Earth formed around 4.6 billion years ago, approximately one-third the age of the universe. Over Earth’s history, life has been changed from the first simple bacteria to the complex creatures that we see on Earth today. The primary and largest cataloged divisions of time are periods called Eons. Eons are divided into eras, which are in turn divided into periods, epochs, and ages. Earth’s history is characterized by four eons; in order from oldest to youngest, these are the Hadeon, Archean, Proterozoic, and Phanerozoic. Collectively, the Hadean, Archean, and Proterozoic are sometimes informally referred to as the “Precambrian”.
Hadeon Eon: 4.6 to 4.0 billion years ago
The Hadean is the first of the four eons of Earth history. Hadean derives from the Greek word “Hades” (the Greek god of the underworld, is the god of the dead and the king of the underworld, or place of the dead, or a word for hell). This is the time that Earth was just being formed. During that time there were a lot more comets, meteoroids, and asteroids in space around our planet, and these objects often crashed into our Earth. As a results, created a lot of heat generated, and oceans on the planet have turned completely into steam. Solidification of the molten material into rock happened as the Earth cooled.
During this time, the Solar System was forming, probably within a large cloud of gas and dust around the sun, called an accretion disc. Hadean, Informal division of Precambrian time occurring between about 4.6 billion and about 4.0 billion years ago. So the history of life on Earth actually begins after the Hadean Era.
Archean Eon: 4.0 billion to 2.5 billion years ago
This is the second major period in the geological history. Archean derives from the ancient Greek word (Arkhē), meaning ‘beginning, origin.’ During this time the atmosphere lacked free oxygen but was probably rich in volcanic gases including carbon dioxide (CO2), and the interior of the Earth had been much hotter. When this Eon began, the Earth’s heat flow was nearly three times as high as it is today.
Life could have arisen before the Archean, but all the conditions necessary to sustain life could not have occurred until the Archean Eon. Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called Prokaryote. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identifying. The Archean Eon has four major divisions, they are: Eoarchean, Paleoarchean, Mesoarchean and Neoarchean.
Proterozoic Eon: 2.5 billion to 541 million years ago
The name Proterozoic comes from Greek word, meaning “earlier life,” (the time from the appearance of oxygen in Earth’s atmosphere to just before the proliferation of complex life on the Earth), Phanerozoic boundary is at 541 million years ago. Proterozoic Eon is subdivided into three geologic eras (from oldest to youngest): the Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. The first known glaciations occurred during the Proterozoic. One of the most important incident of the Proterozoic was the accumulation of oxygen in the Earth’s atmosphere. The oxygen buildup was probably due to two factors. First, a filling of the chemical sinks, and second, an increase in carbon burial, which sequestered organic compounds that would have otherwise been oxidized by the atmosphere. The Proterozoic Eon was a very tectonically active period in the Earth’s history.
Phanerozoic Eon: 541 million years to the present
The Phanerozoic Eon is the current geologic eon in the geologic time scale, and the one during which abundant animal and plant life has existed. Phanerozoic derives from the Ancient Greek words phanerós zōḗ which means “visible life.” The Phanerozoic Eon has spanned from 540 million years ago to present. Some organisms survive changes in the climate, others go extinct when the climate changes too much. At the beginning of this eon, the climate became warm and humid. Since then Earth’s climate has gone through four cycles of frigid glaciers and warm tropical seas. Earth’s climate changed numerous times during the this Eon.The Phanerozoic Eon is made up of the Paleozoic, Mesozoic, and Cenozoic Eras. These are further subdivided into 12 periods.