A(n) clock uses differences in DNA to determine when organisms diverged from a common ancestor.

The concept you are referring to is commonly known as a "molecular clock." A molecular clock is a technique in evolutionary biology that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. Here’s a more detailed breakdown of how it works:

1. DNA Analysis: Scientists compare the DNA sequences of different organisms. By looking at the differences in their DNA, they can quantify how many mutations have occurred since the organisms diverged.

2. Mutation Rate: Every organism has a specific rate at which mutations occur, on average, over time. By estimating this rate (the number of mutations per generation per unit of time), researchers can calculate the time since the last common ancestor.

3. Calibration: In order to use a molecular clock accurately, researchers often calibrate it with fossil records or other established timelines. This is crucial since different genes or organisms may have varying mutation rates.

4. Applications: Molecular clocks are used to build phylogenetic trees, which are diagrams that represent evolutionary relationships. They also help in understanding the timing of evolutionary events, such as the emergence of species in relation to environmental changes.

5. Limitations: It's important to note that while molecular clocks can provide estimates, they are not always precise. Factors like natural selection may affect mutation rates in different lineages, which can lead to discrepancies in timing.

In summary, molecular clocks utilize differences in DNA sequences and known mutation rates to estimate when species diverged from a common ancestor, aiding our understanding of evolutionary timelines.