Photoprotection of the human skin is achieved by extremely efficient internal conversion of DNA, proteins and melanin. Internal conversion is a photochemical process that converts the energy of the UV photon into small, harmless amounts of heat. If the energy of the UV photon were not transformed into heat, then it would lead to the generation of free radicals or other harmful reactive chemical species (e.g. singlet oxygen, or hydroxyl radical).
In DNA this photoprotective mechanism evolved four billion years ago at the dawn of life. The purpose of this extremely efficient photoprotective mechanism is to prevent direct DNA damage and indirect DNA damage. The ultrafast internal conversion of DNA reduces the excited state lifetime of DNA to only a few femtoseconds (10s)—this way the excited DNA does not have enough time to react with other molecules.
For melanin this mechanism has developed later in the course of evolution. Melanin is such an efficient photoprotective substance that it dissipates more than 99.9% of the absorbed UV radiation as heat.
 This means that less than 0.1% of the excited melanin molecules will undergo harmful chemical reactions or produce free radicals.