Paleontologists, archaeologists, geologists, and earth scientists work to determine the different geological eras of the earth's past by studying the accumulation of the earth's strata. The Grand Canyon is one good example of the earth's stratification, with its different colored soils and rocks arising from different times. It is interesting to note that natural catastrophes devastated the world just as each geologic era ended and another era began to form on earth. Along with these catastrophic changes, after which appeared new species, existing species were almost always wiped out. Oceans receded in some areas, while in some place where there was formerly no oceanic cover, new oceans formed. Lush grounds became deserts and deserts became lush forests. The demise of species later created fossils, which scientists have since dug up and used to help them analyze the earth's geological eras.
Eoarchean (4-3.6 billion years ago)
The Eoarchean (4-3.6 billion years ago) era was the earliest time on earth after the initial forming of our planet from the dust and gas that came from the sun. This was the era when earth was a molten mass of lava, followed by the cooling of the earth's surface after water formed in the atmosphere. Then, firm crusts started forming on the earth's surface. This was again followed by a massive meteor-like object that hit the earth, and some parts of the earth were ejected back out as objects to form our Moon. Many meteors and comets may have hit the earth during this time, creating more volcanic activity and forming oceans from ice on the comets.
Paleoarchean (3.6-3.2 billion years ago)
The Paleoarchean (3.6-3.2 billion years ago) era followed the Eoarchean era. The first bacterial life formed during this time, with evidence of such being ascertained and found as microbial mat fossils in Western Australia from the Pilbara Craton crusts. Another similar evidence is the Kaapvaal Craton, in the Limpopo Province in South Africa, pointing to the fact that both areas were once part of the first supercontinent of Vaalbara. Also, at one point in this era, an asteroid the size of about 36 miles hit the earth in the vicinity of South Africa. Although no impact crater was found, evidence of the event is still clearly there.
Mesoarchean (3.2-2.8 billion years ago)
The Mesoarchean (3.2-2.8 billion years ago) era was the beginning of the Pongola glaciation on earth, which occurred in what is now South Africa. This was the first climate disaster on earth, as ice sheets covered areas of Swaziland and South Africa. Stromatolites also started forming at this time, formed by binding sedimentary grains by microorganisms in shallow water. This was a technique adapted by nano-microorganisms and used to build shelters from the materials provided by the severe environment surrounding them. At this time, the supercontinent Vaalbara separated into separate land masses.
Neoarchean (2.8-2.5 billion years ago)
The Neoarchean (2.8-2.5 billion years ago) era is best known as the beginning of the development of oxygenating photosynthesis. During this time, ancient cyanobacteria released oxygen into the surrounding environment by converting sunlight into chemical energy, fixing carbon from carbon dioxide, and releasing oxygen into the atmosphere as a byproduct. This evolution later caused havoc in the Paleoproterozoic era, as the overabundance of oxygen poisoned the atmosphere of those organisms not accustomed to it. The culprits were the photoautotrophs, a variety of organism that still serves as the ultimate food and oxygen sources for humans and other life today. The supercontinent Kenorland also began to form at this time, as a result of a new continental crust formation, although at the close of this era Kenorland broke up into multiple, smaller land masses.
Paleoproterozoic (2.5-1.6 billion years ago)
The Paleoproterozoic (2.5-1.6 billion years ago) era was when the planet started to see the stabilization of the earth's continents. The continental collision belts developed, and these led to another supercontinent named Nuna. These collisions happened on a grand global scale, and also occurred alongside the demise of many of the anaerobic bacteria (which did not need nor adapt to oxygen) following the rise of the multicellular macroscopic organisms that produced oxygen. The anaerobic microorganisms dies as they were subjected to lethal amounts of oxygen in the atmosphere being produced by the photoautotrophs.
Mesoproterozoic (1.6-1 billion years ago)
The Mesoproterozoic (1.6-1 billion years ago) era was the beginning of the evolution of sexual reproduction in single-celled eukaryotes, which continued on into the development of multi-celled organisms. This event resulted in the explosion of populations of eukaryotic and bacterial organisms. The stromatolites also gained a good foothold for a time before their ultimate demise in the Neoproterozoic era. Geologic records show that this era more or less had the same continents as we see today. The Nuna continent of the Paleoproterozoic broke up, and the Rodinia supercontinent subsequently formed. Many micro- and macro-organisms started to evolve in ever more complex ways, adapting as the ocean's chemistry and the atmosphere continued to change.
Neoproterozoic (1-0.541 billion years ago)
The Neoproterozoic (1-0.541 billion years ago) era was the beginning of still more geologic activity, part of which commenced the upwelling of mountains. The earliest animals also appeared at the end of this era. There were four glaciation episodes during this time, and these covered the earth with huge expanses of ice sheets that led to the Sturtian and Marinoan glacial events. Some believe that, at one point, the earth may have looked like a giant snowball from space, with ice sheets even covering the equator. After the glaciation came the Cambrian period, when the first trilobites appeared. More simple lifeforms, including soft-bodied animals, appeared at the end of the era, with some being armored and others tubular.
Paleozoic (541-252 million years ago)
The Paleozoic (541-252 million years ago) era was a time of great change on earth. Defined by four periods, the Cambrian, the first, brought the explosion of invertebrates like trilobites. Then, the Ordovician period brought on similar climate patterns as we see today, with both poles being cold and the earth having a tropically warm equator. Simple plants moved to land, and early forms of fish appeared. Next, the Silurian period warmed the climate across most areas of the planet. Sea scorpions appeared alongside brachiopods and gastropods, although trilobites gradually declined. Moving further into the Paleozoic, the Devonian period asrrived. It was the age of fish, and saw the first sharks. Amphibians also made their marks, and the first terrestrial forests started colonizing land. The aptly named Carboniferous period then saw coal deposits forming from organic matter under the earth's surface. Oxygen levels spiked, and insects proliferated just as the first reptiles evolved. The Permian period started the cycle of the seasonal changes within the now diverse diverse animal and plant life. This period also saw the extinction of many of these lifeforms, likely caused by a massive Siberian volcanic eruption and subsequent climatic changes.
Mesozoic (252-66 million years ago)
The Mesozoic (252-66 million years ago) era is divided into three periods. Namely, these are the Triassic, Jurassic, and Cretaceous. This was a time when there was the explosion of the global reptile population. Large dinosaurs dominated the landscape, and their cousins, birds, also first appeared and began taking flight into the air. The first mammals started their journeys on either water or land. The Mesozoic saw great climate, tectonic, and evolutionary changes happen. The Pangaea supercontinent broke up into several land masses, which we could increasingly recognize as the continents of today. The climate alternated from significantly hotter than today to colder periods. This era ultimately ended with another mass extinction event, and the dinosaur domination ended.
Cenozoic (66 million years ago-Today)
The Cenozoic (66 million years ago-Today) era is significant because we are living in this same era. Continents started moving into their present positions. The beginning of this era was a world of birds, mammals, amphibians, and reptiles. It also saw the age of terror birds, which were flightless yet huge. Land and water alike had small and huge mammals that occupied every level of the increasingly biodiverse ecological food chains. The Cenozoic had also at one point seen lush jungles' expansion, going so far as to have an encroachment that reached both poles! Later, grass and grasslands appeared, which became an essential feeding venue to the rising abundance of grazing animals. Ultimately, primates appeared as well, and these evolved into apes and man.