Rock Cycle - Past Life (5)
Post Lab 

  • Exploring geologic time.
  • Comparing the different eras of time
  • Cenozoic
  • Mesozoic
  • Paleozoic
  • Precambrian
  • Internet
  • time chart

Students use the internet to discover geologic time.

Radiolarian have changed shape through time. 


Geologic time is often very difficult to understand. Most children have trouble understanding the impact of 10 years, not to mention the 4.5 billion years that have passed since the formation of the Earth. It is difficult to date exactly how old the Earth is because no one was there to record the event. There are many lines of evidence for the age of the Earth, within your students' lifetimes this number will probably change as new technology helps us to better date the past.

Students are familiar with dinosaurs and possibly a few other fossils seen in class, but the idea that certain organisms lived at certain times is very difficult for them to comprehend. The oldest unmistakable fossils are about 3.8 billion years old. However, multicellular organisms, the type that we are familiar with did not appear in the fossil record until approximately 650 million years ago. Animals with hard body parts (skeletons) did not appear until about 580 million years ago.

In order to work with the vast time of earth history, geologists have developed the geologic time scale. This scale subdivides geologic time is divided into two eons, the PrePhanerozoic (or Precambrian) and Phanerozoic. The PrePhanerozoic Eon stretches from the formation of the earth, more than 4.5 billion years ago, until the start of the Phanerozoic. There are many fossils in PrePhanerozoic rocks, but they are microscopic. Phanerozoic means "visible life" which covers about the last 545 million years of earth history, are characterized by abundant visible fossils.

The Phanerozoic Eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic. In the Paleozoic or "old life," the first bony fish, amphibians, and reptiles appear. Mammal-like reptiles (our probable ancestors) first evolved toward the end of this era. Ferns and gymnosperms (conifers) were the dominant types of plants. Dinosaurs become abundant in the Mesozoic, which means "middle life." True mammals and birds also appeared during this time period, as do the first angiosperms (flowering plants) In the Cenozoic Era, mammals became dominant, and grasses evolved. The earliest human ancestors first occurred about 3-5 million years ago.

  1. Use the Earth-Life-Plate Configuration chart to discuss the different periods of time. Make sure you make the connection that the distribution of land and water has changed through time. Habitats have changed due to plate tectonics.
  2. Instruct the students to find about five different organisms that lived during the different periods by searching the web.

    There are many web sites on fossils. Here are three good starting points: - the University of California, Berkeley’s Paleontology Museum. This link starts at the geologic time scale. It links from there to specific time periods and organisms. - the Field Museum of Natural History. A link to an on-line version of their Phanerozoic Life exhibit. - a link to a good paleontology link site.

  3. You can also have your students do a web search on any of the search engines. You may want to use the different time periods as a way for students to search for information.

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