.^[1] When FUCA had matured, its genetic code was completely established.^[1]

FUCA was composed by a population of open-systems, self-replicating ribonucleoproteins.^[3] With the arrival of these systems, began the progenote era.^[3] These systems evolved into maturity when self-organization processes resulted in the creation of a genetic code.^[3] This genetic code was for the first time capable to organize an ordered interaction between nucleic acids and proteins through the formation of a biological language.^[3] This caused pre-cellular open systems to then start to accumulate information and self-organize, producing the first genomes by the assembly of biochemical pathways, which probably appeared in different progenote populations evolving independently.^[3]

In the reduction hypothesis, where giant viruses evolved from primordial cells that became parasitic, viruses might have evolved after FUCA and before LUCA.^[2]

Progenotes (also called ribocytes or ribocells)^[7][8][9] are semi-open or open biological systems capable of performing an intense exchange of genetic information, before the existence of cells and LUCA.^[4][3] The term progenote was coined by Carl Woese in 1977,^[10] around the time he introduced the concept of the three domains of life (bacteria, archaea, and eukaryotes) and proposed that each domain originated from a different progenote.^[11][12] The meaning of the term changed with time. In the 1980s, Doolittle and Darnell used the word progenote to designate the ancestor of all three domains of life,^[13] now referred to as the last universal common ancestor (LUCA).^[14]

The terms ribocyte and ribocell refer to progenotes as protoribosomes, primeval ribosomes that were hypothetical cellular organisms with self-replicating RNA but without DNA,^[8][15] and thus with a RNA genome instead of the usual DNA genome.^[5] In Carl Woese's Darwinian threshold period of cellular evolution, the progenotes are also thought to have had RNA as informational molecule instead of DNA.^[7]

The evolution of the ribosome from ancient ribocytes, self-replicating machines, into its current form as a translational machine may have been the selective pressure to incorporate proteins into the ribosome's self-replicating mechanisms, so as to increase its capacity for self-replication.^[16][17] Ribosomal RNA is thought to have emerged before cells or viruses, at the time of progenotes.^[3]

Progenotes composed and were the descendants of FUCA,^[3] and FUCA is thought to have organized the process between the initial organization of biological systems and the maturation of progenotes.^[3] Progenotes were dominants in the Progenote age, the time where biological systems originated and initially assembled.^[3] The Progenote age would have happened after the pre-biotic age of the RNA-world and Peptide-world but before the age of organisms and mature biological systems like viruses, bacteria and archaea.^[3]

The most successful progenotes populations were probably the ones capable to bind and process carbohydrates, amino acids and other intermediated metabolites and co-factors.^[3] In progenotes, compartmentalization with membranes was not yet completed and translation of proteins was not precise. Not every progenote had its own metabolism; different metabolic steps were present in different progenotes. Therefore, it is assumed that there existed a community of sub-systems that started to cooperate collectively and culminated in the LUCA.^[7]

• Abiogenesis
• Alternative abiogenesis scenarios
• Earliest known life forms
• RNP world