The Geologic Time Scale

Age of the Earth According to radioactive dating, the earth is estimated to be 4.6 billion years old. The Geologic Time Scale is a record of the life forms and geologic events in Earth’s history. Simple single celled life is estimated to have formed 3.5 billion years ago. Animals appeared 600 million years ago.

Geologic Time Divisions The Geologic Timeline begins with Precambrian time. Precambrian time takes up 88% of Earth’s history. The 2 other basic units of geologic time are eras and periods. There are 3 eras, and the periods are smaller divisions within the eras.

Precambrian Earliest life was single celled – like bacteria. They contributed oxygen to the atmosphere Microbialites in Pavilion Lake in Canada today are similar to these ancient life forms

Paleozoic Era Many life forms began to evolve during the Cambrian Explosion. Life went from simple invertebrates in the oceans, to more complex and diverse life forms on land. The first plants, fish, amphibians, and reptiles evolved in the Paleozoic era.

Mass Extinction At the end of the Paleozoic era, a mass extinction occurred that killed off almost 90% of marine species. Scientists do not know what caused the extinction, but it could have been caused by climate change resulting from continental drift.

The Mesozoic Era Some plants and animals did survive the Permian extinction, including fish, reptiles, insects and conifers. The Mesozoic Era is known as the Age of Reptiles. The first dinosaur appeared about 225 million years ago.

Mesozoic Era Mammals first evolved during the Triassic period. During the Jurassic period dinosaurs ‘ruled’ the Earth for about 150 million years. Scientists have identified several hundred types of dinosaurs. First birds also evolved during the Jurassic period. Scientists believe that birds evolved from dinosaurs.

Mass Extinction Again! At the end of the cretaceous period, about 65 million years ago, a mass extinction occurred. Scientists hypothesize that this was caused by a massive object from space hitting Earth.

The Cenozoic Era Called the Age of Mammals. They were no longer competing with large reptiles for food, space and survival. The extinction of dinosaurs created an opportunity for mammals.

The Cenozoic Climate During the tertiary period, the Earth was generally warm, and on land flowering plants, insects, and mammals flourished. Many mammals became large. In the oceans whales evolved.

The Ice Ages During the Quartenary period, the Earth’s climate cooled, and species had to evolve to deal with several ice ages. In the ocean, algae, coral, fish, and mammals thrived. On land, flowering plants and mammals such as cats, dogs, cattle, and humans became common.

Early Humans The fossil record shows that modern humans may have evolved 100,000 years ago. By about 12 – 15,000 years ago, humans had reached every continent.