Последний универсальный общий предок всех живых организмов

Most recent common ancestor of all current life on Earth

[[File:Phylogenetic tree of life 1990 LUCA. svg|thumb|upright=1.75|Phylogenetic tree linking all major groups of living organisms, namely the Bacteria, Archaea, and Eukarya, as proposed by Woese et al 1990, An early tree of life was sketched by Jean Baptiste Lamarck in his Philosophie zoologique in 1809. 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." The last sentence of the book begins with a restatement of the hypothesis:
blockquote|text=There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one  |source=

Some other researchers have challenged Weiss et al. 's 2016 conclusions. Sarah Berkemer and Shawn McGlynn argue that Weiss et al. undersampled the families of proteins, so that the phylogenetic trees were not complete and failed to describe the evolution of proteins correctly. There are two risks in attempting to attribute LUCA's environment from near universal gene distribution (as in Weiss et al. 2016). On the one hand, it risks misattributing convergence or horizontal gene transfer events to vertical descent; on the other hand, it risks misattributing potential LUCA gene families as horizontal gene transfer events. 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, NH4+, Fe2+, CO2+, Ni2+, Mg2+, Mn2+, Zn2+, pyrophosphate, and PO3 4 which would imply a terrestrial hot spring habitat. It possibly had a phosphate based metabolism. Further, these proteins were unrelated to autotrophy (the ability of an organism to create its own organic matter), suggesting that the LUCA had a heterotrophic lifestyle (consuming organic matter) and that its growth was dependent on organic matter produced by the physical environment. Nick Lane argues that Na+/H+ antiporters could readily explain the low concentration of Na+ in the LUCA and its descendants. The presence of the energy handling enzymes CODH/acetyl coenzyme A synthase in LUCA could be compatible not only with being an autotroph but also with life as a mixotroph or heterotroph. Weiss et al. 2018 reply that no enzyme defines a trophic lifestyle, and that heterotrophs evolved from autotrophs.

Several lines of evidence now suggest that LUCA was non thermophilic. The content of G + C nucleotide pairs (compared to the occurrence of A + T pairs) can indicate an organism's thermal optimum as they are more thermally stable due to an additional hydrogen bond. As a result they occur more frequently in the rRNA of thermophiles; however this is not seen in LUCA's reconstructed rRNA. The identification of thermophilic genes in the LUCA has been criticized, as they may instead represent genes that evolved later in archaea or bacteria, then migrated between these via horizontal gene transfer, as in Woese's 1998 hypothesis. LUCA could have been a mesophile that fixed CO2 and relied on H2, and lived close to hydrothermal vents. Further evidence that LUCA was mesophilic comes from the amino acid composition of its proteins. The abundance of I, V, Y, W, R, E, and L amino acids (denoted IVYWREL) in an organism's proteins is correlated with its optimal growth temperature. According to phylogentic analysis, the IVYWREL content of LUCA's proteins suggests its ideal temperature was below 50°C.

Studies from 2000 to 2018 have suggested an increasingly ancient time for the LUCA. In 2000, estimates of the LUCA's age ranged from 3.5 to 3.8 billion years ago in the Paleoarchean, a few hundred million years before the earliest fossil evidence of life, for which candidates range in age from 3.48 to 4.28 billion years ago. 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. However, a 2018 study by Holly Betts and colleagues applied a molecular clock model to the genomic and fossil record (102 species, 29 common protein coding genes, mostly ribosomal), concluding that LUCA preceded the Late Heavy Bombardment. They assumed that there was no sterilizing event after the Moon forming event, which they dated as 4.520 billion years ago, and concluded that the most likely date for LUCA was within 50 million years of that.

The origin of viruses remains disputed. Since viruses need host cells for their replication, it is likely that they emerged after the formation of cells. Viruses may even have multiple origins and different types of viruses may have evolved independently over the history of life. Furthermore, extensive virus evolution seems to have preceded the LUCA, since the jelly roll structure of capsid proteins is shared by RNA and DNA viruses across all three domains of life. LUCA's viruses were probably mainly dsDNA viruses in the groups called Duplodnaviria and Varidnaviria. Two other single stranded DNA virus groups within the Monodnaviria, the Microviridae and the Tubulavirales, likely infected the last bacterial common ancestor. The last archaeal common ancestor was probably host to spindle shaped viruses. All of these could well have affected the LUCA, in which case each must since have been lost in the host domain where it is no longer extant. By contrast, RNA viruses do not appear to have been important parasites of LUCA, even though straightforward thinking might have envisaged viruses as beginning with RNA viruses directly derived from an RNA world. Instead, by the time the LUCA lived, RNA viruses had probably already been out competed by DNA viruses. Viruses might have evolved before LUCA but after the First universal common ancestor (FUCA), according to the reduction hypothesis, where giant viruses evolved from primordial cells that became parasitic.