Spontaneous mutations occur in different individuals. Our children will inherit them.
Different individuals pass on different mutations to their offspring.
If different populations of individuals breed separately, the sequences of their DNA will diverge. The mutations most likely to remain without negative selection are silent or neutral mutations. Silent mutations accumulate at an approximately constant rate over generations. In current populations, we cannot distinguish between the original sequence and the versions that mutated. However, by comparing DNA from current populations, we can calculate the most probable sequence of the original.