Last universal common ancestor
All earlier forms of life preceding this divergence and all extant organisms are generally thought to share common ancestry.
On the basis of a formal statistical test, this theory of a universal common ancestry (UCA) is supported versus competing multiple-ancestry hypotheses.
These genes describe a complex life form with many co-adapted features, including transcription and translation mechanisms to convert information from DNA to mRNA to proteins.
[4][5] Charles Darwin more famously proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859: "Therefore I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed.
"[11] The cofactors also reveal "dependence upon transition metals, flavins, S-adenosyl methionine, coenzyme A, ferredoxin, molybdopterin, corrins and selenium.
These were assembled from 20 free amino acids by translation of a messenger RNA via a mechanism of ribosomes, transfer RNAs, and a group of related proteins.
[15] LUCA was likely capable of sexual interaction in the sense that adaptive gene functions were present that promoted the transfer of DNA between individuals of the population to facilitate genetic recombination.
The originator of the three-domain system, Carl Woese, stated that in its genetic machinery, the LUCA would have been a "simpler, more rudimentary entity than the individual ancestors that spawned the three [domains] (and their descendants)".
Weiss and colleagues write that "Experiments ... demonstrate that ... acetyl-CoA pathway [chemicals used in anaerobic respiration] formate, methanol, acetyl moieties, and even pyruvate arise spontaneously ... from CO2, native metals, and water", a combination present in hydrothermal vents.
It is stated that "The late and independent evolution of glycolysis but not gluconeogenesis is entirely consistent with LUCA being powered by natural proton gradients across leaky membranes.
[28] A 2024 study suggests that LUCA's genome was similar in size to that of modern prokaryotes, coding for some 2,600 proteins; that it respired anaerobically, and was an acetogen; and that it had an early CAS-based anti-viral immune system.
A phylogenomic and geochemical analysis of a set of proteins that probably traced to the LUCA show that it had K+-dependent GTPases and the ionic composition and concentration of its intracellular fluid was seemingly high K+/Na+ ratio, NH+4, Fe2+, CO2+, Ni2+, Mg2+, Mn2+, Zn2+, pyrophosphate, and PO3−4 which would imply a terrestrial hot spring habitat.
[10] A 2024 study by Sawsan Wehbi and colleagues directly estimated the order in which amino acids were added into the genetic code from early protein sequences.
[36] For instance, the thermophile-specific topoisomerase, reverse gyrase, was initially attributed to LUCA[11] before an exhaustive phylogenetic study revealed a more recent origin of this enzyme followed by extensive horizontal gene transfer.
[42][43][44][45][46] This placed the origin of the first forms of life shortly after the Late Heavy Bombardment which was thought to have repeatedly sterilized Earth's surface.
[29] In 1990, a novel concept of the tree of life was presented, dividing the living world into three stems, classified as the domains Bacteria, Archaea, Eukarya.
[50][b] In the meantime, numerous modifications of this tree, mainly concerning the role and importance of horizontal gene transfer for its rooting and early ramifications have been suggested (e.g.[54][49]).
Since heredity occurs both vertically and horizontally, the tree of life may have been more weblike or netlike in its early phase and more treelike when it grew three-stemmed.
[55] A modified version of the tree, based on several molecular studies, has its root between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota.
In 1994, on the basis of primordial metabolism (sensu Wächtershäuser), Otto Kandler proposed a successive divergence of the three domains of life[1] from a multiphenotypical population of pre-cells, reached by gradual evolutionary improvements (cellularization).
[62][63][64] These phenotypically diverse pre-cells were metabolising, self-reproducing entities exhibiting frequent mutual exchange of genetic information.
[67] Other authors concur that there was a "complex collective genome"[68] at the time of the LUCA, and that horizontal gene transfer was important in the evolution of later groups;[68] Nicolas Glansdorff states that LUCA "was in a metabolically and morphologically heterogeneous community, constantly shuffling around genetic material" and "remained an evolutionary entity, though loosely defined and constantly changing, as long as this promiscuity lasted.
In 2010, based on "the vast array of molecular sequences now available from all domains of life,"[70] D. L. Theobald published a "formal test" of universal common ancestry (UCA).
This deals with the common descent of all extant terrestrial organisms, each being a genealogical descendant of a single species from the distant past.
His formal test favoured the existence of a universal common ancestry over a wide class of alternative hypotheses that included horizontal gene transfer.
Two other single-stranded DNA virus groups within the Monodnaviria, the Microviridae and the Tubulavirales, likely infected the last bacterial common ancestor.
