History of life

• Geological Timescale


  The earth's history has been divided into a series of time intervals
• Geological Timescale


  1. Divides the vast periods of time into manageable time frames

  2. Geologists have divided the Earth's history into eras, periods and epochs

  3. These time intervals vary in length according to significant events in the history of the Earth

  4. They are not equal divisions of time like years
• Eras


  The smaller time periods that the eons are divided into
• The Phanerozoic eon is divided into three eras: the Cenozoic, Mesozoic and the Paleozoic eras
• The divisions between these eras is determined by very significant events in the history of the Earth
• Periods


  The eras are made up of varying number of periods of different lengths of time
• For example, the Paleozoic is sub divided into the Permian, Carboniferous, Devonian, Silurian, Ordovician and Cambrian Periods
• The earth was formed about 4.5 billion years ago
• It took about 500 million years for the crust to solidify
• The oldest fossils of microorganisms are 3.5 billion years old found embedded in rocks in Western Australia
• Prokaryotes dominated from 3.5 to 2 billion years ago. During this time, the first divergence occurred: Bacteria and Archaea
• Oxygen began accumulating in the atmosphere about 2.7 billion years ago. Cyanobacteria are photosynthetic prokaryotes that are still present today and produced oxygen
• The oldest eukaryotic fossils are about 2 billion years old
• The oldest animal fossils are 700 million years old
• Animal diversity exploded 540 million years ago
• Plants, fungi, and animals began colonizing land 500 million years ago
• First plants transformed the landscape
• Then animals were able to take advantage of new niches
• Mammals evolved 50 to 60 million years ago
• Five Mass Extinctions


  • Ordovician (444 million years ago)
  • Devonian (383-350 million years ago)
  • Permian (252 million years ago)
  • Triassic (201 million years ago)
  • Cretaceous (66 million years ago)
• We are in the crisis of the sixth mass extinction caused by human activities such as legal veld-fires which consume large amounts of valuable oxygen and release smoke and carbon dioxide resulting in global warming and climate change; legal hunting and habitat destruction
• Causes of mass extinctions

  • Environmental disasters: meteor or asteroid impacts, severe volcanism, radiation from exploding stars or killer diseases
  • Organisms failing to adapt to habitat or environmental changes: changing temperatures of ice and warm ages, global sea level changes and continental drift
• Fossil


  The remains or trace of an organism that identifies a plant or animal that lived long ago and has been preserved in rock
• Paleontologist


  A scientist who studies fossils
• Paleontology


  The study of fossils
• Petrification


  The process by which the remains of an organism are replaced by minerals such as silica or calcite
• Sedimentary rock


  A rock formed from the layers of sediment, such as mud, silt, sand, and organic material
• How fossils form


  1. Sediment: An animal is buried by sediment such as volcanic ash or silt shortly after death

  2. Layers Petrification: More sediment layers accumulate above the animal's remains and minerals such as silica slowly replace the calcium phosphate in the bones

  3. Movement: Movement of tectonic plates or giant rock slabs that make up the Earth's surface pushes the sediments and fossils closer together

  4. Erosion: Erosion from the rain, rivers and wind wears away the remaining rock layers, eventually exposing the preserved remains
• Relative dating


  The age of the fossil is worked out by trying to find out how it is related to the age of another fossil or geological event
• Radiometric dating


  Attempts to determine how many years ago the fossil was formed
• Examples of fossils:

  • bones
  • Shells
  • Tracks(footprints),hardened faeces of animals
  • Petrified tree trucks
  • Imprints of leaves and small animals
  • Examples of fossils that occur in tar and ice
  • Fossils of a wooly mammoth (ice)
  • Pits of tar contained bones of the sabre toothed cats
  • Some insects
• Eons: eons are the time periods and are hudres of millions of years long.

  The Phanerozoic is the most recent eon it began 500 million ya