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The Origin of Life: Abiotic Synthesis of Organic Molecules

Two theories - with many subtheories - attempt to account for the origin of life on our earth.

Spontaneous generation in a primeval soup: Miller's Experiment

Stanley Miller, a graduate student in biochemistry, built the apparatus shown here. He filled it with

He hypothesized that this mixture resembled the atmosphere of the early earth. (Some are not so sure.) The mixture was kept circulating by continuously boiling and then condensing the water.

The gases passed through a chamber containing two electrodes with a spark passing between them.

At the end of a week, Miller used paper chromatography to show that the flask now contained several amino acids as well as some other organic molecules.

In the years since Miller's work, many variants of his procedure have been tried. Virtually all the small molecules that are associated with life have been formed: One difficulty with the primeval soup theory is how polymers - the basis of life itself - could be assembled.
Link to a discussion of enantiomers.

This has led to a theory that early polymers were assembled on solid, mineral surfaces that protected them from degradation, and in the laboratory polynucleotides and polypeptides containing about ~50 units have been synthesized on mineral (e.g., clay) surfaces.

An RNA Beginning?

All metabolism depends on enzymes and, until recently, every enzyme has turned out to be a protein. But proteins are synthesized from information encoded in DNA and translated into mRNA. So here is a chicken-and-egg dilemma. The synthesis of DNA and RNA requires proteins. So The discovery that certain RNA molecules have enzymatic activity provides a possible solution. These RNA molecules - called ribozymes - incorporate both the features required of life:
Link to a discussion of ribozymes.

While no ribozyme in nature has yet been found that can replicate itself, ribozymes have been synthesized in the laboratory that can catalyze the assembly of short oligonucleotides into exact complements of themselves. The ribozyme serves as both

(The figure is based on the work of Green and Szostak, Science 258:1910, 1992.)

In principal, the minimal functions of life might have begun with RNA and only later did Several other bits of evidence support this notion of an original "RNA world":

Panspermia: The Murchison Meteorite

Representative amino acids found in the Murchison meteorite. Six of the amino acids (blue) are found in all living things, but the others (yellow) are not normally found in living matter here on earth. The same amino acids are produced in discharge experiments like Miller's.
Glycine Glutamic acid
Alanine Isovaline
Valine Norvaline
Proline N-methylalanine
Aspartic acid N-ethylglycine
This meteorite, that fell near Murchison, Australia on 28 September 1969, turned out to contain a variety of organic molecules including both purines and pyrimidines as well as the amino acids listed here. The amino acids and their relative proportions were quite similar to the products formed in Miller's experiments.

The question is: were these molecules simply terrestrial contaminants that got into the meteorite after it fell to earth.

Probably not:

The ALH84001 meteorite

This meteorite arrived here from Mars. It contained not only a variety of organic molecules, including polycyclic aromatic hydrocarbons, but - some claim - evidence of microorganisms as well.

Furthermore, there is evidence that its interior never rose about 40° C during its fiery trip through the earth's atmosphere. Live bacteria could easily survive such a trip.

Link to a discussion of the possibility of life on Mars and more on the ALH84001 meteorite.

Organic molecules in interstellar space

Astronomers, using infrared spectroscopy, have identified a variety of organic molecules in interstellar space, including

Laboratory Synthesis of Organic Molecules Under Conditions Mimicking Outer Space

A recent report (January 2001) describes the results of taking a mixture of molecules known to be present in space: and exposing it to After as little as a week, a complex mixture of organic molecules had formed.

The bottom line:

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24 March 2001