Background

On the surface of the early Earth, where the ozone layer had not yet formed, intense solar ultraviolet (UV) radiation poured down unfiltered. The academic community has long explained that this harsh UV environment played a role in why life on Earth selected deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) as molecules to store and replicate genetic information. The reason why five types of canonical nucleobases—adenine (A), guanine (G), cytosine (C), thymine (T), and uracil (U)—survived among countless organic molecules was attributed to their unique photostability.

According to previous theories, standard bases exhibit an internal conversion capability, releasing energy as heat in an extremely short time of less than one trillionth of a second (picosecond) immediately after absorbing UV photons. Conversely, the 'photoselection' hypothesis, which has reigned as textbook dogma for decades, posits that non-standard molecules not adopted by life disappeared after degrading following prolonged energy retention. However, this explanation exposed limitations, relying on estimates based on ultrafast laser spectroscopy data of isolated single molecules or theoretical quantum chemical calculations. There was a severe lack of data systematically measuring and comparing the reaction rates of base and nucleoside destruction under conditions of continuous UV exposure, such as actual primitive aqueous environments.

Key Findings

Professor Joy Todd’s research team at the University of Wisconsin–Madison exposed standard and non-standard bases, as well as the nucleosides formed by their binding with sugars, to continuous UV irradiation at a wavelength of 254 nanometers (nm). Successfully achieved precise measurement of molecular concentration attenuation over time and calculation of photodegradation reaction rate constants by concurrently employing high-performance liquid chromatography and spectroscopic analysis techniques. The research team applied a model of Earth's surface UV spectrum that reflected the solar radiation energy and atmospheric transmittance of the early Sun approximately 4 billion years ago to comprehensively compare and analyze relative photostability in primordial natural environments.

The measured data directly refuted the long-standing hypothesis. Surprisingly, the photodegradation reaction rates of standard bases and nucleosides were widely scattered across almost the entire range of the degradation rate distribution exhibited by all non-standard analogs tested. It became clear that the group of bases used in modern biology did not exhibit consistently higher photostability than non-standard molecules. In fact, some non-standard pyrimidine and purine derivatives showed much slower photodegradation rates than standard bases like cytosine or uracil, appearing more stable against UV exposure. This is evaluated as a decisive observation proving that the single physicochemical property of short excited-state lifetime does not directly translate to an advantage in molecular durability under continuous irradiation environments.

Implications and Outlook

The interpretation that UV-induced selection pressure did not act as the decisive sole factor in the process of the primitive biochemical system establishing its molecular repertoire for the genetic code is gaining ground. Research on the origin of life is shifting toward a direction that considers multi-dimensional complex factors, such as the yield of prebiotic synthesis reaction pathways, the efficiency of aggregation and polymerization in aqueous solutions, and the precision of complementary hydrogen bonding, beyond the photochemical durability of single molecules. The possibility that self-shielding in high-concentration organic layers within shallow primordial puddles, or adsorption onto clay minerals, protected against UV degradation is also compelling.

However, certain differences remain between the uniform aqueous environments of the laboratory and the dynamic sedimentary geological environments of the early Earth. This underscores the need for follow-up studies to comprehensively elucidate the combined effects of metal ion catalysts, repeated drying-wetting cycles, and thermodynamic variables in hydrothermal environments on photodegradation rates. The academic community's interest is shifting toward establishing an integrated research framework that evaluates UV resistance at the oligomer and polymer stages, moving beyond measuring the survival of individual monomers.