Earth's eras

Precambrian Era: 4.6 billion to 542 Million Years Ago

5 BCE

There was no life on the earth for billions of years. Single-celled organisms didn't exist until the end of the Precambrian Period. There are several hypotheses on the origin of life on Earth, including the Panspermia Theory, Hydrothermal Vent Theory, and Primordial Soup Theory. At the conclusion of this period, extremely complex marine creatures like jellyfish began to emerge. The atmosphere was only starting to fill with the oxygen required for the existance of higher order species

Paleozoic Era: 542 Million to 250 Million Years Ago

4 BCE

A lengthy age of abundant life on Earth began with the Cambrian Explosion, a relatively quick period of diversification. Huge numbers of marine life types migrated on land. The first organisms to move were plants. Vertebrate animals quickly migrated to the land after that. Numerous new species developed and flourished. The largest mass extinction in the history of life on Earth occurred at the end of the Paleozoic Era, wiping out 95% of marine species and about 70% of land life.

Mesozoic Era: 250 Million to 65 Million Years Ago

3 BCE

The Mesozoic Era, also called the "age of the dinosaurs" since dinosaurs dominated the period after the Permian Extinction caused so many species to go extinct, saw the evolution and flourishing of a vast range of new species.
Dinosaurs were initially little, but as the Mesozoic Era progressed, they became bigger. Birds and small animals both descended from the dinosaurs.
Brought on by a massive meteor another great extinction marked the end of the Mesozoic Era.

Cenozoic Era: 65 Million Years Ago to the Present

1 CE

The Cenozoic Period is the last epoch on the geologic time scale. Smaller mammals that had survived were able to expand and take control once the giant dinosaurs were extinct.
In a relatively short amount of time, the environment radically shifted, becoming more colder and drier than it was during the Mesozoic Era. Over the course of this age, which hasn't finished and most certainly won't until another mass extinction occurs, all species of life—including humans—evolved into their modern forms.