Prebiotic chemistry: a fuzzy field

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Prebiotic chemistrya fuzzy field

• Jacques Reisse
• Université Libre de Bruxelles
• Académie Royale de Belgique

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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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• 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

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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 .

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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

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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.

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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).

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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

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Prebiotic chemistry requires the use of fuzzy or
multivalued logic

• A is not necessarily B or not B but could be
  characterized by a value between
• 1 (set B) and 0 (set not B)
• Zadeh (1965)

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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

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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 quelles présentent, sont des
  moyens tout à fait artificiels. Rien de tout
  cela, je le répète ne se trouve dans la nature.

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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.

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•  Ainsi la nature passe petit à petit des êtres
  inanimés aux êtres doués de vie, si bien que
  cette continuité empêche dapercevoir la
  frontière qui les sépare, et quon ne sait auquel
  des deux groupes appartient la forme
  intermédiaire 
• Aristote (Histoire des animaux)

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• 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

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From non-living to livinga 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 gt 0
• Life index of proteins lpr gt 0
• Life index of polynucleotides lpo gt 0
• Life index of viruses lvi gt 0
• Life index of bacteria lba 1 (my
  assumption)
• with lmolt lprlt lpo ltlvi lt lba

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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 gt 0 ? (my assumption)
• Because pathways leading to mononucleotide in
  prebiotic conditions are unknown

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A prebiotic experiment from 1953 (before
Murchison)
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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

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• The Pantheon syndrome
• The  first ancestor  illusion

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The Pantheon syndrome
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The Pantheon syndrome in prebiotic chemistry
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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)

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The first ancestor illusion
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Less obvious!
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The LUCA illusion
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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

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Prebiotic chemistrya 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 Earths hydrosphere (local versus
  global conditions?)
• Chemical constraints
• Use efficiently the Van der Waals interactions
  to imagine how very peculiar supramolecular
  systems appeared spontaneously

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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

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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 lorigine de la matière organique
  sur la Terre primitive et de son évolution durant
  la période prébiotique
•  Lenvironnement de la Terre primitive 
• Presses Universitaires de Bordeaux (2001)

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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