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Animals are believed to have arisen from protozoans ("first animals" in Greek), which were in existence way back in the Archaean period about 3 billion years ago. These unicellular organisms evolved to multicelluar organisms called metazoa over 600 million years ago. Primitive metazoa can be grouped in three basic categories: sponge-like animals, cnidarians, and worms. The sponges, and cnidarians, are the most primitive with about 11 specialized cell types. Worms and higher metazoa have approximately 55 specialized cells.|
Sponges are the simplest grade of multi-celled animals. In general, sponges have open-topped, sack-like bodies which are fixed to the sea floor. Water is pulled through pores in the body, and food is filtered out; rest of the water exits from the opening.
The cnidarians include corals, hydras, sea anemones, and jellyfish. Their basic body plan is also a sack-like form, but at one end there is a mouth/anus, which can be opened and closed, and tentacles which direct food to the mouth.
Figure 10-33 Animal Evolution [view large image]
Figure 10-33 shows animal diversification over the age. All this diversity stems from successive branchings, starting from a single bacterium-like ancestor. Each branching event is called a speciation: a breeding population splits into two, and they go their separately evolving ways. Among sexually reproducing species, speciation is said to have occurred when the two gene pools have separated so far that they can no longer interbreed. Speciation begins by accident, but natural selection operates in a non-random way. When separation has reached the stage where there is no interbreeding even without a geographical barrier, we have the origin of a new species.
Further details of animals evolving through the geological periods from Cambrain to the present are described in an appendix - Age of Animals. The living animals are described in the Anatomy of Animals.