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
- 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
- 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
- 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.
- 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.