Last universal ancestor

The last universal ancestor (LUA), also called the last universal common ancestor (LUCA), cenancestor, or progenote, is the most recent organism from which all organisms now living on Earth have a common descent.^[1] Thus it is the most recent common ancestor (MRCA) of all current life on Earth. The LUA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era).^[2][3] The earliest evidence for life on Earth is biogenic graphite found in 3.7 billion-year-old metasedimentary rocks discovered in Western Greenland^[4] and microbial mat fossils found in 3.48 billion-year-old sandstone discovered in Western Australia.^[5][6]

Charles Darwin proposed the theory of universal common descent through an evolutionary process in his book On the Origin of Species in 1859, saying, "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."^[7]

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Life timeline

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Axis scale: Millions of years.
also see {{Nature timeline}}

Considering what is known of the offspring groups (see phylogenetic bracketing), the LUA is thought to have been a small, single-cell organism. It likely had a ring-shaped coil of DNA floating freely within the cell, like modern bacteria. Morphologically, it would likely not have been exceptionally distinctive among a collection of generalized, small-size, modern-day bacteria. However, Carl Woese et al, who first proposed the currently-used three domain system based on an analysis of the 16S rRNA sequences of bacteria, archaea, and eukaryotes, stated that the LUA would have been a "...simpler, more rudimentary entity than the individual ancestors that spawned the three [domains] (and their descendants)" regarding its genetic machinery.^[8]

While the gross anatomy of the LUA must be reconstructed with much uncertainty, its internal mechanisms may be described in some detail, based on the properties currently shared by all independently living organisms on Earth:^{[9][10][11][12]}

• The genetic code was based on DNA.^[13] Note, however, that some studies suggest that the LUCA may have lacked DNA and been defined wholly through RNA.^[14]
  • The DNA was composed of four nucleotides (deoxyadenosine, deoxycytidine, deoxythymidine, and deoxyguanosine), to the exclusion of other possible deoxynucleotides.
  • The genetic code was composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids were used, multiple codons code for the same amino acids.
  • The DNA was kept double-stranded by a template-dependent DNA polymerase.
  • The integrity of the DNA was maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase and other DNA repair enzymes. The DNA was also protected by DNA-binding proteins such as histones.
• The genetic code was expressed via RNA intermediates, which were single-strand.
  • RNA was produced by a DNA-dependent RNA polymerase using nucleotides similar to those of DNA with the exception that thymidine in DNA was replaced by uridine in RNA.
• The genetic code was expressed into proteins.
• Proteins were assembled from free amino acids by translation of an mRNA by ribosomes, tRNA and a group of related proteins.
  • Ribosomes were composed of two subunits, one big 50S and one small 30S.
  • Each ribosomal subunit was composed of a core of ribosomal RNA surrounded by ribosomal proteins.
  • The RNA molecules (rRNA and tRNA) played an important role in the catalytic activity of the ribosomes.
  • Only 20 amino acids were used, to the exclusion of countless other amino acids.
  • Only the L-isomers of the amino acids were used.
• ATP was used as an energy intermediate.
• There were several hundred protein enzymes that catalyzed chemical reactions that extract energy from fats, sugars, and amino acids, and that synthesize fats, sugars, amino acids, and nucleic acid bases using arbitrary chemical pathways.
• The cell contained a water-based cytoplasm that was surrounded and effectively enclosed by a lipid bilayer membrane.
• Inside the cell, the concentration of sodium was lower, and potassium was higher, than outside. This gradient was maintained by specific ion transporters (also referred to as ion pumps).
• The cell multiplied by duplicating all its contents followed by cellular division.
• The cell used chemiosmosis to produce energy. It also reduced CO₂ and oxidized H₂ (methanogenesis or acetogenesis) via acetyl-thioesters ^[15][16]

Hypotheses

A cladogram linking all major groups of living organisms to the LUA (the black trunk at the bottom). This graph is derived from ribosomal RNA sequence data.^[17]

In 1859, Charles Darwin published The Origin of Species in which he twice stated the hypothesis that there was only one progenitor for all life forms. In the summation he states, "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."^[18] The very last sentence is a restatement of the hypothesis: "There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one."^[18]

When the LUA was hypothesized, cladograms based on genetic distance between living cells indicated that Archaea split early from the rest of life. This was inferred from the fact that all known archaeans were highly resistant to environmental extremes such as high salinity, temperature or acidity, and led some scientists to suggest that the LUA evolved in areas like the deep ocean vents, where such extremes prevail today. Archaea, however, were discovered in less hostile environments and are now believed to be more closely related to eukaryotes than bacteria, although many details are still unknown.^[19][20]

Current (2005) tree of life showing horizontal gene transfers, giving rise to a phylogenetic network

In 2010, based on "the vast array of molecular sequences now available from all domains of life,"^[21] a formal test of universal common ancestry was published.^[1] The formal test favored the existence of a universal common ancestor over a wide class of alternative hypotheses that included horizontal gene transfer. While the formal test overwhelmingly favored the existence of a single LUA, this does not imply that the LUA was ever alone. Instead, it was a member of the early microbial community.^[1] Given that many other nucleotides are possible besides adenine (A), thymine (T, DNA only), guanine (G), cytosine (C), and uracil (U, RNA only), it is extremely unlikely that organisms descendent from separate abiogenesis events (that is to say separate incidents where organic molecules initially came together to form cell-like structures) would be able to complete a horizontal gene transfer without garbling each other's genes, converting them into noncoding segments. Also, many more amino acids are chemically possible than the twenty found in modern protein molecules. These lines of chemical evidence, taken into account for the formal statistical test by Theobald (2010), point to a single cell having been the LUA in that, although it was a member of the early microbial community, only its descendents survived beyond the Paleoarchean Era. With a common framework in the AT/GC rule and the standard twenty amino acids, horizontal gene transfer would have been feasible and may have been very common later on among the progeny of that single cell.

In 1998, Carl Woese proposed (1) that no individual organism can be considered a LUA, and (2) that the genetic heritage of all modern organisms derived through horizontal gene transfer among an ancient community of organisms.^[22] While the results described by the later papers Theobald (2010) and Saey (2010) demonstrate the existence of a single LUCA, the argument in Woese (1998) can still be applied to Ur-organisms. At the beginnings of life, ancestry was not as linear as it is today because the genetic code took time to evolve.^[23] Before high fidelity replication, organisms could not be easily mapped on a phylogenetic tree. Not to be confused with the Ur-organism, however, the LUCA lived after the genetic code and at least some rudimentary early form of molecular proofreading had already evolved. It was not the very first cell, but rather, the one whose descendents survived beyond the very early stages of microbial evolution.

Location of the root

The most commonly accepted location of the root of the tree of life is between a monophyletic domain Bacteria and a clade formed by Archaea and Eukaryota of what is referred to as the "traditional tree of life" based on several molecular studies starting with C. Woese.^[24] A very small minority of studies have concluded differently, namely that the root is in the Domain Bacteria, either in the phylum Firmicutes^[25] or that the phylum Chloroflexi is basal to a clade with Archaea+Eukaryotes and the rest of Bacteria as proposed by Thomas Cavalier-Smith.^[26]

See also

• Abiogenesis
• Bacterial phyla
• Common descent
• Most recent common ancestor
• Origin of the first cell
• Protocell
• Panspermia
• Timeline of evolution

References

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2. Doolittle WF (February 2000). "Uprooting the tree of life". Scientific American 282 (2): 90–5. doi:10.1038/scientificamerican0200-90. PMID 10710791.
3. Glansdorff N, Xu Y, Labedan B (2008). "The last universal common ancestor: emergence, constitution and genetic legacy of an elusive forerunner". Biology Direct 3: 29. doi:10.1186/1745-6150-3-29. PMC 2478661. PMID 18613974.
4. Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; Nagase, Toshiro; Rosing, Minik T. (2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience 7: 25–8. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025.
5. Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013.
6. Noffke N, Christian D, Wacey D, Hazen RM (December 2013). "Microbially induced sedimentary structures recording an ancient ecosystem in the ca. 3.48 billion-year-old Dresser Formation, Pilbara, Western Australia". Astrobiology 13 (12): 1103–24. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812.
7. Darwin, C. (1859), The Origin of Species by Means of Natural Selection, John Murray, p. 490
8. Woese, C. R.; Kandler, O.; Wheelis, M. L. (1990-06-01). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya.". Proceedings of the National Academy of Sciences 87 (12): 4576–4579. doi:10.1073/pnas.87.12.4576. ISSN 0027-8424. PMC 54159. PMID 2112744.
9. Wächtershäuser, Günter (1998). "Towards a Reconstruction of Ancestral Genomes by Gene Cluster Alignment". Systematic and Applied Microbiology 21 (4): 473–4, IN1, 475–7. doi:10.1016/S0723-2020(98)80058-1.
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17. Woese CR, Kandler O, Wheelis ML (June 1990). "Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America 87 (12): 4576–9. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
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19. Xie Q, Wang Y, Lin J, Qin Y, Wang Y, Bu W (2012). "Potential key bases of ribosomal RNA to kingdom-specific spectra of antibiotic susceptibility and the possible archaeal origin of eukaryotes". PLoS ONE 7 (1): e29468. doi:10.1371/journal.pone.0029468. PMC 3256160. PMID 22247777.
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21. Steel M, Penny D (May 2010). "Origins of life: Common ancestry put to the test". Nature 465 (7295): 168–9. Bibcode:2010Natur.465..168S. doi:10.1038/465168a. PMID 20463725.
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