Download Origin of life on Earth Two approaches: • bottom-up

Origin of life on Earth
Two approaches:
• bottom-up - which of the chemical / structural parts
of modern life could have formed from abiotic
processes on the early Earth?
• top-down - which of the constituents of current cells
could have been part of earlier, simpler life
forms?
Basic requirements:
Miller-Urey experiment
1953 experiment:
mixture of several
simple gases (water,
hydrogen, methane
and ammonia) was
exposed to sparks
(“lightning”) and
cycled through a
model of the ocean /
atmosphere
• genetic information (DNA)
• catalytic molecules (proteins)
• cell membranes (lipids)
Extraterrestrial Life: Spring 2008
Result: large number of complex
organic molecules formed in
the experiment, including a
number of amino acids
Extraterrestrial Life: Spring 2008
Experiment has been repeated many times: results stand,
under these conditions amino acids are synthesized easily
Are the conditions a realistic depiction of the early Earth?
H2O, H2, CH4 and NH3 are major constituents of the
atmosphere of the giant planets. Chemically, this is
a reducing atmosphere, very different from the oxidizing
conditions on the Earth today.
…within weeks of the discovery
of the structure of DNA…
Early Earth was likely devoid of oxygen, atmospheric
composition would have been controlled by volcanic
outgassing.
Large CO2 concentration (needed for strong greenhouse
effect to offset the faint young Sun) leads to less
efficient synthesis of organic molecules.
Extraterrestrial Life: Spring 2008
Oceanic synthesis
Extraterrestrial Life: Spring 2008
Meteoritic delivery
Amino acids can also be formed
under conditions similar to
hydrothermal vents
Comets or meteorites can also be
a source: Murchison meteorite
may contain as many as 70
amino acids
Protected against impacts, common,
thermophiles seem like simple
organisms…
However, complex organic
chemicals are also destroyed
by the high temperatures - today
water cycles through such systems on a timescale
of only ~10 million years
Extraterrestrial Life: Spring 2008
Synthesis of small molecules: site is very uncertain,
but there are several plausible candidates (possibly
the atmosphere, ocean vents, extraterrestrial delivery)
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The RNA-world hypothesis
What about chirality?
Currently:
• DNA encodes information for building proteins
• Proteins catalyze the cellular mechanisms that
lead to their formation
RNA (ribonucleic acid) can fulfill both functions:
Both meteoritic amino acids and those synthesized in
Miller-Urey type experiments tend to be almost racemic
mixtures: equal amounts of left-handed and right-handed
versions
Additionally, the set of 20 amino acids used in biology
today is not particularly favored
• carry information that can be copied
• can catalyze reactions, including the formation
of proteins (though less efficiently than
current mechanisms)
Hypothesis: first life may have been based around RNA,
with the DNA / protein symbiosis evolving later
Extraterrestrial Life: Spring 2008
Formation of more complex molecules
Getting to the RNA-world (or similar schemes based on
other molecules) from simple precursors is the hardest step:
Proteins
alanine + glycine " di - peptide + H 2O
Extraterrestrial Life: Spring 2008
Forming RNA
Constituents of RNA:
• sugar (ribose)
• base (adenine, guanine,
cytosine, uracil)
• phosphate
Presence of water as a product means that in water,
dissociation rather than polymerization is favored…
!
Moreover: more or less random order of the monomers
is obtained even under conditions where proteins form
Synthesis of the bases (especially adenine) appears to
be relatively easy - HCN can yield adenine in water
when exposed to ultraviolet light
Extraterrestrial Life: Spring 2008
Sugars: pre-biotic synthesis of sugars has been
demonstrated experimentally from formaldehyde
(CH2O) in the presence of mineral catalysts
BUT - these reactions yield a mix of many sugars
(as many as 40) with no chiral preference
Chirality of ribose affects the 3D structure of RNA,
upon which the hereditary system rests… how did
one form of ribose come to dominate?
Speculation: perhaps ribose was not part of the
first `RNA’-like molecules?
Extraterrestrial Life: Spring 2008
Extraterrestrial Life: Spring 2008
Polymerization of nucleotides
Neither of the critical steps:
• reactions of the bases with ribose
• reaction to join in the phosphate
…have been demonstrated to occur under plausible
early-Earth conditions, though there are many
possible pathways.
In particular, water and high temperatures are
unfavorable for the survival of RNA…
Where might early life have started?
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Role of mineral catalysts
Often suggested that mineral catalysts such as clays
may have played a critical role
Early chemistry took place on surfaces
Chirality may have been inherited from surface defects
Enclosure within cell membranes came later…
Extraterrestrial Life: Spring 2008
1) Organic molecules formed, either in the atmosphere,
at undersea vents, or via delivery from space
2) Short strands of RNA formed with the help of
catalytic materials (perhaps clays)
3) RNA became capable of self-replication
4) Membranes formed to enclose RNA
5) Natural selection led to an increase in complexity,
until eventually something recognizably living
formed
Extraterrestrial Life: Spring 2008
Prospects for progress
Clearly this topic involves too much extrapolation from
`known’ conditions to allow for robust conclusions.
Life started somehow - can we ever know how?
• better understanding of the the atmospheres of
planets (from observing others)
• deeper knowledge of the function of all the
molecules involved in life
• lab experiments
• discovery of life elsewhere
Extraterrestrial Life: Spring 2008
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