Precambrian Times Occurred from 4.6 BYA to 542 MYA The period of Earth history known as the “Precambrian Times” is broken up into three eons, which are the largest segments on the geologic time scale. These three eons make up 90% of Earth history!

Absolute age dating of rocks and meteorites indicate that the Earth was likely formed 4.6 billion years ago from the accumulation of small, rocky bodies orbiting the Sun. A stellar nebula has enough material for thousands of stars and solar systems The material in a nebula is drawn together due to gravity, and begins spinning like a disk. As bodies collide, they begin to form progressively larger objects, and eventually planets

The early Earth was very hot, but it eventually cooled enough for liquid water to exist on the surface.

This water may have come from the many meteorites that collided with Earth during its early history. Water in the form of ice would have vaporized on impact, forming a haze around the planet – our early atmosphere. Volcanoes would have also contributed to the early atmosphere through eruptions containing CO2 and nitrogen.

The amount of liquid water on Earth is actually very small when compared to its total size!

In the Archean eon the first single-celled organisms began to appear. This includes the appearance of cyanobacteria, which were the first producers of oxygen.

Cyanobacteria in the oceans formed large mats called stromatolites, which filled the atmosphere with oxygen. The landmasses on Earth were joined together at this time to make the first supercontinent, Rodinia.

By the end of the Proterozoic eon, many marine multi- cellular organisms began to appear. Life was becoming more complex.

Paleozoic Era Occurred from 542 MYA to 251 MYA. Means “Ancient Life”

The beginning of this period is known as the “Cambrian explosion” due to the rapid diversification of marine species during this time.

During the Ordovician period, the first fish began to appear, and many coral reefs.

During the Silurian period, the first land plants appeared, paving the way for future life on land.

During the Devonian period, animals with backbones began to appear, such as the first amphibians. Armored fish swam the oceans, and insects appear in the rock record.

The Carboniferous (Mississipian/Pennsylvanian) gets its name from the coal that is found in rock layers dating to this period, and formed due to the many swamps and rainforests on Earth at this time.

Carboniferous insects and reptiles

During the Permian period, the climate was much warmer, and reptiles became the dominant species. This is the time of the supercontinent Pangaea.

The Paleozoic ended with the largest mass extinction event in Earth’s history, which wiped out 90% of organisms living on the planet at that time. Also known as “The Great Dying”, the Permian-Triassic extinction may have been caused by long-lasting eruptions in a region known as The Siberian Traps. Watch the video at to learn more.

Mesozoic Era Occurred from 251 MYA to 65 MYA. Means “Middle Life”

The first dinosaurs appeared during the Triassic period. Seed plants also appeared during this time, and the first mammals, though they were very small and nocturnal.

During the Jurassic period, dinosaurs became the dominant organisms on the planet. Birds also appear in the rock record.

During the Cretaceous period, flowers appear for the first time. Dinosaurs continued to diversify.

Dinosaurs with feathers??

At the end of the Mesozoic era there was another mass extinction event. Based on a crater found dating to this time period, it is believed there was a large meteorite impact. The amount of rock blown into the atmosphere by this event would have significantly cooled the Earth.

Cenozoic Era Occurred from 65 MYA to the present. Means “Recent Life”

Overall, the Cenozoic was cooler in temperature than the previous era. During this time the continents began to move into their present positions.

The Cenozoic is known as the “Age of Mammals”, because they rapidly diversified and dominated this era. In the late Cenozoic, the first primates appeared, and later the first humans.