Fuke

Hi all, im new... I'm Fuad and i come from Bosnia. I study biology. Also, my english is bad, so please bear with me. Ok, about my question. Well, they always teach you that RNA has Uracil base, and that DNA has Thymine instead. But I never learned why! Why did DNA molecule evolved that way... why didn't it keep Uracil, why replace it with Thymine?