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Extraterrestrial Life: Spring 2008
Origin of life on Earth
Two approaches:
• bottom-up - which of the chemical / structural partsof modern life could have formed from abioticprocesses on the early Earth?
• top-down - which of the constituents of current cellscould have been part of earlier, simpler lifeforms?
Basic requirements:
• genetic information (DNA)• catalytic molecules (proteins)• cell membranes (lipids)
Extraterrestrial Life: Spring 2008
Miller-Urey experiment
1953 experiment:mixture of severalsimple gases (water,hydrogen, methaneand ammonia) wasexposed to sparks(“lightning”) and cycled through a model of the ocean /atmosphere
Extraterrestrial Life: Spring 2008
Result: large number of complexorganic molecules formed in the experiment, including a number of amino acids
…within weeks of the discoveryof the structure of DNA…
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 isa reducing atmosphere, very different from the oxidizingconditions on the Earth today.
Early Earth was likely devoid of oxygen, atmospheric composition would have been controlled by volcanicoutgassing.
Large CO2 concentration (needed for strong greenhouseeffect to offset the faint young Sun) leads to less efficient synthesis of organic molecules.
Extraterrestrial Life: Spring 2008
Oceanic synthesis
Amino acids can also be formedunder conditions similar to hydrothermal vents
Protected against impacts, common,thermophiles seem like simple organisms…
However, complex organic chemicals are also destroyedby the high temperatures - todaywater cycles through such systems on a timescaleof only ~10 million years
Extraterrestrial Life: Spring 2008
Meteoritic delivery
Comets or meteorites can also bea source: Murchison meteoritemay contain as many as 70 amino acids
Synthesis of small molecules: site is very uncertain,but there are several plausible candidates (possiblythe atmosphere, ocean vents, extraterrestrial delivery)
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Extraterrestrial Life: Spring 2008
What about chirality?
Both meteoritic amino acids and those synthesized in Miller-Urey type experiments tend to be almost racemicmixtures: equal amounts of left-handed and right-handedversions
Additionally, the set of 20 amino acids used in biologytoday is not particularly favored
Extraterrestrial Life: Spring 2008
The RNA-world hypothesis
Currently:• DNA encodes information for building proteins• Proteins catalyze the cellular mechanisms that
lead to their formation
RNA (ribonucleic acid) can fulfill both functions:
• 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
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alanine + glycine " di - peptide + H2O
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
Extraterrestrial Life: Spring 2008
Forming RNA
Constituents of RNA:
• sugar (ribose)• base (adenine, guanine,
cytosine, uracil)• phosphate
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 thefirst `RNA’-like molecules?
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 plausibleearly-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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Extraterrestrial Life: Spring 2008
Role of mineral catalysts
Often suggested that mineral catalysts such as claysmay 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-replication4) Membranes formed to enclose RNA5) Natural selection led to an increase in complexity,
until eventually something recognizably livingformed
Extraterrestrial Life: Spring 2008
Prospects for progress
• better understanding of the the atmospheres ofplanets (from observing others)
• deeper knowledge of the function of all themolecules involved in life
• lab experiments• discovery of life elsewhere
Clearly this topic involves too much extrapolation from`known’ conditions to allow for robust conclusions.
Life started somehow - can we ever know how?