Synthesis of uracil and thymine under simulated prebiotic conditions.

Evaporation and mild heating of solutions of beta-alanine and urea in the presence of clay minerals leads to the formation of 5,6-dihydrouracil (DHU). Ultraviolet irradiation of DHU in the presence of clay minerals and water vapor leads to the synthesis of uracil. Addition of acetate to this system results in the synthesis of thymine. The entire pathway could plausibly occur in pools subject to cycles of dehydration and rehydration and would result in the formation of significant concentrations of uracil and thymine from indisputably prebiotic materials.