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Ultraviolet Light and Its Role in the Origin of Life


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Introduction

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.


Evidence

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 UV damage.
  • 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?"

Conclusion

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.


References

  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.
  2. Riziq et al. "Photochemical Selectivity in quanine-cytosine base-pair structures." http://www.pnas.org/cgi/content/full/102/1/20.
  3. 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.
  4. 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.
  5. Miller, S. L., and Orgel, L.E. The Origins of Life on the Earth Englewood Cliffs, NJ: Prentice Hall, 1974.
  6. Encyclopedia entry for Miller-Urey.
  7. Braterman, P. S., et al. Nature 303 and Borowska, Z. K., et al. Origin of Life 17
  8. 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