Introduction to the Evolutionary History of the Animal Kingdom

What you’ll learn to do: Discuss the evolutionary history of the animal kingdom

The illustration shows a sea bed abundant with odd organisms, including tube-shaped worms anchored to the sea floor and animals that resemble cockroaches crawling along it. Swimming creatures somewhat resemble modern insects.

Figure 1. An artist’s rendition depicts some organisms from the Cambrian period.

Many questions regarding the origins and evolutionary history of the animal kingdom continue to be researched and debated, as new fossil and molecular evidence change prevailing theories. Some of these questions include the following: How long have animals existed on Earth? What were the earliest members of the animal kingdom, and what organism was their common ancestor? While animal diversity increased during the Cambrian period of the Paleozoic era, 530 million years ago, modern fossil evidence suggests that primitive animal species existed much earlier.