Origin And Classification

The first animals probably arose in the sea. The structural characteristics of invertebrates suggest that they were the first multicellu-lar animals and that they evolved from protists. Because protists are both heterotrophic and eukaryotic, scientists have inferred that mul-ticellular invertebrates may have developed from colonies of loosely connected, flagellated protists, such as the one shown in Figure 32-2.

What path did cell specialization take in these early organisms? Colonial protists may have lost their flagella over the course of evolution as individual cells in the colony grew more specialized. They may have been similar to modern colonial protists that do show some degree of cell specialization, such as some species of algae. In these species, the gametes are distinct from nonreproductive cells. A similar division of labor in early colonial protists may have been the first step toward multicellularity.

Scientists often use a type of branching diagram called a phylogenetic diagram, such as the one in Figure 32-3, to show how animals are related through evolution. Taxonomists have grouped animals into several phyla (singular, phylum) by comparing animals' fossils, body symmetry, patterns of embryo development, and ribosomal RNA (rRNA) and other macromolecules. Taxonomy is an ever-changing branch of science; therefore, it should not be surprising that the actual number and names of animal phyla continue to change and be debated. Many taxonomists recognize 30 or more different animal phyla, though some phyla contain a very small number of species.

figure 32-2

The first animals may have evolved from colonial protists similar to the one shown in this drawing.

figure 32-2

The first animals may have evolved from colonial protists similar to the one shown in this drawing.

figure 32-3

This phylogenetic diagram represents a hypothesis for the relationship among members of the animal kingdom based on rRNA analysis. Notice on the diagram the locations of similarities in body tissues, body symmetry, and embryo development. For updates on phylogenetic information, visit go.hrw.com. Enter the keyword HM6 Phylo.

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