the molecular clock

Suppose two towns started from one settlement, and each began keeping its own slightly drifting calendar. After many years their dates no longer match, and the gap between their calendars grows steadily. If you knew how fast the calendars drifted apart per year, you could measure today's gap and work backward to when the towns separated. The molecular clock applies this idea to DNA and proteins: differences between two species' sequences pile up at a roughly steady rate, so the size of the difference is a rough timer for how long ago they shared an ancestor.

Here is the mechanism in plain steps. In two lineages that have split, each independently accumulates substitutions over the generations. For sequences not strongly shaped by changing selection — especially neutral or nearly-neutral positions — these substitutions arrive at an approximately constant average rate, because the rate is set mostly by the underlying mutation rate, not by the day-to-day struggle for survival. Count the differences between two species, divide by the per-year rate (calibrated using a fossil or geological event of known age), and you estimate the divergence time. This is how molecular biologists date splits that left no fossils at all — when the human and chimp lineages parted, when birds and crocodiles diverged, when a virus first jumped into humans.

The honest part is the unevenness. The clock is not a precise Swiss watch; it 'ticks' at different speeds in different genes (fast in some, slow in conserved ones), in different lineages (faster in organisms with short generations or sloppier replication), and over different epochs. So molecular dates carry real uncertainty and must be calibrated and cross-checked, ideally using many genes and several fossil anchors. Treated honestly — as an approximate, calibrated estimate rather than an exact readout — the molecular clock is one of the most powerful tools for putting a timeline on the tree of life.