"> Introduction - Animals Past and Present - Univeristy of Illinois Extension
University of Illinois Extension


Over earth’s history, there have been many different types of animal life. We know this because of fossils that have been found in many parts of the world. Amphibians, reptiles, and mammals have dominated the earth at different times.


Animals Past and Present takes a look at what types of animals were present during different time periods. It also explains how such things as plate tectonics, climate, and other factors affected these animals and the ecosystems they lived in.

A special section focuses on Illinois, and what animals could be found in the state at three different time periods: 65 million years ago, 10,000 years ago, and today.

Got the Time?

Earth is estimated to be 4.6 billion years old. This is a very, very long time! Scientists have divided earth’s geologic and biologic history into four time eras. These are called the Precambrian, Paleozoic, Mesozoic, and Cenozoic. These are divided into smaller time frames, based on evidence of what was happening then. The closer we are to present time, scientists can divide time into smaller pieces, because they have more evidence of what was going on.

graph of Cezonoic Era

For almost one half of earth’s history, there is little or no evidence of any type of life on earth. The oldest rocks are around 3.8 billion years old, and the oldest fossils are around 3.5 billion years old.

An explosion of life seemed to have occurred around 2.5 billion years ago.

Since then, there have been several times when there have been large extinctions of animals and other forms of life. For the most part, earth has undergone long periods where there is very slow, endless change, mixed together with shorter periods of catastrophic, rapid change. Species that survived and adapted to rapid changes became dominant.