To prop up the argument that ancestors did precede the organisms we see in the fossil record, scientists look at DNA sequences in living creatures and try to calculate rates of divergence or becoming more different from each other. These calculations are based on presumed mutation rates which might have occurred in the past and which would make organisms carrying a given mutation more different from other contemporary organisms. The idea is that all organisms originally shared the same order of nucleotides in the DNA, but that with occasional mutations in various localized populations, the groups became more and more different over time, and this was reflected in increasingly different body plans. In the case of the Cambrian explosion, the answers initially obtained from these calculations on rates of change suggested an evolutionary process that took too long a time. So, the mathematicians adjusted these calculations and their answers then suggested a process that was too fast to produce such dramatic differences. So, no viable explanations for the Cambrian explosion have been developed.