7.5 The Evolutionary History of the Animal Kingdom
7.5 The Evolutionary History of the Animal Kingdom
Mary Ann Clark; Jung Choi; and Matthew Douglas
Learning Objectives
By the end of this section, you will be able to do the following:
- Describe the features that characterized the earliest animals and approximately when they appeared on Earth
- Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity that took place during that time
- Describe some of the unresolved questions surrounding the Cambrian explosion
- Discuss the implications of mass animal extinctions that have occurred in evolutionary history
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.
Pre-Cambrian Animal Life
The time before the Cambrian period is known as the Ediacaran Period (from about 635 million years ago to 543 million years ago), the final period of the late Proterozoic Neoproterozoic Era (see figure below). Ediacaran fossils were first found in the Ediacaran hills of Southern Australia.
There are no living representatives of these species, which have left impressions that look like those of feathers or coins (see figure below). It is believed that early animal life, termed Ediacaran biota, evolved from protists at this time.
Most Ediacaran biota were just a few mm or cm long, but some of the feather-like forms could reach lengths of over a meter. Recently there has been increasing scientific evidence suggesting that more varied and complex animal species lived during this time, and likely even before the Ediacaran period.
Fossils believed to represent the oldest animals with hard body parts were recently discovered in South Australia. These sponge-like fossils, named Coronacollina acula, date back as far as 560 million years, and are believed to show the existence of hard body parts and spicules that extended 20–40 cm from the thimble-shaped body (estimated about 5 cm long). Other fossils from the Ediacaran period are shown in the figure below.
Another recent fossil discovery may represent the earliest animal species ever found. While the validity of this claim is still under investigation, these primitive fossils appear to be small, one-centimeter long, sponge-like creatures, irregularly shaped and with internal tubes or canals. These ancient fossils from South Australia date back 650 million years, actually placing the putative animal before the great ice age extinction event that marked the transition between the Cryogenian period and the Ediacaran period. Until thi