What's Here—What's Not?
Trilobites, crinoids, corals, and a multitude of shelled animals lived in the tropical seas that covered Illinois. Some were similar to animals you can see today. Others were bizarre creatures that have since become extinct. There were no mammals, birds, or land plants yet.
When Did Life Evolve?
The first evidence of life is preserved in the fossils of single-celled cyanobacteria. They appeared 3.5 billion years ago and began to generate the oxygen-rich atmosphere that we depend on today. From this simple beginning, complex, multi-cellular life evolved.