The Geologic Time Scale

Geologic Time Scale : Scientists have formed a chronology of Earth’s history based on evidence from the Earth’s rocks and fossils. Geologic Time Scale is divided into four large segments called Eras. The Eras are subdivided into Periods. The periods are Further subdivided into epochs.

The Precambrian Era billion to 543 million years ago. (It accounts for 87% of the Earth’s history. If the Earth’s history was compared to a 12 month calendar, the Precambrian Era would last until the middle of October.) During this Era, the first cells appeared. They were bacteria cells that could not use oxygen and were heterotrophic.

Also in the Precambrian Era: autotrophic prokaryotes appeared (chemosynthesizers and photosynthesizers). O 2 levels rise & Ozone forms. Eventually eukaryotic cells formed (about 2.1 billion years ago).

By the end of the Precambrian era, multicellular eukaryotic organisms appeared, such as the invertebrate animals: sponges and jellyfish.

The Paleozoic Era (543 – 248 million years ago) The Paleozoic Era was rich in marine life. Many types of animals and plants developed in the oceans and on land.

The earliest part of the Paleozoic Era is the Cambrian Period. Paleontologists often refer to the “Cambrian explosion” of life shown in the fossil record. During this period, many animals including worms, sea stars, and unusual arthropods appeared.

During the Paleozoic, The first land plant, such as ferns, appeared. Insects developed. Fish appeared (the first vertebrates). Amphibians developed and primitive seed plants arrived. Then reptiles appeared. (Ancient Paleozoic Forest)

The end of the Paleozoic Era was marked by the largest mass extinction recorded in the fossil record. 90% of the Earth’s marine life and 70% of land species disappeared. It was called the Permian Extinction. (Some theorize it was the result of rapid global warming and cooling)

The Mesozoic Era began about 248 million years ago. (If the world’s history is reduced to a 12 month calendar, it would start on December 10.) It is divided into three periods: Triassic, Jurassic, and Cretaceous. The Mesozoic Era was the “Age of the Dinosaurs” Mammals appeared and were small and mouselike at the time. The first birds evolved from a group dinosaurs. The first flowering plants appeared.

Dinosaurs thrived during the much of the Mesozoic Era. Dinosaurs also Became extinct during this Era. (about 65 mil. yrs ago.)

During the Mesozoic, scientists believe that the continents, which had been merged into a supercontinent, broke up and drifted apart. The theory that explains this is called plate tectonics which states that the Earth’s surface consists of crustal plates that are continually moving on top of a partially molten layer.

The Cenozoic Era began about 65 million years ago. (About December 26 on the hypothetical calendar). Mammals began to flourish. Primates first appeared. Modern humans appeared perhaps as recently as 200,000 years ago. (Shortly before midnight on Dec. 31 st on the calendar) This cave painting was created By Cro-Magnons In France about 40,000 yrs ago.