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Prebiotic chemistry: a fuzzy field

Prebiotic chemistry: a fuzzy field. Jacques Reisse Université Libre de Bruxelles Académie Royale de Belgique. Prebiotic chemistry. Ideally : the study of the chemical steps leading to the first living systems But no fossils, no traces … nothing from these first living systems remains.

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Prebiotic chemistry: a fuzzy field

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  1. Prebiotic chemistry:a fuzzy field Jacques Reisse Université Libre de Bruxelles Académie Royale de Belgique

  2. Prebiotic chemistry Ideally : the study of the chemical steps leading to the first living systems But no fossils, no traces … nothing from these first living systems remains

  3. “Results rarely specify their causes unambiguously. If we have no direct evidences of fossils or human chronicles, if we are forced to infer a process only from its modern results, then we are usually reduced to speculation about probabilities.” S.J. Gould

  4. Prebiotic chemistry Study of the spontaneous generation of the first living cells. Study of the progressive transition from “non living systems” to “living systems” But There is no consensus as to what a “living system” is .

  5. It is not a problem not having a clear-cut definition of “living systems” “In the world of human thought generally, and in physical science in particular, the most important and most fruitful concepts are those to which it is impossible to attach a well-defined meaning.” Hendrik A. Kramers

  6. What is certain! Prebiotic chemistry is related to synthetic chemistry and to synthetic biology but is fundamentally different from these two fields: there is no chemist at work, “only” the laws of physics and chemistry acting on sub-systems (atoms, small molecules) under not well known conditions.

  7. The strange status of prebiotic chemistry Prebiotic chemistry is not the search for the unknown pathways from non living to living systems. It consists in inventing possible pathways (which will never be fully confirmed).

  8. Prebiotic chemistry requires that Aristotelian logic be put aside • The law of contradiction A cannot be both “B” and “non B” • The law of excluded middle A must be either “B” or “non B” • The law of identity A will always be A These axioms are considered as self evident

  9. Prebiotic chemistry requires the use of fuzzy or multivalued logic A is not necessarily B or “not B” but could becharacterized by a value between 1 (set “B”) and 0 (set “not B”) Zadeh (1965)

  10. A molecular system can be “partially” living • This was necessarily true during the prebiotic period (progressive spontaneous generation) • This is still true today (viruses, spores) • Analogy between the “non living – living” problem and the species problem, as clearly identified by Lamarck, Darwin… and Aristotle

  11. Lamarck and the concept of species “Mais ces classifications, dont plusieurs ont été si heureusement imaginées par les naturalistes, ainsi les divisions et sous-divisions qu’elles présentent, sont des moyens tout à fait artificiels. Rien de tout cela, je le répète ne se trouve dans la nature….”

  12. Darwin and the concept of species “I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other….”

  13. « Ainsi la nature passe petit à petit des êtres inanimés aux êtres doués de vie, si bien que cette continuité empêche d’apercevoir la frontière qui les sépare, et qu’on ne sait auquel des deux groupes appartient la forme intermédiaire » Aristote (Histoire des animaux)

  14. The concept of species is contradictory with the concept of evolution. This is true for organisms but also for molecular or supramolecular systems evolving from the “non living” state to the “living” state

  15. From “non-living” to “living”a continuous scale The scale is necessarily arbitrary (and dependent on scientific knowledge) • “Life index” of H, C, O, N atoms: lat = 0 • “Life index” of liquid water: lwa = 0 • “Life index” of aminoacids:laa = 0 (my assumption) • “Life index” of mononucleotides: lmo > 0 • “Life index” of proteins: lpr> 0 • “Life index” of polynucleotides: lpo > 0 • “Life index of viruses: lvi > 0 • “Life index” of bacteria:lba = 1 (my assumption) with lmo< lpr< lpo <lvi < lba

  16. “Life index” of aminoacids versus “Life index” of mononucleotides • Why laa = 0 ? (my assumption) Because (non racemic) AA are present in carbonaceous chondrites (Murchison, 1969) • Why lmo> 0 ?(my assumption) Because pathways leading to mononucleotide in “prebiotic conditions” are unknown

  17. A prebiotic experiment from 1953 (before Murchison)

  18. Why a living index higher for a polymer than for a monomer? H-K-OH + H-L-OH + H-M-OH + H-Q-OH → H-K-L-M-Q-OH + 3 HOH • Because the condensation of monomers under “prebiotic conditions” : - requires activation - requires confinement • Because hydrolysis (←) is always a competitive reaction

  19. The Pantheon syndrome • The « first ancestor » illusion

  20. The Pantheon syndrome

  21. The Pantheon syndrome in prebiotic chemistry

  22. A dramatic example of the Pantheon syndrome « Radio-astronomers have discovered a vast array of organic molecules in the interstellar medium. We are thus led to the inescapable conclusion that life must be common place in the cosmos » C. Ponnamperuma (1993)

  23. The “first ancestor” illusion

  24. Less obvious!

  25. The LUCA illusion

  26. LUCA is definitively not the first living cell It is “simply” the ancestor of present-day living species and (most probably) the ancestor of all the organisms known as fossils or microfossils LUCA was just the best fit in a large collection of “pre-LUCA” living systems

  27. Prebiotic chemistry:a fuzzy field under strong constraints • Epistemological constraints - Not forget what we are searching for - Abandon Aristotelian logic • Geochemical constraints Take into account the conditions prevailing in the young Earth’s hydrosphere (local versus global conditions?) • Chemical constraints Use efficiently the Van der Waals interactions to imagine how very peculiar supramolecular systems appeared spontaneously

  28. The greatest difficulties in prebiotic chemistry • To imagine possible pathways for the spontaneous formation of a population of far-from equilibrium supramolecular systems able to evolve by Darwinian evolution • To imagine how a language (not only the support of) can appear spontaneously

  29. Merci pour votre attention! • Comment definir la vie? Editeurs: Hugues Bersini et Jacques Reisse. Vuibert (2007) • Jacques Reisse. La longue histoire de la matière PUF (2006) • From Suns to Life as co-editor and co-author of two chapters. ( Springer, 2006) • John Cronin and Jacques Reisse Chirality and the Origin of Homochirality “Lectures in Astrobiology, vol.1”. Springer (2005) • Jacques Reisse A propos de l’origine de la matière organique sur la Terre primitive et de son évolution durant la période prébiotique « L’environnement de la Terre primitive » Presses Universitaires de Bordeaux (2001)

  30. Exaptation: an efficient concept in biological evolution « Exaptation: features that now enhance fitness but were not built by natural selection for their present role » Gould and Urba (1982) Prebiotic chemical evolution requires exaptation, at least in its « last stages », when natural selection was already an efficient process

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