The Eras of the Geologic Time Scale

The four periods ranged from 4.6 billion years ago to the present

Geological time

United States Geological Survey/Wikimedia Commons/Public Domain

The Geologic Time Scale is the history of the Earth broken down into spans of time marked by various events. Markers such as the types of species and how they evolved help distinguish one era from another. Four main time spans generally mark the Geologic Time Scale. The first, Precambrian Time, is not an actual era on the scale because of the lack of diversity of life, but the other three are defined eras:

Precambrian Time: 4.6 billion to 542 million years ago

A stromatolite fossil
John Cancalosi / Getty Images

Precambrian time started at the beginning of the Earth 4.6 billion years ago. For billions of years, there was no life on Earth. It wasn't until the end of this time period that single-celled organisms came into existence. No one knows for sure how life on Earth began, but theories include the Primordial ​Soup TheoryHydrothermal Vent Theory, and Panspermia Theory.

The end of this time span saw the rise of a few more complex animals in the oceans, such as jellyfish. There was still no life on land, and the atmosphere was just beginning to accumulate the oxygen needed for higher-order animals to survive. Life didn't take off and diversify until the next era.

Paleozoic Era: 542 million to 250 million years ago

Trilobites are an index fossil from the Paleozoic Era

Jose A. Bernat Bacete/Getty Images

The Paleozoic Era began with the Cambrian Explosion, a relatively rapid period of speciation that kicked off a long period of life flourishing on Earth. Great amounts of life from the oceans moved onto the land. Plants made the move first, followed by invertebrates. Not long afterward, vertebrates took to the land. Many new species appeared and thrived.

The end of the Paleozoic Era came with the largest mass extinction in the history of life on Earth. The Permian Extinction wiped out 95 percent of marine life and nearly 70 percent of life on land. Climate changes were most likely the cause of this extinction as the continents all drifted together to form Pangaea. The mass extinction paved the way for new species to arise and a new era to begin.

Mesozoic Era: 250 million to 65 million years ago

Mesozoic sea life
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After the Permian Extinction caused so many species to go extinct, many new species evolved and thrived during the Mesozoic Era. It's also known as the "age of the dinosaurs" because dinosaurs were the dominant species for much of this period. Dinosaurs started off small and got larger as the Mesozoic Era continued.

The climate during the Mesozoic Era was very humid and tropical, and many lush, green plants sprouted all over the Earth. Herbivores especially thrived during this time period. Small mammals came into existence, and birds evolved from the dinosaurs.

Another mass extinction marked the end of the Mesozoic Era, triggered by a giant meteor or comet impact, volcanic activity, more gradual climate change, or various combinations. All the dinosaurs and many other animals, especially herbivores, died off, leaving niches to be filled by new species in the next era.

Cenozoic Era: 65 million years ago to the present

Smilodon and mammoth evolved during the Cenozoic Era

Dorling Kindersley/Getty Images

The last time period on the Geologic Time Scale is the Cenozoic Period. With large dinosaurs now extinct, smaller mammals that survived were able to grow and become dominant. Human evolution also happened during this era.

The climate changed drastically over a relatively short period of time, becoming much cooler and drier than during the Mesozoic Era. An ice age covered most temperate parts of the Earth with glaciers, causing life to adapt relatively rapidly and the rate of evolution to increase.

All species of life evolved into their present-day forms. The Cenozoic Era hasn't ended and most likely won't until another mass extinction occurs.