GENETICS × EVOLUTION

The Molecular Clock

1962 to 1965 CE

Phanerozoic · Cenozoic · Quaternary · Holocene · Meghalayan

The molecular clock, proposed between 1962 and 1965, is a method that dates the separation of two species by counting the differences between their molecules, on the premise that mutations accumulate at a roughly constant rate.

The molecular clock is a method for dating the divergence of two species from their molecules. Its premise is that mutations, heritable changes in the sequence of a molecule such as a gene, accumulate at a roughly constant rate. The number of differences between the molecules of two species then measures the time since they diverged. Molecules thereby became documents of the history of species, which can be read without a single fossil. Palaeontologists resisted the idea for twenty years, and were partly right. The rate differs between lineages, the lines of descent, and between genes. The clock must also be calibrated, that is, set against dated fossils, before it can give an age in years.

Why it matters

Half the pre-Cambrian dates on this timeline, those older than the Cambrian period of geology, come from molecular clocks. Where the timeline reports that the ages from genes and the ages from rocks disagree, the ages from genes come from this method. Because the clock is calibrated on fossils, the two kinds of evidence, genes and rocks, are never entirely independent.

Dating & uncertainty

The molecular clock was formulated between 1962 and 1965 and accepted only two decades later. A clock must first be calibrated on fossils, so every age it gives depends on the fossil record, the set of known fossils.

Earth history as one day

23:59:59.99881.2 ms before midnight

if Earth’s 4.54 billion years were compressed into a single day