A simple introduction to this answer can revolve around a general statement about geological time scale.

The Earth’s geological time scale is a crucial tool used by geologists, paleontologists, and other scientists to investigate the planet’s history. 

It is a system that divides the history of the Earth into discrete intervals of time, based on events, such as the evolution and extinction of different living beings and processes that have occurred.

It is divided into five broad categories:

(i) Eons

(ii) Epochs

(iii) Eras

(iv) Periods

(v) Ages

Body

You may incorporate some of the following points in the body of your answer:

How has the geological time scale evolved over time?

The geological time scale takes hundreds of years to evolve. The root of its origin goes back to the 1500s and 1600s when the first major breakthrough came in 1669 after Danish scientist Nicolas Steno published the first laws of stratigraphy — the science of interpreting the strata, or layers of rock, in the Earth’s outer surface.

He laid out two basic geologic principles:

(i) Sedimentary rocks (formed on or near the planet’s surface, in contrast to metamorphic and igneous rocks, which are formed deep within the Earth) are laid down in a horizontal manner.

(ii) Younger rock units were deposited on top of older rock units.

The second principle essentially means that layers closer to the Earth’s surface must be younger than layers below them. This allowed scientists of the time to conclude that each rock layer represented a specific interval of geologic time.

Italian geologist Giovanni Arduino classified the four main layers of the Earth’s crust as:

(i) Primary (the lowest metamorphic and volcanic layers),

(ii) Secondary (hard sedimentary rocks),

(iii) Tertiary (less hardened sedimentary rocks), and

(iv) Quaternary (the most recently laid rock layer, which is quite soft in comparison to other strata).

There were some issues with Arduino’s classification and Steno’s principles like:

(i) The rocks were locally described by colour, texture, or even smell, comparisons between rock sequences of different areas were often not possible.

(ii) Unlike tree-ring dating, in which each ring is equivalent to one year’s growth, rock layers don’t tell the specific length of geologic time, meaning no one layer can convey how long a certain period lasted on the Earth.