Geologic Timeline

A spiral representing Earth's history with various species representing the progression of evolution. — Earth is over 4.5 billion years old and life has existed on its surface for only a fraction of that time, but it has still grown an unfathomable diversity.
United States Geological Survey
Public domain

Scientists have estimated that the Earth is about 4.6 billion (4,600,000,000) years old! Geologists and paleontologists have made a timeline of the Earth's history called the geologic timeline. This timeline is divided into chunks of time based on fossils we find from each one. These large chunks of time are called eras. Sometimes eras are further divided into periods. The Mesozoic era, when dinosaurs ruled, is divided into the Triassic, Jurassic, and Cretaceous periods. You can look at how the different eras are divided in the geologic timeline shown.
Each era is represented by a unique group of organisms that lived together during that time. Each era and period has been given a designated range of years. For example, the Triassic period lasted from 252 million years ago to 200 million years ago. Sometimes an organism only lived for part of an era or period. If a species only lived from 215 million to 200 million years ago, we would say that it lived in the Early Triassic.
Now you get to make your own geologic timeline!

You will need:

Picture of the geologic timeline
Prehistoric Species Index with pictures (check out our lesson plan to get the document)
String
Paperclips or clothespins

A string tied between two chairs sitting apart with their backs facing each other. — The string is tied between two chairs to represent the time life has existed on Earth up to present day.
Florissant Fossil Beds National Monument

Instructions:

Step One:

Find a place to stretch out your string so it is off the floor and straight across. A good option is to tie it between two chairs and move the chairs apart from each other so the string is taut.

Your string now represents the entire length of life's history on Earth. You will be arranging the pictures of prehistoric organisms according to when they appear in geologic time. The oldest will be at the left end of the line and the youngest will be on the right.

A laminated image of stromatolites hanging on a string by a paperclip next to the far left chair. — Stromatolites are at the very beginning on our geologic timeline with the start of life on Earth.
Florissant Fossil Beds National Monument

For the first billion years of the planet's history, life wasn't yet present on Earth. It was only around 3.6 billion (3,600,000,000) years ago that the first forms of life finally appeared! Colonies of single celled cyanobacteria in the oceans were the first life forms to appear on Earth. They formed algal mats and nodules called stromatolites.

Step Two:

Clip the picture of the stromatolites to the left end of the string with a paperclip or clothespin. This is when life first appears on your timeline.
During the Cambrian Period, at the beginning of the Paleozoic Era, there was a sudden explosion of life as new species started appearing rapidly. At this time, life was still restricted to the oceans. But sea creatures like corals, fish, and trilobites flourished. As the Paleozoic Era went on life continued to evolve and the first amphibians, land plants, and reptiles emerged.

Step Three:

Look at the Prehistoric Species Index. Follow the instructions to mark the important information about each animal. Each species is labeled with the geologic period they lived in. Use this to put them in order of appearance on your geologic timeline.
Around 250 million years ago the largest mass extinction the Earth has ever seen wiped out most of life on the planet. Almost all of the sea creatures were killed off and things on land changed drastically. This extinction event marked the end of the Paleozoic Era and the beginning of the Mesozoic Era. The Mesozoic was marked by the long rule of the dinosaurs starting with their emergence in the Triassic Period. They then thrived until the end of the Cretaceous Period. During the Mesozoic Era, the earliest birds and mammals also evolved in the shadow of the titanic and bizarre reptiles that ruled. Most of today's modern plant life (flowering plants known as angiosperms) also appeared.
Then 65 million years ago another mass extinction wiped out the dinosaurs marking the end of the Mesozoic Era. Mammals took their new place as the dominant group on land. This marks the start of the Cenozoic Era which continues today. Human-like mammals have only been around for the past 2 million years. However, it has been known that humans have had a greater impact on Earth's history than any other species.

Sixteen laminated pictures of various prehistoric organisms hang from a string between two chairs with paperclips. — Sixteen pictures of prehistoric organisms are arranged in a geologic timeline showing the progression of life on Earth.
Florissant Fossil Beds National Monument

Step Four:

A key for your geologic timeline is provided below. When you finish clipping all of your pictures to your string, look at the key to check where each species goes.