Abiogenesis

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Transcript Abiogenesis

Science and Creationism
6. Abiogenesis
© Colin Frayn, 2008-2011
www.frayn.net
Abiogenesis
• Literally, the emergence of life from
non-living origins
• This is organic chemistry, not
biological evolution
• There are several competing ideas
• Bottom line: we really don’t know
how this happened yet
• Yes, science has the guts to say “we
don’t know”!
© Colin Frayn, 2008-2011
www.frayn.net
Miller-Urey
• Stanley L. Miller & Harold C. Urey
– 1953, University of Chicago
• Experiment designed to simulate the early
Earth
• Synthesised organic compounds
– The building blocks of life
• First of many experiments
– Using varying atmospheric assumptions
– All 20 necessary amino acids can be
synthesised easily
© Colin Frayn, 2008-2011
www.frayn.net
Miller-Urey Apparatus
© Colin Frayn, 2008-2011
www.frayn.net
Reducing Atmosphere (1)
• Miller & Urey assumed a ‘reducing’
atmosphere
– Low in Oxygen, high in Hydrogen
• Some studies suggested this was not the
case on the early Earth
• Latest studies favour reducing atmosphere
– See e.g. Tian et al (2005), Schaefer & Fegley (2007)
– Atmosphere was approximately 30-40% Hydrogen
– Exactly as required
• Proofs:
– Banded iron, Uraninite, paleosoils, pyrite
© Colin Frayn, 2008-2011
www.frayn.net
Reducing Atmosphere (2)
• A reducing atmosphere is vulnerable
to ultra-violet (UV) radiation
– No oxygen to form ozone layer
• Ozone layer protects against UV radiation
– Recent studies
• Effect of UV is less than was originally thought
• Aerosol haze can efficiently block UV radiation
• E.g. Zahnle (1986), Sagan & Chyba (1997) and Pavlov et
al (2000)
• If life developed close to deep-sea hydrothermal
vents then the UV was not a problem!
© Colin Frayn, 2008-2011
www.frayn.net
Autocatalysis
• What came first, DNA or proteins?
– DNA is needed as a pattern for proteins
– Proteins are needed to catalyse DNA
– Chicken and egg?
• RNA can auto-catalyse
– RNA is very similar to DNA
– We still have RNA in our body cells
– It can catalyse its own reactions
– RNA is a plausible precursor to DNA
© Colin Frayn, 2008-2011
www.frayn.net
Hydrothermal Vents
• Discovered in 1949
• First imaged in 1977
• Unique ecosystems
• Under many kilometres of water
© Colin Frayn, 2008-2011
www.frayn.net
How did RNA arise?
• RNA is (probably) too complex to have arisen in one step
– Plausible precursors are required
• Benefits of RNA
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Can replicate itself
Is simpler than DNA
Can self-catalyse
Cell replication apparatus is RNA
• Formation of RNA through ribonucleotides
– Problematic as ribose is difficult to form
– Maybe ribonucleotides arose through a different process?
– Nature, 459, pp239-242, Powner, Gerland & Sutherland
(2009)
© Colin Frayn, 2008-2011
www.frayn.net
From RNA to Cells
• Lipids spontaneously form bilayer ‘vesicles’
– These are spherical shells on a molecular scale
– They can contain self-replicating RNA strands
– This provides an ideal, self-contained environment
• Problems
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How to form sufficient concentrations of lipids?
Work is on-going
E.g. Szostak & team (Harvard)
Hydrothermal vents?
© Colin Frayn, 2008-2011
www.frayn.net
A Likely Model
• Amino acids are formed
– See the Miller-Urey Experiment
• Lipid bilayers form
– These are observed to form spontaneously
• Self-replicating RNA strings arise
– This stage is uncertain, but plausible
– RNA is able to catalyse its own replication
• RNA strings merge with bilayer shells
– Lipid bilayers form ‘vesicles’ or shells
• First cells form
– All the components are held together in one place
– Facilitates chemical processes
© Colin Frayn, 2008-2011
www.frayn.net
Progress
• Recent studies have made solid progress,
but abiogenesis remains a very
complicated topic
• We are very slowly filling in the remaining
gaps in our knowledge
• Don’t expect this to be ‘solved’ any time
soon
• ‘Unknown’ ≠ ‘unknowable’
• Don’t give in to intellectual cowardice and
say “we can never explain this”!
© Colin Frayn, 2008-2011
www.frayn.net