Reaction scheme of the chemoton, showing the interplay of metabolism, information and structural closure. Based on Fig. 1.1 of Gánti (2003)[1]

The term chemoton (short for 'chemical automaton') refers to an abstract model for the fundamental unit of life introduced by Hungarian theoretical biologist Tibor Gánti. Gánti conceived the basic idea in 1952 and formulated the concept in 1971 in his book The Principles of Life (originally written in Hungarian, and translated to English only in 2003).[1][2] He suggested that the chemoton was the original ancestor of all organisms.

The basic assumption of the model is that life should fundamentally and essentially have three properties: metabolism, self-replication, and a bilipid membrane.[3] The metabolic and replication functions together form an autocatalytic subsystem necessary for the basic functions of life, and a membrane encloses this subsystem to separate it from the surrounding environment. Therefore, any system having such properties may be regarded as alive, and it will be subjected to natural selection and contain a self-sustaining cellular information. Some consider this model a significant contribution to origin of life as it provides a philosophy of evolutionary units.[4]

Property

The chemoton is a protocell that grows by metabolism, reproduces by biological fission, and has at least rudimentary genetic variation. Thus, it contains three subsystems, namely an autocatalytic network for metabolism, a lipid bilayer for structural organisation, and a replicating machinery for information. Unlike cellular metabolic reactions, the metabolism of the chemoton is in an autonomous chemical cycle and is not dependent on enzymes. Autocatalysis produces its own structures and functions. Hence, the process itself has no hereditary variation. However, the model includes another molecule (T in the diagram) that is spontaneously produced and is incorporated into the structure. This molecule is amphipathic like membrane lipids, but it is highly dynamic, leaving small gaps that close and open frequently. This unstable structure is important for new amphipathic molecules to be added, so that a membrane is subsequently formed. This will become a microsphere. Due to metabolic reaction, osmotic pressure will build up inside the microsphere, and this will generate a force for invaginating the membrane, and ultimately division. In fact, this is close to the cell division of cell wall-less bacteria, such as Mycoplasma. Continuous reactions will also invariably produce variable polymers that can be inherited by daughter cells. In the advanced version of the chemoton, the hereditary information will act as a genetic material, something like a ribozyme of the RNA world.[5]

Significance

Origin of life

The primary use of the chemoton model is in the study of the chemical origin of life. This is because the chemoton itself can be thought of as a primitive or minimal cellular life as it satisfies the definition of what a cell is (that it is a unit of biological activity enclosed by a membrane and capable of self-reproduction). Experimental demonstration showed that a synthesised chemoton can survive in a wide range of chemical solutions, it formed materials for its internal components, it metabolised its chemicals, and it grew in size and multiplied itself.[6]

Unit of selection

As it is scientifically hypothesised that the first replicating systems must be simple structure, most likely before any enzymes or templates existed, chemoton provides a plausible scenario. As an autocatalytic but non-genetic entity, it predates the enzyme-dependent precursors of life, such as RNA World. But being capable of self-replication and producing variant metabolites, it possibly could be an entity with the first biological evolution, therefore, the origin of the unit of Darwinian selection.[7][8][9]

Artificial life

The chemoton has laid the foundation of some aspects of artificial life. The computational basis has become a topic of software development and experimentation in the investigation of artificial life.[10] The main reason is that the chemoton simplifies the otherwise complex biochemical and molecular functions of living cells. Since the chemoton is a system consisting of a large but fixed number of interacting molecular species, it can effectively be implemented in a process algebra-based computer language.[11]

Comparison with other theories of life

The chemoton is just one of several theories of life, including the hypercycle of Manfred Eigen and Peter Schuster,[12] [13] [14] which includes the concept of quasispecies, the (M,R) systems[15] [16] of Robert Rosen, autopoiesis (or self-building)[17] of Humberto Maturana and Francisco Varela, and the autocatalytic sets[18] of Stuart Kauffman, similar to an earlier proposal by Freeman Dyson.[19] All of these (including the chemoton) found their original inspiration in Erwin Schrödinger's book What is Life?[20] but at first they appear to have little in common with one another, largely because the authors did not communicate with one another, and none of them made any reference in their principal publications to any of the other theories. (Gánti's book[1] does include a mention of Rosen, but this was added as an editorial comment, and was not written by Gánti.) Nonetheless, there are more similarities than may be obvious at first sight, for example between Gánti and Rosen.[21] Until recently[22][23][24] there have been almost no attempts to compare the different theories and discuss them together.

Last Universal Common Ancestor (LUCA)

Some authors equate models of the origin of life with LUCA, the Last Universal Common Ancestor of all extant life.[25] This is a serious error resulting from failure to recognize that L refers to the last common ancestor, not to the first ancestor, which is much older: a large amount of evolution occurred before the appearance of LUCA.[26]

Gill and Forterre expressed the essential point as follows:[27]

LUCA should not be confused with the first cell, but was the product of a long period of evolution. Being the "last" means that LUCA was preceded by a long succession of older "ancestors."

References

  1. ^ a b c Gánti, Tibor (2003). Eörs Száthmary; James Griesemer (eds.). The Principles of Life. Oxford University Press. ISBN 9780198507260.
  2. ^ Gánti, Tibor (31 December 2003). Chemoton Theory: Theory of Living Systems. Translated by Elisabeth Csárán. Kluwer Academic/Plenum Publishers. ISBN 9780306477850.
  3. ^ Van Segbroeck S, Nowé A, Lenaerts T (2009). "Stochastic simulation of the chemoton". Artif Life. 15 (2): 213–226. CiteSeerX 10.1.1.398.8949. doi:10.1162/artl.2009.15.2.15203. PMID 19199383. S2CID 10634307.
  4. ^ Hoenigsberg HF (2007). "From geochemistry and biochemistry to prebiotic evolution...we necessarily enter into Gánti's fluid automata". Genet Mol Res. 6 (2): 358–373. PMID 17624859.
  5. ^ John Maynard Smith; Eörs Száthmary (1997). The Major Transitions in Evolution. Oxford University Press. pp. 20–24. ISBN 9780198502944.
  6. ^ Csendes T (1984). "A Simulation study of the chemotron". Kybernetes. 13 (2): 79–85. doi:10.1108/eb005677.
  7. ^ Laurent Keller (1999). Levels of Selection in Evolution. Princeton University Press. p. 52. ISBN 9780691007045.
  8. ^ Munteanu A, Solé RV (2006). "Phenotypic diversity and chaos in a minimal cell model". J Theor Biol. 240 (3): 434–442. Bibcode:2006JThBi.240..434M. doi:10.1016/j.jtbi.2005.10.013. PMID 16330052.
  9. ^ Pratt AJ (2011). "Prebiological Evolution and the Metabolic Origins of Life". Artificial Life. 17 (3): 203–217. doi:10.1162/artl_a_00032. PMID 21554111. S2CID 6988070.
  10. ^ Hugues Bersini (2011). "Minimal cell: the computer scientist's point of view". In Muriel Gargaud; Purificación López-Garcìa; Hervé Martin (eds.). Origins and Evolution of Life: An Astrobiological Perspective. Cambridge University Press. pp. 60–61. ISBN 9781139494595.
  11. ^ Zachar I, Fedor A, Szathmáry E (2011). "Two different template replicators coexisting in the same protocell: stochastic simulation of an extended chemoton model". PLOS ONE. 6 (7): 1380. Bibcode:2011PLoSO...621380Z. doi:10.1371/journal.pone.0021380. PMC 3139576. PMID 21818258.
  12. ^ Eigen, M; Schuster, P (1977). "The hypercycle: a principle of natural self-organization. A: emergence of the hypercycle". Naturwissenschaften. 64 (11): 541–565. doi:10.1007/bf00450633. PMID 593400. S2CID 42131267.
  13. ^ Eigen, M; Schuster, P. "The hypercycle: a principle of natural self-organization. B: the abstract hypercycle". Naturwissenschaften. 65 (1): 7–41. doi:10.1007/bf00420631. S2CID 1812273.
  14. ^ Eigen, M; Schuster, P. "The hypercycle: a principle of natural self-organization. C: the realistic hypercycle". Naturwissenschaften. 65 (7): 41–369. doi:10.1007/bf00420631. S2CID 1812273.
  15. ^ Rosen, R. (1958). "The representation of biological systems from the standpoint of the theory of categories". Bull. Math. Biophys. 20 (4): 317–341. doi:10.1007/BF02477890.
  16. ^ Rosen, R. (1991). Life Itself: a comprehensive inquiry into the nature, origin, and fabrication of life. New York: Columbia University Press.
  17. ^ Maturana, H. R.; Varela, F. (1980). Autopoiesis and cognition: the realisation of the living. Dordrecht: D. Reidel Publishing Company.
  18. ^ Kauffman, S. A. (1969). "Metabolic stability and epigenesis in randomly constructed genetic nets". J. Theor. Biol. 22 (3): 437–467. Bibcode:1969JThBi..22..437K. doi:10.1016/0022-5193(69)90015-0. PMID 5803332.
  19. ^ Dyson, F. J. (1982). "A model for the origin of life". J. Mol. Evol. 18 (5): 344–350. Bibcode:1982JMolE..18..344D. doi:10.1007/bf01733901. PMID 7120429. S2CID 30423925.
  20. ^ Schrödinger, Erwin (1944). What is Life?. Cambridge University Press.
  21. ^ Cornish-Bowden, A. (2015). "Tibor Gánti and Robert Rosen: contrasting approaches to the same problem". J. Theor. Biol. 381: 6–10. Bibcode:2015JThBi.381....6C. doi:10.1016/j.jtbi.2015.05.015. PMID 25988381.
  22. ^ Letelier, J C; Cárdenas, M L; Cornish-Bowden, A (2011). "From L'Homme Machine to metabolic closure: steps towards understanding life". J. Theor. Biol. 286 (1): 100–113. Bibcode:2011JThBi.286..100L. doi:10.1016/j.jtbi.2011.06.033. PMID 21763318.
  23. ^ Igamberdiev, A.U. (2014). "Time rescaling and pattern formation in biological evolution". BioSystems. 123: 19–26. doi:10.1016/j.biosystems.2014.03.002. PMID 24690545.
  24. ^ Cornish-Bowden, A; Cárdenas, M L (2020). "Contrasting theories of life: historical context, current theories. In search of an ideal theory". BioSystems. 188: 104063. doi:10.1016/j.biosystems.2019.104063. PMID 31715221. S2CID 207946798.
  25. ^ Jheeta, S.; Chatzitheodoridis, E.; Devine, Kevin; Block, J. (2021). "The Way forward for the Origin of Life: Prions and Prion-Like Molecules First Hypothesis". Life. 11 (9): 872. Bibcode:2021Life...11..872J. doi:10.3390/life11090872. PMC 8467930. PMID 34575021.
  26. ^ Cornish-Bowden, A; Cárdenas, M L (2017). "Life before LUCA". J. Theor. Biol. 434: 68–74. Bibcode:2017JThBi.434...68C. doi:10.1016/j.jtbi.2017.05.023. PMID 28536033.
  27. ^ Gill, S.; Forterre, P. (2016). "Origin of life: LUCA and extracellular membrane vesicles (EMVs)". Int. J. Astrobiol. 15 (1): 7–15. Bibcode:2016IJAsB..15....7G. doi:10.1017/S1473550415000282. S2CID 44428292.

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