Hard problems of the origin of life
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Hard problems of the origin of life
Eörs Szathmáry
Collegium Budapest Eötvös University
München
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Chemical evolution
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Units of evolution
hereditary traits affecting survival and/or reproduction
1. multiplication
2. heredity
3. variation
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Gánti’s chemoton model (1974)
ALL THREE SUBSYSTEMS ARE AUTOCATALYTIC
template copying
metabolism
membrane growth
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The latest edition: OUP 2003
• After several editions in Hungarian
• Two previous books (the Principles and Contra Crick) plus one essay
• Essays appreciating the biological and philosophical importance
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Pathways of supersystem evolution
boundary
template
metabolism M B
B T
M T M B T
INFRABIOLOGICAL SYSTEMS
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What about replication?
• Replication from a chemical point of view always rests on autocatalysis
• The basic form is A + X 2 A + Y
• very important for biology • Much more general than DNA
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The formose ‘reaction’
formaldehyde
glycolaldehyde
autocatalysis
Butlerow, 1861
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Replication in the formose reaction
• Replication is non-informational• Autocatalysis – YES• Heredity – NO• Good for metabolism• Not good for genetics• Butlerow was born on the 15th Sept, 1829• He was regarded as one of the best lecturers of his
time. His lectures were lucid and thorough, yet his language was colourful. Local society often preferred his lectures to the theatre
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Primitive ancestry of the reverse citric acid cycle
• Was proposed by Günter Wächtershäuser (1990)
• Coupled to CO2 fixation and pyrite formation around deep-sea hydrothermal vents
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The main problem of the origin of life is metabolite channelling
• Enzymes speed up reactions relative to the unwanted reactions
• Spontaneous decay reactions abound• Maintenance, not only reproduction,
requires autocatalysis dx/ dt = k x – d x = 0
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All network models neglecting side reactions were seriously incomplete
• E.g. protein networks• In model assumptions, a reaction is either
good or neutral for the system – but the number of harmful transformations is in fact much higher
• Did life emerge from a chemical canyon?
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Chemical evolution was a race between tar formation and life
formation
Chemical networks
Life Tar
What fraction of planets would end up with just tar?
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Another case: von Kiedrowski’s replicators
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Von Kiedrowski’s replicator
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Peptide replicator networks
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• Theory with experiment• J. Mol. Evol.,
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Does temperature cycling work?
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Elongation taxes the system badly
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Classification of replicatorsLimited heredity
Unlimited heredity
Holistic formose
Modular Von Kiedrowski
genes
Limited (number of individuals) > (number of types)
Unlimited (# of individuals) << (# of types)
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A crucial insight: Eigen’s paradox (1971)
• Early replication must have been error-prone
• Error threshold sets the limit of maximal genome size to <100 nucleotides
• Not enough for several genes• Unlinked genes will compete• Genome collapses• Resolution???
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Molecular hypercycle (Eigen, 1971)
autocatalysis
heterocatalytic aid
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Parasites in the hypercycle (JMS)
parasite
short circuit
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The stochastic corrector model for compartmentation
Szathmáry, E. & Demeter L. (1987) Group selection of early replicators and the origin of life. J. theor Biol. 128, 463-486.
Grey, D., Hutson, V. & Szathmáry, E. (1995) A re-examination of the stochastic corrector model. Proc. R. Soc. Lond. B 262, 29-35.
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Dynamics of the SC model
• Independently reassorting genes• Selection for optimal gene composition between
compartments• Competition among genes within the same
compartment• Stochasticity in replication and fission generates
variation on which natural selection acts• A stationary compartment population emerges
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Group selection of early replicators
• Many more compartments than templates within any compartment
• No migration (fusion) between compartments
• Each compartment has only one parent• Group selection is very efficient• Selection for replication synchrony Chromosomes!
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Kauffman: Reflexively autocatalytic protein networks (1986)
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Current investigations
• Evolvability is possible only in compartments • Occasionally new autocatalytic loops appear• Can be inherited from one cell to the daugther• Can be selected for, give some evolution• GARD is shadow of protein networks is a shadow
of template replicators
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Open questions
• Origin of efficient replication• Origin of full protocells• Origin of transcription • Origin of highly specific enzymes• Origin of translation