Geologic Time of Earth

Geologic time is the vast period that has passed since the beginning of the Earth. The history of the planet Earth goes back as far as 4.5 billion years. It took many more years for life to form on Earth. About 3.5 billion years ago, first, living organisms were originated in the oceans. According to scientists, marine life evolved for another million years, and some later spread to land. The first to spread on land were plants, not animals. Man, compared to plants and animals, was born much later.

In typical day to day life, we use time frames such as hours, days, weeks, and years. Something happened a hundred years ago is called as old while something occurred a thousand years ago is known as ancient. However, in terms of geologic time, the duration is different. There, a period of 1000 million years ago, is probably called old. Hence, this long time is difficult to interpret using ordinary time frames. Because of this, geologists use different time scale called the geologic time scale. Here, Geologists divide the entire geologic time into different magnitude units. Simply, it is the calendar used by geologists. However, the geologic time scale is a dynamic tool, and it can always update with new findings.

Structure of the Geologic time scale

In descending order, the geologic time scale divides into several subdivisions, namely Eons, eras, periods, and epochs. On this scale, the earliest events recorded at the bottom.

The Geologic Time Scale v.5.0.

Eon	Era	Period	Millions of years ago	Key geological and biological events
Phanerozoic	Cenozoic	Quaternary	2.58	Humans develop
		Neogene	23.03	Ice age begins
		Paleogene	66	The rise of the mammals
	Mesozoic	Cretaceous	145	Extinction of dinosaurs
		Jurassic	201.3	First birds
		Triassic	251.9	First mammals First dinosaurs
	Paleozoic	Permian	298.9	Formation of Pangaea
		Pennsylvanian	323.2	Abundant coal-forming swamps
		Mississippian	358.9	First reptiles
		Devonian	419.2	First amphibians
		Silurian	443.8	First upright-growing land plants
		Ordovician	485.4	First jawless fish
		Cambrian	541	Golden age of invertebrates
Precambrian			4000	Earliest shelled animals Earliest fossil records of life

Source: The Geological Society of America and
Essentials of Geology, Frederick K. Lutgens, Edward J. Tarbuck, 11th edition

Eons

First, geologic time divides into Eons. An Eon is a substantial period. The geologic time of the Earth can divide into three Eons. Namely, Archean, Proterozoic, and Phanerozoic.

The first two Eons are commonly known as the Precambrian (4000 – 541 million years ago). It comprises the first 4 billion years of Earth’s history. This period accounted for 88 percent of Earth’s history. During this period, the Earth’s continental crust was created. Moreover, first, living organisms appeared in the Precambrian oceans. Phanerozoic is the Eon in which we live today. And it began about 541 million years ago.

Eras

Next, Eons again split into eras. The Phanerozoic Eon that we live in today divide into three main eras; Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life). As the names suggest, these eras experienced a profound worldwide change in life forms.

The Paleozoic era (541 -251.9 million years ago) saw a variety of new life forms. During the early Paleozoic era, all major invertebrate groups formed in the oceans. Invertebrates are creatures without backbones such as jellyfish, worms, and mollusks. These life forms diversified dramatically. Later in this era, insects and plants migrated into lands, and the first amphibians emerged on the Earth.

The Mesozoic era (251.9 – 66 million years ago) is often termed as the “Age of Reptiles.” During this period, reptiles, especially Dinosaurs, ruled the Earth. However, at the end of this era, Dinosaurs and many other giant reptiles were extinct from Earth.

During the Cenozoic era (66 million years ago – present), mammals became the dominant life form. Therefore, this era is commonly known as the “Age of Mammals.” Also, this era saw the emergence of first flowering plants.

Periods

Each era again subdivides into periods. Compared to eras, each period is distinguished by a slightly less profound change in life forms.

Epochs

Finally, each period subdivides into smaller units called epochs. There are seven epochs in the Cenozoic era. Among the seven, the last three epochs are the most notable periods in the realm of human history.  During the Tertiary period, humans genetically separated from their apelike predecessors. In the Pleistocene epoch, human evolution saw a significant change. Subsequently, the genus homo emerged on the African continent. Also, the Pleistocene epoch experienced the last ice age. During this time, 30 percent of the Earth’s surface was covered with thick glaciers. However, the beginning of the Holocene epoch marked the end of the last ice age. And this new warmer climate induces agriculture and led to the Agricultural Revolution. At present, we live in the Holocene epoch.

Time scale of Cenozoic Era

Era	Period	Epoch
Cenozoic	Quaternary	Holocene (0.012 million years ago - present)
		Pleistocene (2.58 million – 0.012 million years ago)
	Tertiary	Pliocene (5.3 – 2.58 million)
		Miocene (23.0 -5.3 million years ago)
		Oligocene (33.9 – 23.03 million years ago)
		Eocene (56 – 33.9 million years ago)
		Paleocene (66 – 55 million years ago)

Source: The Geological Society of America and
Essentials of Geology, Frederick K. Lutgens, Edward J. Tarbuck, 11th edition