User:Bmcarruthers/Abiogenesis: Difference between revisions – Wikipedia

The last universal common ancestor (LUCA) is the cellular population from which all extant life today emerged. Its genetic material was DNA, requiring the 4-nucleotide genetic code, messenger RNA, transfer RNA, and ribosomes to translate the code into proteins such as enzymes. It was evidently already a complex organism, and must have had precursors; it was not the first living thing^[1][2]. Starting with the work of Carl Woese from 1977, genomics studies have placed LUCA between Bacteria and a clade formed by Archaea and Eukaryota in the phylogenetic tree of life. It lived over 4 Gya.^[3][4] A minority of studies have placed LUCA in Bacteria, proposing that Archaea and Eukaryota are evolutionarily derived from within Eubacteria^[5]. Thomas Cavalier-Smith suggested in 2006 that the phenotypically diverse bacterial phylum Chloroflexota contained the LUCA.^[6]

The central approach to studying the physiology of LUCA is by reconstructing its genome through phylogenetic reconstruction, where ancestral genes are inferred from the modern diversity of existing genes. Previous research identified 60 proteins common to all life.^[7] Using phylogenetic reconstruction approaches, a study in 2016 identified a set of 355 genes, out of 6.1 million genes from Bacteria and Archaea, that were likely present in LUCA^[1]. From these genes, metabolic reactions inferred were the incomplete reverse Krebs cycle, gluconeogenesis, the pentose phosphate pathway, glycolysis, reductive amination, and transamination^[1][8][9]. The cofactors of the inferred enzymes suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and transition metals. This study suggested that LUCA was anaerobic and acetogenic (producing acetate from anaerobic respiration or fermentation) with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, and thermophilic.

A separate study in 2024 used alternative reconstruction methods and identified ~2,600 protein-coding genes in the genome of LUCA^[10]. Their results support the idea that LUCA was anaerobic and an acetogen, but argued against the existence of nitrogen-fixing machinery and thermophily, instead suggesting that LUCA was a mesophile. They also found evidence for a primitive CRISPR immune system in LUCA’s genome, supporting the idea that LUCA existed in an ecosystem with other cells and primitive viruses^[11][10].

These reconstruction studies often use the inferred physiology of LUCA to hypothesize the environment in which LUCA inhabited. Because the physiology of LUCA is in dispute^[12][13][14], its habitat is also highly contested. Most recent evidence indicates that LUCA was a mesophile, primed for moderate temperatures, rather than a thermophile based on global characteristics of the reconstructed genome^[15]. Many deeply-branching archaea clades are hyperthermophiles, implying that the earliest life inhabited hydrothermal vent environments^[3], but the disputed presence of reverse gyrase, a hallmark gene of hyperthermophiles, in LUCA’s genome complicates this argument^[15][10].

Important to note is that abiogenesis and LUCA are thought to have occurred in different environments, since many prebiotic reaction schemes are more favorable under colder, UV-shielded conditions while LUCA’s reconstructed genome indicates a preference for warmer habitats^[15].