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The largest defined unit of time is the supereon, composed of eons.Eons are divided into eras, which are in turn divided into periods, epochs and ages.

Dominantly fluid planets, such as the gas giants, do not preserve their history in a comparable manner.

Apart from the Late Heavy Bombardment, events on other planets probably had little direct influence on the Earth, and events on Earth had correspondingly little effect on those planets.

Leonardo da Vinci (1452-1519) concurred with Aristotle's interpretation that fossils represented the remains of ancient life.

The 11th-century Persian geologist Avicenna (Ibn Sina, died 1037) and the 13th-century Dominican bishop Albertus Magnus (died 1280) extended Aristotle's explanation into a theory of a petrifying fluid.

This clock representation shows some of the major units of geological time and definitive events of Earth history.

The Hadean eon represents the time before fossil record of life on Earth; its upper boundary is now regarded as 4.0 Ga (billion years ago).

Different spans of time on the GTS are usually marked by changes in the composition of strata which correspond to those, and indicate major geological or paleontological events, such as mass extinctions.

For example, the boundary between the Cretaceous period and the Paleogene period is defined by the Cretaceous–Paleogene extinction event, which marked the demise of the non-avian dinosaurs and many other groups of life.

In the late 17th century Nicholas Steno (1638-1686) laid down the principles underlying geologic (geological) time scales.

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