Due to simple chemistry, the nucleotide base cytosine undergoes a common, but unintended reaction with water, resulting in loss of its ammonia and conversion of the base from cytosine to uracil. This is known as cytosine deamination and is a natural process, that results in a constant assault on our DNA, causing damage due to the aqueous environment surrounding the DNA molecule. This leads to a point mutation, but is usually easily picked up by the proof-reading properties of DNA polymerase (the enzyme that copies the DNA molecule).
It is easily picked up because uracil is not a natural base in DNA. It is found normally in RNA, and it is quite possible that RNA evolved first in the earliest life on earth, and when DNA took over the role it has today of storing genetic information thymine was substituted in place of uracil so that the deamination process could be detected and repaired. In other words, if uracil was a natural base in DNA, there would be no way for the proof-reading machinery to detect that a base change occurred. Pretty ingenious of nature, don’t you think?