It seems that the evolution of all successful animals began during the Cambrian Period. Here we show a very simplified view of the relationships between the main animal groups we find on Earth today.
Note that in general animals which do not have backbones are called invertebrates.
The main division in the animals we are likely to meet in our everyday lives is between the group of animals called the (such as molluscs and arthropods) and the (to which the vertebrates belong).
Unusual Animal Groups
During the “explosion” of animals during the Cambrian, many forms existed which seem unrelated to modern animals, such as this:
Opabinia reconstruction courtesy of Nobu Tamura
In this story, however, we will concentrate on those creatures which appeared in the Cambrian and would later become important in Earth history.
Cells specialized even more than and tissues (groups of similar cells) began with the jellyfish family. Jellyfish might have appeared 650 mya. Because of their soft bodies, they have left few fossil traces.
Nerves and muscles first appear in these animals, although they had no brain. Their behavior is very simple, responding automatically to touch. Nerves can carry more information between cells than can. Nerves are the main feature which makes animals different from plants.
Jellyfish image courtesy of US National Oceanic and Atmospheric Administration
They belong to the group of animals called Cnidaria or Coelenterata. This group includes corals, hydras, jellyfish, Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans.
They are closely related to the comb jelly group, or Ctenophora.
Jellyfish might have evolved into .
Early forms of seem to have evolved into various different kinds of , one of which was the ancestor of . They, in turn, probably evolved into higher animals. Ancestral flatworms may have first appeared around 570 mya.
Turbellarian flatworm image by Ivy Livingstone courtesy of BIODIDAC
Here we see the basic plan laid out for the structure of the body which would be used and modified by all later groups of animals.
Tissues (first seen in jellyfish) evolved into organs (parts of the body devoted to doing a specialized job). So, for example, the flatworms evolved a stomach. The stomach of the flatworms was just a sac. The worms took food in and send out waste through their mouths. This was not as efficient as the more advanced system we are used to, where the digestive system forms a tube with an outlet separate from the inlet! However it was a big step forward from the jellyfish. The cells of the stomach specialized in secreting digestive enzymes, making processing of food more efficient.
Flatworms crawled forwards and so evolved sensors for smell and touch at their front end. An organ, the brain, developed at the front to process this information, with nerves going back to control the flatworm's body.
Their bodies evolved so the two sides were the same (bilaterally symmetrical). This was quite different from the radially symmetrical bodies of the jellyfish. This has been the basic design of most the higher animals. It was probably more efficient for the embryo of the worm to grow in this way rather than any other way.
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