There are two main theories, with many sub-theories, that try account for the origin of life on
Planet Water.
# Spontaneous generation of life somewhere on the planet from organic precursors
# Panspermia: life brought to earth from space.
Spontaneous generation in a primeval soup: Miller's Experiment
Stanley Miller, a graduate student in biochemistry, built an apparatus to simulate what he
believed was the atmosphere and conditions on the primitive earth.
He filled the glass chamber with:
* water (H2O
* methane (CH4)
* ammonia (NH3) and
* hydrogen (H2)
------ but NO oxygen.
Miller 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 (simulated lightning).
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:
* all the amino acids used in protein synthesis
* all the purines and pyrimidines used in nucleic acid synthesis.
But abiotic synthesis of ribose - and thus of nucleosides - has been much more
difficult.
One problem with the - 'It all happened in the primeval soup theory' - is how
polymers - the basis of life itself - could be assembled:
* Polymers could grow in solution but hydrolysis would limit the size it could reach.
Another problem is that abiotic synthesis produces a mixture of L and D enantiomers.
Each inhibits the polymerization of the other. So, for example, the presence of D amino
acids inhibits the polymerization of L amino acids (the ones that make up proteins here
on earth).
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.
There is a problem with the link between RNA and proteins (enzymes)
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 in another guise - the RNA-protein dilemma.
The synthesis of DNA and RNA requires proteins and so:
* proteins cannot be made without nucleic acids and
* nucleic acids cannot be made without proteins.
However it has been found that certain RNA molecules have enzymatic activity provides a
possible solution. These RNA molecules - called ribozymes - incorporate both the
features required of life: storage of information and the ability to act as catalysts.
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 as the template on which
short lengths of RNA ("oligonucleotides") are assembled following the rules of base pairing
and as the catalyst for covalently linking these oligonucleotides.
In principal, the minimal functions of life might have begun with RNA and only later did
proteins take over the catalytic machinery of metabolism and DNA take over as the
repository of the genetic code.
Several other bits of evidence support this notion:
# Many of the cofactors that play so many roles in life are based on ribose; for example:
o ATP
o NAD
o FAD
o coenzyme A
o cyclic AMP
o GTP
# In the cell, all deoxyribonucleotides are synthesized from ribonucleotide precursors.
The Bottom Line
While it has been shown that organic molecules and many of the building blocks of living
things can be produced in a flask, it is a huge leap from the ingredients to the living cake.
The step required is akin to getting a mob of monkeys to use type like mad to produce one
of Shakespeare's plays - even if they could type. It is the organisation that is the problem.
Similarly if this soup produced this life through such a random process under
the conditions on this planet at this time then what hope is there that it
could have occurred elsewhere.
Similarly if we started again with the atmosphere we have now, the soup would have had too
much oxygen, and everything would be oxidized, and the result would be different.
Also if we changed our present atmosphere back to what it was at the beginning when life
first appeared - most living things including us would die. ( No oxygen )
Life as we know it is ultimately linked with the atmosphere and climate on Planet Water
and these features have been altered and shaped by living things.
See How Plants Changed the Atmosphere
Another aspect to consider is that once the conditions on earth became suitable for life
with a drop in temperature and lessening of the volcanic activity and bombardment from
meteorites and comets, life quickly established.
The period of time was about 200-500 million years.
See Evolution of Life as We Know It