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

Early Life

It took the earth almost a billion years to become favorable for any type of life.  Even then, the earth was nothing like it is today.  The atmosphere contained almost no oxygen, and harmful rays from the sun mercilessly struck the earth’s surface.


Some of the first types of life that can be found in fossils are cyanobacteria.  These microscopic, single celled life forms produced energy from sunlight, and released oxygen.  This type of bacteria is found on earth even today!

These bacteria helped raise oxygen levels in the atmosphere.  However, it would take over 2.5 billion more years before there would be enough oxygen in the atmosphere to promote other forms of life.

A Life Explosion and Implosion

About 565 million years ago, when the earth was already over 4 billion years old, an explosion of life occurred.  During this time, much of earth’s land was near the equator, and large shallow seas were present.  It was in the seas where almost all life was present.


Tribolites, a type of arthropod, was dominant during this time.  It had a hard shell and jointed legs.  There were many species of this creature. Most were believed to be predators, hunting worms, or were scavengers.

The first fish were present about 505 million years ago, although it would be several more hundred million years before they become prolific.

At about 440 million years ago, the earth went through a major Ice Age, which is believed to have made many species of life go extinct.