The origin of life on Earth - JILA

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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)


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


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…


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.


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


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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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


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


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


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


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?


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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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…


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


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?

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The origin of life on Earth - JILA