Our timeline has been turned upright to reflect the viewpoint of a geologist, with older, deeper layers of rock at the bottom. The lower half is the Paleozoic, containing the early steps of life's history. The Mesozoic "Age of Dinosaurs" lies above, and only the top 10% belongs to the Cenozoic -- our current era.

Eras are subdivided into Periods -- often named after the rock formations that were first used to describe them. Each has its own distinctive character, and represents a fascinating chapter in the saga of life on our planet.

The story starts 543 million years ago in the Cambrian period of the Paleozoic, when the first explosion of animal diversification occurred. The Ordovician was a water world -- full of shallow seas filled with myriad creatures that would be abruptly destroyed in one of the "big five"extinction events that forever altered the course of life on Earth. The Silurian showed the survivors rushing to take complex new forms, including the first insects and land plants. The Devonian "Age of Fish" saw the oceans populated with giant armored fish, the first creatures with true jaws and backbones. By the Carboniferous, the Earth's first forests had appeared, inhabited by insects and crawling salamanders. The Permian period witnessed the development of land animals, including the fin-backed Dimetrodon that was a forerunner to modern mammals. Evolution seemed to be literally hitting its stride when the Paleozoic suddenly ended with a bang -- the worst catastrophe ever to hit life on Earth. The cause remains one of science's greatest mysteries -- and it would leave the world an empty desert.

The Mesozoic era begins in the red sands of the Triassic, on a planet almost sterilized by an event that killed virtually every living thing -- including 96% of marine species, and 83% of the genera of animal types. From this disaster, life would take off in a new direction. Although mammals had scored an early advantage, by the Jurassic our planet would belong to dinosaurs, including the colossal, long-necked Sauropods. In the final years of the Cretaceous, our forests would contain flowering plants, pollinating insects, and be ruled by packs of fearsome Tyrannosaurs. Yet this too would reach a spectacular climax -- with the K-Pg meteor impact, one of the most dramatic events in Earth's history.

The final 50 million years of the Cenozoic start in the Paleogene, with the recovery of Mammals and birds in their many forms. The Neogene includes the appearance of modern ice ages, and -- late upon the stage -- our own species.

Click "Next" to read the whole story of life's beginnings.