4. Hadean Eon (Earth's formation-4 billion years ago)
The Hadean Eon (referring to the Greek word Hades meaning hell in Hebrew) refers to the Precambrian time period, beginning with the Earth’s formation and extending until around 4.0 billion years ago. This is the time when the Earth was forming, doing so from the accumulation of dust and gases resulting from the collision of extraterrestrial bodies. The frequent impact of celestial objects with the Earth generated high levels of heat as well, which in turn slowed up the solidification of the Earth’s crust. Our planet was highly unstable during this period and is believed to have appeared like a giant cauldron of molten rock, hot gases, and boiling liquids. Heavier elements like iron sank down to form the core of the planet, while lighter elements such as silicon started forming the crust. Though it is difficult to tell when the first stable outer crust of the Earth formed, the discovery of a few grains of zircon in the Jack Hills of Australia dated 4.4 billion years ago points to the fact that stable continents, liquid water, and temperatures lower than the boiling point of water could have existed during the Hadean Eon. The early atmosphere of the eon is predicted to have been made up of escaping hydrogen and helium gases, while ammonia, neon and methane appeared at later stages, especially as the crust started cooling. Volcanic "outgassing" was responsible for adding water vapor, hydrogen, and nitrogen to the atmosphere. Additional water vapor could have been supplied by comets striking the planet. The water on the surface of the Earth probably appeared when this water vapor started condensing to form clouds and rain. The Hadean Eon is also the time when the moon was born. Among the several theories proposed to explain the formation of the Moon, the most popularly accepted one states that a collision between Earth and another planet could have ejected small chunks of then-planet Earth that ultimately became the Moon we see today.
3. Archean (4-2.5 billion years ago)
The period between the Hadean and Proterozoic Eons, spanning the time between 4 billion and 2.5 billion years ago, is known as the Archean Eon. The oldest rock formations on Earth belong to this eon. During the beginning of this eon, the heat flow on the planet was estimated to be three times of what it is now. The extra heat probably came from planetary accretion, radioactive elements, and the heat produced during the stabilization of the Earth’s core. There was also substantial volcanic and tectonic activity during this period. Most rocks belonging to this eon are of the metamorphic or igneous types, and large continents probably did not form until late in the Archean. Small protocontinents are believed to have prevailed during the early periods of this eon, which were hindered from coalescing into larger units by the high degree of geologic activity prevailing on the Earth. One of the major features of this eon was the appearance of the earliest lifeforms on Earth. Prokaryotic fossils belonging to this eon have even been recovered by paleontologists as evidence of such life. These earliest fossils, representing individual cyanobacterial cells and stromatolites (layered mounds of cyanobacterial colonies) have been discovered and dated as having formed throughout the eon, becoming increasingly more common towards the end of the Archean. Besides cyanobacteria, a few fossils have also been discovered which probably represent Eubacteria and Archaebacterial cells. The cyanobacteria existing today is almost identical to the ones that would have likely occurred in the Archean Eon, proving that these single-celled organisms have not undergone much evolution over the years. Besides these non-nucleated prokaryotes, the eon exhibits no evidence of the presence of eukaryotes. Fossil evidence also does not point to the existence of viruses in this era.
2. Proterozoic (2.5-0.542 billion years ago)
The Proterozoic Eon extended between 2.5 billion years and 0.542 billion years ago. It is subdivided into the three eras of Paleoproterozoic, Mesoproterozoic, and Neoproterozoic. This eon witnessed some significant and exciting events in the history of the Earth. The first stable continents began to form on our planet, and protozoans thrived during this period. Towards the end of this eon, fossil evidences of the earliest eukaryotic forms of life were also discovered. The first pollution crisis on Earth also happened in the Proterozoic Eon which, contrary to the current scenario related to carbon dioxide, this one was caused by excessive oxygen in the Earth’s atmosphere. Oxygen levels in the atmosphere during the Archean Eon was only 1% of the current levels, while in the Proterozoic it was 15% higher that the current levels. The entry of oxygen into the Earth’s atmosphere probably triggered the formation of eukaryotic cells which used oxidative respiration to generate energy. This, however, meant doom to the existing anaerobic bacterial cells which completely perished in the presence of oxygen. The stromatolites which were widely distributed in the fossil records of the late Archean Eon started diminishing about 700 million years ago. The proliferating herbivorous eukaryotes and earliest multi-cellular animals feeding on these cyanobacterial colonies could have been responsible for their decreased abundance. Several fossils, primarily belonging to the Neoproterozoic era in the form of carbon films, are believed to resemble what we would now know as seaweeds and eukaryotic algae. The origin and diversification of the first soft-bodied animals occurred somewhere between 635 million and 542 million years ago, with the discovery of fossils in the Ediacara Hills of Southern Australia supporting this theory.
1. Phanerozoic (0.542 billion years ago-Today)
We are currently living in the Phanerozoic Eon, which started 0.542 billion years ago and continues into our own present times. This eon is also referred to as the ‘eon of visible life’ as it has thrived with life forms of great diversity. The eon has been further divided into different stages depending on the assemblages of life forms on Earth during the respective stages. These are the Paleozoic (541-242 million years ago), Mesozoic (252- 66 million years ago) and, finally, Cenozoic (66 million years ago to the present time). Even though the earliest forms of life occurred during the Archean period, and the evolution of eukaryotes took place during the Proterozoic Eon, the majority of the complex life forms on Earth appeared in our Phanerozoic Eon and evolved and diversified to occupy every niche available on the planet. The first major boost to life on Earth occurred when plants developed the process of photosynthesis, generating oxygen as a by-product of this process, and then releasing it into the atmosphere. This oxygen-laden atmosphere could now begin to support species dependent on oxygen for their energy needs. Besides proliferation of life forms on Earth, the eon also witnessed major geological events. The included the continental drifts, mountain formations, continental glaciation, and others that have shaped our world into the form we know it today. Despite representing only one-eighth of our planet’s time since its formation, the Phanerozoic Eon represents a period of extreme significance on Earth, as it is the one with the most biodiversity.