Life on earth

• Geologic Time Scale


  Divides up the history of the earth based on life-forms that have existed during specific times since the creation of the planet
• Geochronologic units

  • Eon
  • Era
  • Period
  • Epoch
• Eon


  • Longest subdivision; based on the abundance of certain fossils
• Era


  • Next to longest subdivision; marked by major changes in the fossil record
• Period


  • Based on types of life existing at the time
• Epoch


  • Shortest subdivision; marked by differences in life forms and can vary from continent to continent
• Life on Earth began around 3.8 billion years ago with single-celled prokaryotic cells such as bacteria
• Multicellular life forms evolved much later, starting with arthropods that made their appearance more than 500 million years ago (Ma), followed by land plants, 475 Ma, and forest plants, 385 Ma
• Mammals started evolving into existence 180 Ma and Homo sapiens only 200, 000 years ago
• Mass extinction


  The extinction of a large number of species within a relatively short period of geologic time, thought to be due to factors such as catastrophic global events or widespread environmental change that occurred too rapidly for most species to adapt
• Possible theories for mass extinction

  • Catastrophic methane release
  • Floods
  • Basalt eruptions
  • Climate change
  • Impact events
• The geologic time scale distinguishes Earth's history based on life-forms that existed at certain times since the planet's formation
• The geologic time scale is divided into four divisions: eon, era, period, and epoch