Geological time scale: how was it made?

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The geological time scale has its origins in 1815, when a canal builder named William Smith noticed a relationship between rock strata and the fossils found within them. At the time, fossils were thought to be evil artifacts or inorganic mineral arrangements, but these fossils were the key to the advent of this time scale.
Being a canal builder, Smith looked at the rocks quite frequently. He began to associate certain fossils with certain strata. The field of geology was thriving at the time, directing attention towards the subject. Scientists around the world began to observe the strato-fossil correlation, but it wasn’t until the mid-19th century that Darwin’s Origin of Species was published and the theory of evolution began to gain credence.

Evolutionarily speaking, it became clear why different rock strata contained different fossils. Fossils were organisms that lived in varying periods of time. The rock layers and the times in which they were formed acquired names and classifications. This is the geological time scale.

A classification scheme was formulated based on eras, periods and epochs, each representing a progressively shorter division of time. The first era on the geological time scale is the Precambrian, of which there are barely any fossils. Next is the Paleozoic Era (ancient life), followed by the Mesozoic Era (middle life) and the Cenozoic Era (modern life).

The subdivisions of eras, periods, were generally named after the places where fossils of that respective period were found. For example, the Devonian period is named after Devonshire in southwest England and the Permian after the Permian kingdom in Russia. An exception, the Cretaceous period, derives from the Latin word creta, meaning ‘chalk’. This is because associated fossils have been discovered in chalk cliffs on the coast of England. The geological periods of the time scale have a variety of reasons behind their names.

Epochs of the geological time scale are given specific names only for the Cenozoic era, the most recent. In the Pleistocene epoch, which takes its name from the Greek words meaning ‘most’ and ‘recent’, 90% of the fossils discovered represent species still living today. First was the Eocene epoch, of which only 1% – 5% of species are still alive. Of all the species that have ever lived on earth, those that currently live represent only a small percentage.

Thanks to radioactive dating, scientists are now able to identify fossil age with pinpoint accuracy, conclusively associating each fossil with its specific period or era on the geological time scale. This helps us trace the relics of the evolutionary tree as it branched out in billions of directions.

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