Evolution by Natural Selection and the Origin of Life

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Evolution by Natural Selection andthe Origin of
Life

• Mauro Santos
• Universitat Autònoma de Barcelona
• Collegium Budapest (Institute for Advanced Study)

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There is grandeur in this view of life, with
its several powers, having being breathed into a
few forms or into one and that, whilst this
planet has gone cycling on according to the fixed
law of gravity, from so simple a beginning
endless forms most beautiful and most wonderful
have being, and are being, evolved. Darwin
1859. The Origin of Species
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Units of evolution

• Multiplication
• Heredity
• Variation

Hereditary traits affecting survival and/or
reproduction
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Origin of life The genetics- or
replication-first approach
At some point a particularly remarkable
molecule was formed by accident. We will call it
the Replicator. It may not have been the biggest
or the most complex molecule around, but it had
the extraordinary property of being able to
create copies of itself. Richard Dawkins
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In actual cells replication means doubling of the
DNA (exponential growth), but this requires a
quite complicate enzymatic machinery. Earlier
replicators had to be far more simple.
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Replicator-first theorists must explain how
such a complicate molecule could have formed
before the process of evolution was under way.
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Origin of life The metabolism-first scenario
Life, in a deep sense, crystallized as a
collective self-reproducing metabolism in a space
of possible organic reactions. Stuart Kauffman
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Metabolism-first proponents must show that
reaction networks capable of growing and evolving
could have formed when the earth was young.
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But what is life?

• This may be a philosophical question
• Better to ask what a living system is!
• Autonomous life is always cellular
• But there are several types of cells
• The main divide is between eukaryotes (cells with
  nuclei) and prokaryotes (bacteria)

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The eukaryotic cell is very complextoo complex!
It took ?2 billion years for life to reach this
complexity!
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The simplest cells are bacterial

• THUS we want to explain the origin of some
  primitive bacterium-like cell
• Even present-day bacteria are far too complex
• The main problem is the genetic code

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DNA first? Protein first?
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The RNA world is a nice idea, because

• You do not have to solve the problem of the
  ORIGIN OF LIFE and that of the ORIGIN OF THE
  GENETIC CODE at once
• In the case of RNA information flows from gene to
  enzyme and back (lack of translation)
• Goes back to Woese (1967), Crick (1968) and Orgel
  (1968)

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Early replication is still a problem

• Early replication must have been error-prone
• Error threshold sets the limit of maximal genome
  size to lt100 nucleotides
• Not enough for several genes
• Unlinked genes will compete
• Genome collapses
• Resolution???

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Eigens paradox (1971)

• Suppose that to increase the maintainable amount
  of
• information, an evolving (Darwinian) system must
• acquire a more complex molecular mechanism to
• reduce the mutation rate. However, to have such a
• complex molecular mechanism the system must
• maintain a longer sequence in the first place.
• The system will encounter
• a barrier in the evolution
• of complexity

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Molecular hypercycle (Eigen, 1971)
autocatalysis
heterocatalytic aid
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Parasites in the hypercycle (Maynard Smith, 1979)
short cuts
parasite
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Population structure is necessary!

• Compartments are, by clonal selection, not
  only the best countermeasures against molecular
  parasites, but the best vehicles for the
  selection of molecular function, such as
  catalytic aid in metabolism

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The stochastic corrector model
metabolic gene
replicase
membrane
Szathmáry and Demeter (1987) J. Theor.
Biol. Zintzaras et al. (2002) J. Theor.
Biol. Santos et al. (2003) OLEB
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Dynamics of the SC model

• Independently reassorting genes (with gene
  redundancy)
• 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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Robustness to deleterious mutations
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Microfluidic device to mimic the life cycle of a
protocell
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Microfluidics Manipulation of droplets
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RNA secondary structure

• There is more structure than sequence
• Neutral paths
• The phenotype is more easily maintained than the
  genotype.

Phenotypic error threshold
which is lower than the genotypic error
threshold. Hypothesis More information can be
maintained
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Peter Schuster 2001. Biol. Chem. 3821301-1314
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Neurospora Varkund Satellite Ribozyme
N 144
83/144 (57) of the positions were mutated, we
used 183 mutants
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Hairpin Ribozyme
N 50
39/50 (78) of the positions were mutated, we
used 142 mutants
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Maintainable genome size
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Holmes, E.C. Nat. Genet. NewsViews
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Microfluidics Manipulation of droplets
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Lethal selfish parasite
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I have long regretted that I truckled to public
opinion, and used the Pentateuchal term of
creation, by which I really meant appeared by
some wholly unknown process. It is mere rubbish,
thinking at present of the origin of life one
might as well think of the origin of
matter. Darwin (letter written in 1863 to J. D.
Hooker, the most important British botanist of
the nineteenth century).
The origin of life, being an event that had
occurred in nature, needs to be understood in
terms of natural processes.
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Coauthors
Ádám Kun
Elias Zintzaras
Eörs Szathmáry