The last universal common ancestor (LUCA), also called the last universal ancestor (LUA), or concestor, is the most recent population of organisms from which all organisms now living on Earth have a common descent, the most recent common ancestor of all current life on Earth. (A related concept is that of progenote.[R 1]) LUCA is not thought to be the first life on Earth but only one of many early organisms, all the others becoming extinct.
Charles Darwin first 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." Later biologists have separated the problem of the origin of life from that of the LUCA.
By analysis of the presumed LUCA's offspring groups, the LUCA appears to have been a small, single-celled organism. It likely had a ring-shaped coil of DNA floating freely within the cell, like modern bacteria. Morphologically, it would likely not have stood out within a mixed population of small modern-day bacteria. However, Carl Woeseet al., who first proposed the currently-used three domain system based on an analysis of ribosomal RNA (rRNA) sequences of bacteria, archaea, and eukaryotes, 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)".
While the gross anatomy of LUCA can only be reconstructed with much uncertainty, its biochemical mechanisms can be described in some detail, based on the properties currently shared by all independently living organisms on Earth.
The genetic code was expressed into proteins. These were assembled from free amino acids by translation of a messenger RNA via a mechanism of ribosomes, transfer RNAs, and a group of related proteins. The ribosomes were composed of two subunits, a big 50S and a small 30S. Each ribosomal subunit was composed of a core of ribosomal RNA surrounded by ribosomal proteins. Both types of RNA molecules (ribosomal and transfer RNAs) played an important role in the catalytic activity of the ribosomes. Only 20 amino acids were used, only in L-isomers, to the exclusion of countless other amino acids. ATP served as an energy intermediate. Several hundred protein enzymes catalyzed chemical reactions to extract energy from fats, sugars, and amino acids, and to synthesize fats, sugars, amino acids, and nucleic acid bases through various chemical pathways.
A 1990 phylogenetic tree linking all major groups of living organisms to the LUCA (the black trunk at the bottom), based on ribosomal RNA sequence data.
In 1859, Charles Darwin published On 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." The very last sentence begins with 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..."
When the LUCA 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 the archaeans known at that time were highly resistant to environmental extremes such as high salinity, temperature or acidity, leading some scientists to suggest that the LUCA evolved in areas like the deep ocean vents, where such extremes prevail today. Archaea, however, were later discovered in less hostile environments, and are now believed to be more closely related to the Eukaryota than to the Bacteria, although many details are still unknown.
In 2010, based on "the vast array of molecular sequences now available from all domains of life," a formal test of universal common ancestry was published. The formal test favored the existence of a universal common ancestor over a wide class of alternative hypotheses that included horizontal gene transfer. Basic biochemical principles make it overwhelmingly likely that all organisms do have a single common ancestor. It is extremely unlikely that organisms descended from separate incidents of cell-formation would be able to complete a horizontal gene transfer without garbling each other's genes, converting them into noncoding segments. Further, many more amino acids are chemically possible than the twenty found in modern protein molecules. These lines of chemical evidence, incorporated into the formal statistical test point to a single cell having been the LUCA. While the test overwhelmingly favored the existence of a single LUCA, this does not imply that the LUCA was ever alone: instead, it was one of many early microbes but the only one whose descendants survived beyond the Paleoarchean Era.
With the later gene pool of the LUCA's descendants, with their common framework of the AT/GC rule and the standard twenty amino acids, horizontal gene transfer would have been feasible and could have been very common.
In an earlier hypothesis, Carl Woese (1988) had proposed that: (1) no individual organism can be considered a LUCA, and (2) the genetic heritage of all modern organisms derived through horizontal gene transfer among an ancient community of organisms. While the results described by Theobald (2010) and Saey (2010) demonstrate the existence of a single LUCA, Woese's argument can still be applied to Ur-organisms (initial products of abiogenesis) before the LUCA. At the beginnings of life, ancestry was not as linear as it is today because the genetic code had not evolved. Before high fidelity replication, organisms could not be easily mapped on a phylogenetic tree. However, the LUCA lived after the origin of the genetic code and at least some rudimentary early form of molecular proofreading.
These findings could mean that life on Earth originated in such hydrothermal vents, but it is also possible that life was restricted to such locations at some later time, perhaps by the Late Heavy Bombardment. The identification of these genes as being present in LUCA has also been criticized, as they may simply represent later genes which migrated via horizontal gene transfers between archaea and bacteria.
^There is a common misconception that definitions of LUCA and progenote are the same; however, progenote is defined as an organism “still in the process of evolving the relationship between genotype and phenotype”, and it is only hypothesized that LUCA is a progenote. Gogarten, Johann Peter; Deamer, David (25 November 2016). "Is LUCA a thermophilic progenote?". Nature Microbiology. 1 (12): 16229. doi:10.1038/nmicrobiol.2016.229. PMID27886195.
^ abcdWeiss, M. C.; Sousa, F. L.; Mrnjavac, N.; Neukirchen, S.; Roettger, M.; Nelson-Sathi, S; Martin, W. F. (2016). "The physiology and habitat of the last universal common ancestor". Nat Microbiol. 1 (9): 16116. doi:10.1038/nmicrobiol.2016.116. PMID27562259.
^Gogarten, J.P.; Taiz, L. (1992). "Evolution of Proton Pumping ATPases: Rooting the Tree of Life". Photosynthesis Research. 33 (2): 137–46. doi:10.1007/BF00039176. PMID24408574.
^Gribaldo, S.; Cammarano, P. (1998). "The Root of the Universal Tree of Life Inferred from Anciently Duplicated Genes Encoding Components of the Protein-Targeting Machinery". Journal of Molecular Evolution. 47 (5): 508–16. doi:10.1007/pl00006407. PMID9797401.