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Earth formed when gravity pulled swirling gas and dust in space in to a mass. Precambrian time started with the formation of Earth.
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The oldest fossil evidence of life on Earth is found in rock. These ancient life-forms were simple, single-celled organisms, much like bacteria on Earth today.
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Some photosynthetic bacteria start pulling carbon dioxide out of the atmosphere and releasing oxygen into the atmosphere. Prior to this, there were gases lots of gases in the atmosphere, but oxygen was lacking.
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These fossils of multicellular organisms were rare since they had soft bodies. These organisms were not like any organisms on Earth today. These life-forms lived at the bottom of the sea and many of these species became extinct at the end of Precambrian time.
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The ozone layer blocks most of the damaging ultraviolet light from the Sun. Until this formed, any organism on the surface of the Earth would have been killed and all life must have existed under at least several centimeters of water or underground. As oxygen in the atmosphere increased, the ozone layer formed and began to shield the surface of the Earth from the harmful UV light. This allowed life to evolve to survive on the surface of the oceans and on land...the Cambrian "explosion" of life.
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This is part of the Paleozoic era explosion of life in warm, shallow seas. Invertebrates are animals without backbones. The sudden appearance of new, multicellular, complex life-forms had hard body parts (shells) which made them more easily preserved. For example, there is more evidence of trilobites in the fossil record because of their hard body parts. So many kinds of invertebrates lived in the oceans that this time is often called the age of invertebrates.
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Paleozoic means "ancient life". Commonly referred to as the age of fish.
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Land plants evolve, drastically changing Earth's landscape and creating new habitats.
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Vertebrates are animals with backbones. Some of the earliest vertebrates were fish. So many types of fish lived during this time that it is often called the age of fish. During this time on land, cockroaches, dragonflies, and other insects also evolved.
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Some fish-like organisms spent part of their lives on land. Ancient amphibian species developed lungs and could breathe which made them better adapted to land. They also had thin, moist skin to help take in oxygen. Strong limbs also helped them move on land. These early amphibians (like today) had to return to water to mate and lay eggs.
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Reptile species evolved. The entire life of a reptile could be spent on land. They were the first animals that did not need water to reproduce. They could lay eggs on land. Their eggs (like today) had tough, leathery shells that kept them from drying out.
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Dense, tropical forests grew in swamps along shallow inland seas. When trees and other plants died, they sank into the swamps. A coal swamp is an oxygen-poor environment where, over time, plant material changes into coal. These coal swamps are the major sources of coal we use today.
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This was the LARGEST mass extinction ever recorded in history. This major catastrophe marked the end of the Paleozoic Era. Approximately 95% of all life disappears. One theory is global-climate change due to massive volcanic explosions.
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Earth’s continents formed a giant supercontinent called Pangaea. Land masses came together close to Earth’s equator. The continents pushing up against one another resulted in some mountain formation.
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This is the start of the Mesozoic Era, also known as the "Age of Reptiles." Dinosaurs dominated the landscape (hundreds of species of many sizes existed), small mammals (no larger than present-day cats) evolved, and first birds appeared (evolving from reptiles). The mammals were not large because dinosaurs were ruling the world!
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Mesozoic means "middle life" since it is the time between the Paleozoic and Cenozoic Eras. It is commonly referred to as the age of reptiles. It consists of the Triassic, Jurassic, and Cretaceous periods.
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Pangaea starts to separate and continents begin to move to where they are now. The climate was getting warmer, there were no ice caps and no glaciers, so more water filled the oceans. Some of this water flowed onto continents as Pangaea began to split apart, creating narrow channels that eventually became oceans.
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The North American continent moved slowly westward. Its western edge collided with another landmass. As the plate subducted downward beneath the North American continent, the crust buckled inland, slowly pushing up the Rocky Mountains.
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The Mesozoic Era also ended with a major global catastrophe - another mass extinction. The leading theory at this time is an asteroid hit the Earth, This leads to global-climate change with the idea that as the asteroid hit, it threw up a large amount of gas, dust, and debris into the atmosphere, blocking out the sunlight that living things need. Dinosaur species and other large vertebrate species could not survive in this changed environment (fewer plants).
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This is the beginning of the Cenozoic era. With the dinosaurs gone, mammals took over and became the dominant species. Mammals were abundant and diversified into a variety of species including land, sea, and flying mammals. Some of the largest mammals were called mega-mammals and lived from 34 mya to 5 mya. Others, such as the woolly mammoths, giant sloths, and saber-toothed cats, lived in more recent years (5 mya to 10,000 years ago) and many fossils from these animals have been discovered.
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Cenozoic means "recent life". Commonly referred to as the age of mammals.
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Fossils of humans showed up only 100,000 years ago. This is not very long in geologic time!
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