This excerpt from MATCMadison,edu on absorbing UV light.
http://matcmadison.edu/biotech/resources/methods/labManual/unit_4/exercise_15.ht...

"Most biological molecules do not intrinsically absorb light in the visible range, but they do absorb ultraviolet light. Biologists take advantage of UV absorbance to quickly estimate the concentration and purity of DNA, RNA, and proteins in a sample... It is also possible to quantify the amount of DNA in a sample by looking at its absorbance at a wavelength of 260nm or 280nm (in the UV region)...

Proteins have two absorbance peaks in the UV region, one between 215-230 nm, where peptide bonds absorb, and another at about 280 nm due to light absorption by aromatic amino acids (tyrosine, tryptophan and phenylalanine). Certain of the subunits of nucleic acids (purines) have an absorbance maximum slightly below 260 nm while others (pyrimidines) have a maximum slightly above 260 nm. Therefore, although it is common to say that the absorbance peak of nucleic acids is 260 nm, in reality, the absorbance maxima of different fragments of DNA vary somewhat depending on their subunit composition. "

What if UV is a selective force at the start of life. If purines, and pyrimidines have slightly different absorbance maximums, then wouldn't each have a selective advantage under certain UV conditions?

Thoughts?

Tom Hendricks