Abiogenesis - Current Models - Origin of Organic Molecules - Ultraviolet and Temperature-assisted Replication Model

Ultraviolet and Temperature-assisted Replication Model

From a thermodynamic perspective of the origin of life, springs the ultraviolet and temperature-assisted replication (UVTAR) model. Karo Michaelian of the National Autonomous University of Mexico points out that any model for the origin of life must take into account the fact that life is an irreversible thermodynamic process which arises and persists because it produces entropy. Entropy production is not incidental to the process of life, but rather the fundamental reason for its existence. Present day life augments the entropy production of Earth by catalysing the water cycle through evapotranspiration. Michaelian argues that if the thermodynamic function of life today is to produce entropy through coupling with the water cycle, then this probably was its function at its very beginnings. It turns out that both RNA and DNA when in water solution are very strong absorbers and extremely rapid dissipaters of ultraviolet light within the 200–300 nm wavelength range, which is that part of the sun's spectrum that could have penetrated the dense prebiotic atmosphere. Cnossen et al. have shown that the amount of ultraviolet (UV) light reaching the Earth's surface in the Archean eon could have been up to 31 orders of magnitude greater than it is today at 260 nm where RNA and DNA absorb most strongly. Absorption and dissipation of UV light by the organic molecules at the Archean ocean surface would have significantly increased the temperature of the surface skin layer and led to enhanced evaporation and thus to have augmented the primitive water cycle. Since absorption and dissipation of high energy photons is an entropy producing process, Michaelian argues that non-equilbrium abiogenic synthesis of RNA and DNA utilizing UV light would have been thermodynamically favored.

A simple mechanism that could explain the replication of RNA and DNA without resort to the use of enzymes could also be provided within the same thermodynamic framework by assuming that life arose when the temperature of the primitive seas had cooled to somewhat below the denaturing temperature of RNA or DNA (based on the ratio of 18O/16O found in cherts of the Barberton greenstone belt of South Africa of about 3.5 to 3.2 Ga., surface temperatures are predicted to have been around 70±15 °C, close to RNA or DNA denaturing temperatures). During the night, the surface water temperature would drop below the denaturing temperature and single strand RNA/DNA could act as a template for the formation of double strand RNA/DNA. During the daylight hours, RNA and DNA would absorb UV light and convert this directly to heat the ocean surface, thereby raising the local temperature enough to allow for denaturing of RNA and DNA. The copying process would have been repeated with each diurnal cycle. Such a temperature assisted mechanism of replication bears similarity to polymerase chain reaction (PCR), a routine laboratory procedure employed to multiply DNA segments. Michaelian suggests that the traditional origin of life research, that expects to describe the emergence of life from near-equilibrium conditions, is erroneous and that non-equilibrium conditions must be considered, in particular, the importance of entropy production to the emergence of life.

Since denaturation would be most probable in the late afternoon when the Archean sea surface temperature would be highest, and since late afternoon submarine sunlight is somewhat circularly polarized, the homochirality of the organic molecules of life can also be explained within the proposed thermodynamic framework.