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A number of recent studies suggest that ultraviolet radiation may be a necessary
ingredient in the origin of life.1, 2, 3, 4. Further, one may
hypothesize that the daily solar cycle and the sun's UV radiation were important
in all stages of the origin of on Earth. Moreover, it is suggested that
life can be defined as that chemical reaction forced by the energy cycle
of the sun, under very specific circumstances.
In support of these claims are the following assumptions and lines of evidence:
The assumption that all stages of the origin of life took place at
or near the surface and close to or on land.
The assumption that there was a lower solar luminosity, higher UV
flux, and no ozone layer in the period of the origin.
The assumption that there was a shorter diurnal (day/night cycle),
due to the faster rotation of the Earth following the presumed collision
that preceded the formation of the Moon.
The assumption that chemical "selection" was for stability under the
sun heat cycle.
The assumption that solar radiation far exceeded all other energy
available for organic synthesis, including electrical discharges, shock
waves, radioactivity to a depth of 1 km, volcanoes, and cosmic rays.5
The assumption that there were wet/dry cycles that drove phosphorylation
of nucleotides and perhaps other condensation reactions.
The assumption that there was at first a primitive, environmentally
forced, PCR-like replication process of alternating heat and cold that
denatured then annealed RNA paired strands. It is assumed that the sun
cycle (day and night) caused a cycle of primitive denaturing and annealing
of paired RNA nucleotide strands [and possibly folded (annealed) and
unfolded (denatured) nucleotide strands]. It is assumed that this provided
a large number of variations of paired RNA strands with variations of
properties, the most stable of which possessed the best Watson-Crick
(W-C) pairing. It is assumed that instead of a self replicator, there
was at first a primitive, sun-forced replication process. It is assumed
that 'proof reading' would at first have been limited to W-C pairing
over non W-C pairing for stability. Note also the assumption that paired
bases may have better protected the ribose-phosphate backbones from
The assumption that the first coded 'information' would have been
for that molecule which was most stable in the sun/heat cycle environment.
Note: in Watson-Crick base pairing in RNA there are two sets of nucleotide
bases: G bonds to C and A bonds to U. It is assumed that Watson-Crick
base pairing is more stable in this environment than non Watson-Crick
base pairing. And of the two sets of bases, It is assumed that the G-C
bonds would have been more stable than A-U bonds because G-C bonds have
3 hydrogen bonds instead of the 2 of A-U. It is also assumed that A-U
would have been more stable than non Watson-Crick base pairing. Further
it is assumed that high G-C base pairing would have supported more stability
than high A-U base pairing. Additionally it is assumed that A-U base
pairing would have supported more stability than non Watson-Crick base
pairing, or no base pairing at all. It is also assumed that high A-U
base pairing would allow for more variation than G-C base pairing, because
A-U bonds are more likely to denature in heat and more likely to denature
quicker than G-C bonds and thus more likely to anneal with other RNA
strands in cooler temperatures. It is assumed that overall the G-C plus
A-U sets of nucleotides would promote both general stability with the
G-C set, and variety with the A-U set of nucleotides.
The assumption that RNA acted as a receptor and transducer of UV radiation.1
The assumption that there was a cyanobacteria-like lifestyle for the
earliest confirmed true organisms so far, and that this earliest remnant
of life is very near the likely origin of life.
The assumption that there was a pyrimidine dimer impact on the genetic
code. It is assumed that because of the high UV during this period,
UV-caused pyrimidine dimers would also be highly likely. This further
assumes that this would not favor any code with adjacent pyrimidines
that would lead to the likelihood of pyrimidine dimers. This further
assumes that the most likely first codons would be either purine/pyrimidine/purine,
or pyrimidine/purine/pyrimidine – coding that prevents adjacent
pyrimidines and thus pyrimidine dimers. Later it is assumed that this
would lead to 'information' coding beginning in the 2nd position, or
middle position – the most protected position of the 3 base codon
and anticodon. It is assumed that this initial coding may have been
limited to 2 classes or sets of amino acids – hydrophilic (XAX
with "A" in 2nd protected postion) and hydrophobic (XUX with U in 2nd
protected position). There is also the assumption that there was a pyrimidine
dimer impact on tRNA which, it is further assumed, was one of the earliest
forms of RNA.
The assumption that the Miller/Urey experiments are seen as an illustration
of a heat cycle, "energized by a cyclical electrical discharge apparatus
to represent UV radiation from the sun."6
The assumption that the first mechanism that used sunlight energy
to remove hydrogen from water may have been UV radiation on ferrous
ions. Magnetite, a mixed oxide of ferrous and ferric iron found in banded
iron formations (BIF) may be remnants of that process.7
This hypothesis avoids problems in competing theories:
The problem that heterotrophic lifestyles rapidly deplete the 'soup'
of nutrients, thus forcing an implausibly rapid invention of photosynthesis.
Instead it is assumed that life started out on a phototrophic path,
rather than having to invent it in such a short span of time.
The problem of 'self' replication.It is assumed that RNA would not
at first replicate on its own, and would need some kind of environmental
energy forcing it to replicate. It is assumed that the sun cycle would
provide planet wide forced energy daily. It is assumed that it would
provide stable but variable energy that could force into existence a
replicating chemical system by denaturing paired RNA strands in high
temperatures, then annealing single RNA strands in low temperatures.
The problem of UV as a major detriment to the origin. It is assumed
that UV would no longer be seen as a detriment to the origin of life
but instead as a necessary component of the origin and a necessary part
of the selection process during the origin.
The two problems that hydrothermal vent scenarios have: 1) no assumed
necessary dry phase component for condensation synthesis, 2) no assumed
necessary UV component. The sun is a more stable, longer lasting, energy
source than any vent, especially if the origin occured during part of
the long bombardment phase – a bombardment that would have upset
the constancy and stability of most vents. And the sun does not sterilize
the water through high temperatures like the vents do.
The problem of how the complex mixture of necessary molecues for the
origin of life, came about.8
James Lovelock has said this regarding the claim that ultraviolet would
have been detrimental to the origin of life:
'This belief that ultraviolet radiation is unconditionally
lethalto life on Earth has sustained a distorted view of the Archaean
... And it is a view still deeply entrenched in scientific thinking. I
found it to be common among the scientists who sought life on Mars. I
could not help wondering how they could think that there was life on the
intensely irradiated surface of Mars and at the same time believe that
the land beneath the thick and murky Archean atmosphere of Earth was sterile.
How could they fit into their minds two such contrary ideas?"
It is suggested that experimenters in prebiotic chemistry look to how chemicals
react to a sun/heat/uv cycle in ways that lead to life processes as a response
to that forced energy.
1. Mulkidjanian, A. Y., et al. "Survival of the fittest before the beginning
of life Selection of the first oligonucleotide-like polymers by UV light." BMC
Evolutionary Biology. Senanayake and Idriss et al. "Titanium dioxide Crystals Might Have
Helped Trigger Life on Earth." http://www.rsc.org/chemistryworld/News/2006/January/18010601.asp.
2. Riziq et al. "Photochemical Selectivity in quanine-cytosine base-pair
3. >Buccino, A. P., Lemarchand, G. A., and Mauas, P. J. D. "UV Radiation
Constraints Around the Circumstellar Habitable Zone." http://arxiv.org/abs/astro-ph/0512291.
4. >Miller, S. L., and Orgel, L.E. The Origins of Life on the Earth Englewood Cliffs, NJ: Prentice Hall, 1974.
5. Encyclopedia entry for Miller-Urey.
6. Braterman, P. S., et al. Nature 303 and Borowska, Z. K.,
et al. Origin of Life 17
7. Dworkin, Jason P., David W. Deamer, Scott A. Sandford, and Louis J.
Allamandola. "Self-assembling amphiphilic molecules: Synthesis in simulated
interstellar/precometary ices." http://www.pnas.org/cgi/content/full/98/3/815