How DNA becomes a molecular clock
When DNA is copied to make eggs, sperm, or new cells, tiny copying errors can occur. Most have no noticeable effect, but some persist and are passed on. Over many generations, these inherited changes gradually distinguish one family line from another.
Geneticists can compare DNA sequences in people or species and count the differences that accumulated in a stretch of DNA. If they also have an estimate of how often changes arise there, they can use that rate to estimate when the two lineages last shared an ancestor. The clock is not a literal timer in a cell; it is a statistical inference from changes preserved in descendants.
Why the clock is useful—and imperfect
A steady average rate makes comparisons possible, but mutation rates are not identical everywhere. They vary among DNA regions and can shift with factors such as age and the number of generations. Natural selection also affects what remains: harmful changes are less likely to spread, while neutral changes can accumulate more freely. For that reason, researchers choose suitable stretches of DNA and calibrate estimates with other evidence, such as fossils or known dates in a family tree.
The result is usually a range, not an exact birthday for an ancestor. Still, the method can reveal relationships too old or too distant to trace through written records. A handful of tiny differences, interpreted with care, can help reconstruct how populations branched apart and how long those branches have been evolving separately.
