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Scientists believe the Earth's life began around 4.6 billion years ago. The Earth formed as cosmic dust clumped together to form larger and larger particles over a period of 150 million years. Lighter gases, such as hydrogen and helium, were able to escape into space. Because of the high temperatures, all water was held in the atmosphere as vapor.The lower atmosphere is characterized by the continued release of gases from the lithosphere and the formation of water vapor clouds. Chemosynthetic bacteria first appeared on Earth 3.9 billion years ago. Life begins to alter the environment.
As the Earth cooled, the water vapor in the atmosphere condensed to form the oceans and other bodies of fresh water on the continents. Oxygen began to accumulate as a result of photo-dissociation from water and, eventually, photosynthesis. The appearance of living organisms was critical in the formation of atmospheric oxygen and ozone. Life on land would be impossible without ozone due to harmful ultraviolet radiation.
The evolution, development, and growth of life raises the amount of oxygen in the atmosphere from 1% to 21%. The concentration of atmospheric oxygen peaked five hundred million years ago. Around the year 1700, humans began modifying the concentrations of certain gases in the atmosphere.
GEOLOGIC TIME SCALE
The ideas of superposition and evolution provide the basis of the geological time scale, which was developed in a somewhat random fashion (mostly in Europe) during the 19th century. The geologic time scale is a system of chronological dating that relates geological strata to time. It is used by geologists, paleontologists, and other Earth scientists to describe the timing and relationships of events that have occurred during Earth’s history. The geological time scale divides the history of Earth into eons, eras, periods, and epochs.
EONS
Eon has the largest intervals of geologic time. A single eon covers a period of several hundred million years. The history of the Earth has been divided into four eons: Hadean, Archaean, Proterozoic and Phanerozoic.
Hadean Eon (4.6 bya - 4.0 bya)
This time represents Earth’s earliest history, during which the planet was characterized by a partially molten surface, volcanism, and asteroid impacts. Several mechanisms made the newly forming Earth incredibly hot: gravitational compression, radioactive decay, and asteroid impacts.
Archaean Eon (4 bya - 2.5 bya)
It is the period where life first formed on Earth, archaea and bacteria. Earth cooled down and was able to support continents and oceans.
Proterozoic Eon (2.5 bya - 541 mya)
It is the period just before the proliferation of complex life on Earth. There were extensive shallow epicontinental seas and rocks are less metamorphosed than in the Archean age.
Phanerozoic Eon (541 mya - Present)
It is the period of visible life where rapid expansion and evolution of life forms occur and fill the various ecological niches available on Earth.
ERAS
It is the subdivision of eons. The Phanerozoic Eon is divided into three eras – Paleozoic (time of ancient life), the Mesozoic (time of middle life) and the Cenozoic (time of recent life).
Paleozoic era (542 mya - 251 mya)
A time of great change on Earth. Paleozoic era began with the breakup of one supercontinent and the formation of another. Plants became widespread. And the first vertebrate animals colonized land. Began with the Cambrian explosion, an extraordinary diversification of marine animals, and ended with the end-Permian extinction, the greatest extinction event in Earth history.
The Mesozoic Era (251 mya - 66 mya)
Known as the age of reptiles, Mesozoic Era is made up of three periods: Triasic, Jurassic and Creataceous. The most significant event was the rise of the dinosaurs. A famous Jurassic deposit is the Morrison Formation, within which the world’s richest storehouse of dinosaurs was preserved. True pines and red woods appeared and rapidly spread. Flowering plants arose and their emergence accelerated the evolution of insects. A major event of this era was the breakup of Pangea. At the end of this era, the dinosaurs and reptiles were completely wiped out.
The Cenozoic Era (65 mya - Present)
This era is known as the “age of mammals” because mammals replaced the reptiles as the dominant land animal. It is also sometimes called the “ age of flowering plants” because angiosperms replaced gymnosperms as the dominant land plants. This era is made up of two periods: Tertiary and Quartenary. From oldest to youngest the periods are broken up into the Paleocene, Eocene, Oligocene, Miocene and Pliocene for the Tertiary period and the Pleistocene and Holocene for the Quarternary period.
PERIODS
Eras are divided into shorter time units called periods. Each period is characterized by specific fossils and is usally named for the location in which fossils were first discovered.
PALEOZOIC ERA PERIODS
Cambrian Period
(542 mya - 488 mya)
Almost all marine organisms came into existence as evidenced by abundant fossils. One important event is the development of organisms having the ability to secrete calcium carbonate and calcium phosphate for the formation of shells. The evolution of chordates, animals with dorsal nerve cord, hard resembled clams and arthropods ancestors of spiders, insects, and crustaceans. There were two supercontinents during this period, Gondwana and Laurentia.
Ordovician Period
(488 mya - 444 mya)
This period marked the earliest appearance of vertebrates and the jawless fish known as Agnatha. Ordovician rocks have the distinction of occurring at the highest elevation on Earth – the top of Mount Everest. During this period, the level of carbon dioxide was several times higher than today. There were four major continents separated by three major oceans.
Silurian Period
(444 mya - 416 mya)
This period brought the emergence of terrestrial life, the earliest well developed circulatory system (vascular plants) known as Cooksonia. As plants move ashore so did other terrestrial organisms. Airbreathing scorpions and millipedes were common during the period. Romundina, a primitive armoured fish with a cartilage skeleton is the earliest fish known to have jaws. Three northern continents collided forming the new supercontinent Euramerica.
Devonian Period
(416 mya - 359 mya)
This period was known as the “age of fishes”. Lowland forests of seed ferns, scale trees and true ferns flourished. Sharks and bony fish developed. Today the lung fishes and coelacanth, a “living fossil” have such internal nostrils and breathe in a similar way. The first amphibians made their appearance, although able to live on land, they needed to return to water to lay their eggs. The Kellwasser Event was largely responsible for the demise of the great coral reefs, jawless fishes, and trilobites.
Mississippian Period (359 mya - 318 mya)
This period is sometimes called the “Age of Crinoids” because the fossils of these invertebrates are major components of much Mississippian-age limestone. Also noteworthy in this period is the first appearance of amphibians.
Pennsylvanian Period (318 mya - 299 mya)
Warm, moist climate conditions contributed to lash vegetation and dense swampy forests. Insects under rapid evolution led to such diverse forms of giant cockroaches and dragonflies. The evolution of the first reptiles took place with the development of the amniotic egg, a porous shell containing a membrane that provided an environment for embryo. The Coal Age, the formation of organic deposits of coal in plant debris formed the world’s first extensive coal deposits.
Permian Period
(299 mya - 251 mya)
A dramatic climatic shift may have partially triggered the assembly of smaller continents into a supercontinent, Pangea, which was surrounded by an immense ocean called Panthalassa. The reptiles were well-suited to their environment that they ruled the Earth for 200 million years. The two major groups of reptiles – diapsids and synapsids dominated this period. Diapsids gave rise to the dinosaurs while synapsids gave rise to mammals.
MESOZOIC ERA PERIODS
The Triassic Period
(251 mya - 200 mya)
The Triassic followed on the heels of the largest mass extinction in the history of the Earth. This event occurred at the end of the Permian, when 85 to 95 percent of marine invertebrate species and 70 percent of terrestrial vertebrate genera died out. During the recovery of life in the Triassic Period, the relative importance of land animals grew. Reptiles increased in diversity and number, and the first dinosaurs appeared.
The Jurassic Period
(200 mya - 145 mya)
Was characterized by a warm, wet climate that gave rise to lush vegetation and abundant life. Many new dinosaurs emerged—in great numbers. Among them were stegosaurs, brachiosaurs, allosaurs, and many others. During the Jurassic Period dinosaurs became the dominant animals on land.
The Cretaceous Period (145 mya - 65 mya)
The Cretaceous is usually noted for being the last portion of the "Age of Dinosaurs", but that does not mean that new kinds of dinosaurs did not appear then. It is during the Cretaceous that the first ceratopsian and pachycepalosaurid dinosaurs appeared.
CENOZOIC ERA PERIODS
Tertiary Period
(65 mya - 2.6 mya)
Covers the major demise of dinosaurs and the beginning of the most recent ice age. At the start of the tertiary period, reptiles were replaced by mammals as dominant vertebrates.
The Quaternary Period (2.6 mya - Present)
Famous for the many cycles of glacial growth and retreat, the extinction of many species of large mammals and birds, and the spread of humans. The Quaternary Period is divided into two epochs, from youngest to oldest: the Holocene and Pleistocene. We are living in the Holocene epoch.
EPOCHS
Where the rock record is most complete and least deformed: a detailed fossil record may allow scientists to divide periods into shorted time units called epochs.
TERTIARY PERIOD EPOCHS
Paleocene
Paleocene
First epoch of the tertiary period. Lasted from 65 to 55.8 million years ago. This epoch marks the beginning of the Cenozoic era and the tertiary period.
Eocene
Eocene
The Second epoch of the tertiary period. Lasted from about 55.8 to 33.9 million years ago.
Oligocene
Oligocene
Third epoch of the tertiary period. Lasted from about 33.9 to 23 million years ago.
Miocene
Miocene
Fourth epoch of the tertiary period. Lasted from about 23 to 5.3 million years ago.
Pliocene
Pliocene
Fifth epoch of the tertiary period. Lasted from about 5.3 to 2.6 million years ago.
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