Last universal common ancestor

The oldest evidence of life on Earth is graphite , found in metamorphosed sedimentary rocks from West Greenland with an age of 3.7 billion years ^[7] , as well as fossil remains of bacterial mats found in sandstone in Western Australia with an age of 3.48 billion years ^{[ 8] [9]} . In 2015, the discovery of carbon of potentially biogenic origin in ancient stones 4.1 billion years old was described, however, this find may indicate other than the current conditions on the Earth at that time and indicate an earlier origin of life ^{[10] [ 11]} . In 2017, a description of the alleged fossilized microbial remnants of at least 3.77 billion years old and possibly 4.28 billion years old from rusty sedimentary rocks in Quebec , Canada was published ^[12] .

An analysis of the alleged offspring of LUCA showed that it was a small, unicellular organism , probably with circular DNA floating freely in the cell , like in modern bacteria . However, Karl Wöze , who proposed a three-domain living world system based on rRNA sequences of bacteria, archaea, and eukaryotes , argues that LUCA was simpler than individual ancestors that gave rise to three life domains ^[13] .

While the anatomy of LUCA can only be reconstructed in general terms, its internal biological mechanisms can be described in more detail based on the properties of modern organisms ^{[14] [15] [16] [17]} .

The genetic code was most likely based on DNA ^[18] . Some researchers believe that LUCA could be devoid of DNA, and only RNA was its genome ^[19] . If he had DNA, then it consisted of four nucleotides ( deoxyadenosine , deoxycytidine , deoxy thymidine and deoxyguanosine ), excluding the remaining possible deoxynucleotides. DNA was double-stranded due to the matrix-independent DNA polymerase enzyme . DNA integrity was maintained by a group of enzymes, including DNA topoisomerase , DNA ligase, and other DNA repair enzymes. DNA was protected by like histones . The genetic code consisted of trinucleotide codons, and a total of 64 different codons were possible. Since only 20 amino acids were used to construct proteins, some amino acids were encoded by several codons ^{[14] [15] [16] [17]} . Gene expression was carried out through the intermediate formation of single-stranded RNA. RNA was formed by the enzyme DNA-dependent RNA polymerase using nucleotides similar to DNA nucleotides, with the exception of thymidine, which was replaced by uridine in RNA ^{[14] [15] [16] [17]} .

Genetic material was expressed as proteins . They were assembled from amino acids by translation of messenger RNA ( mRNA ) using ribosomes , transport RNA ( tRNA ), and a group of other proteins . Ribosomes consisted of two subunits: 30S (small) and 50S (large). Each subunit consisted of ribosomal RNA ( rRNA ) surrounded by ribosomal proteins. Both types of RNA molecules (tRNA and rRNA) played an important role in the catalytic activity of ribosomes. Only 20 amino acids were used to construct the proteins, and only their L-isomers were used exclusively. ATP molecules were used as an energy carrier. There were several hundred protein enzymes that catalyzed chemical reactions that release energy from fats , sugars, and amino acids, as well as the synthesis of fats, sugars, amino acids, and nitrogenous bases that make up nucleic acids ^{[14] [15] [16] [17]} .

The cell contained a cytoplasm , mainly consisting of water, which was surrounded by a membrane represented by a lipid bilayer . Inside the cell, the concentration of sodium was lower, and potassium was higher than outside. This gradient was supported by ion channels , also known as ion pumps. The cell multiplied by duplication of the contents before division ^{[14] [15] [16] [17]} . The cell used chemosmosis to generate energy. It also formed CO ₂ and oxidized H ₂ ( methanogenesis or acetogenesis ) through acetyl thioethers ^{[20] [21]} .

The cell supposedly lived in deep - water hydrothermal springs formed during the interaction of sea water with magma under the ocean floor ^{[22] [23]} .

In 1859, Charles Darwin published his book The Origin of Species, in which he twice formulated the hypothesis that all life forms on Earth have one common ancestor. When the LUCA hypothesis was put forward, cladograms based on the genetic distance between living species showed that the archaea separated very early from the rest of life. This statement was formulated on the basis that the archaea, known at that time, were very resistant to extreme environmental conditions, such as high salinity, temperature and acidity . This led some scientists to the idea that LUCA lived in habitats similar to deep-sea hydrothermal springs. However, later archaea were discovered in less hostile environments, and now it is believed that they are more related to eukaryotes than bacteria, although many details are unknown ^{[24] [25]} .

In 2010, based on the DNA sequences of organisms of various domains ^[26], it was established that there was a single ancestor of all living things. However, this does not mean that LUCA was the only organism of those ancient times: it was one of several early microbes ^[1] . However, the fact that, along with several nucleotides of DNA and RNA used by all modern life forms, other nucleotides are possible, it almost certainly follows that all organisms have one common ancestor. It is improbable that all organisms descended from different ancestors in which organic molecules combined to form cell-like structures capable of horizontal gene transfer do not spoil each other's genes, turning them into non-coding regions. In addition, much more amino acids are chemically possible than those used by modern organisms for protein synthesis. This chemical evidence suggests that all other organisms originated from LUCA cells, with only LUCA descendants surviving the Paleoarchean era ^[27] .

In 1998, Karl Vöze suggested that LUCA was not the only one organism, and the genetic material of all living organisms is the result of horizontal gene transfer between communities of ancient microorganisms ^[28] . At the dawn of life, kinship was not as linear as it is now, because the appearance of the modern genetic code took time ^[29] .

Scientists from the University of Bristol in the UK calculated that the common ancestor of all modern representatives of life on Earth (Last Universal Common Ancestor, LUCA), whose traces were preserved in the DNA of absolutely all existing organisms, lived at hot springs on land and was extremophile 4.52— 4.47 billion years ago, even before the late heavy bombardment of the Earth began 3.9 billion years ago - shortly after the collision of the Earth's germ with Teia - the "great-grandmother" of the Moon ^[4] .

According to the most generally accepted point of view, the root of the tree of life is between the monophyletic domain of the bacterium and the treasure formed by archaea and eukaryotes. This tree is considered the traditional tree of life and is based on the molecular biological studies of Karl Woese ^[31] . A small number of studies have shown that the root of the tree of life lies in the bacterial domain, in the firmum Firmicutes ^[32] or , which make up the basal clade in relation to the combined group of archaea and eukaryotes, as well as other bacteria. This hypothesis was proposed by Thomas Cavalir-Smith ^[33] .

A 2016 study by William Martin et al., Based on sequencing of 6.1 million protein-coding genes of various prokaryotes , showed that 355 protein clusters out of 286,514 studied protein clusters were present in LUCA. According to these data, LUCA was an anaerobic organism that fixes CO ₂ , is dependent on H ₂ , with a Wood-Ljungdahl pathway capable of N ₂ fixation and thermophilic . As cofactors, he used transition metals , flavins , S-adenosylmethionine , coenzyme A , ferredoxin , , corrins and selenium . He had modifications of nucleosides and S-adenosylmethionine-dependent methylation . This study showed that the basal group is methanogenic clostridia , and LUCA lived in anaerobic hydrothermal springs in a geochemically active environment enriched in hydrogen, carbon dioxide, and iron ^[23] .

• Goryainova, Oksana; Dry, Olga. In the wild: how the last universal ancestor of LUCA lived (neopr.) . // Biomolecula.ru website (March 6, 2017). Date of treatment March 20, 2018.
• Lapochkin, Yuri. Viral genomes in the evolution system (neopr.) . // Biomolecula.ru website (November 28, 2014). Date of treatment March 20, 2018.
• Korzhova, Victoria. The appearance and evolution of the cell membrane (neopr.) . // Biomolecula.ru website (December 31, 2014). Date of treatment March 20, 2018.