Most of the animals we interact with directly—such as domesticated animals, our pets, and many forms of wildlife—are vertebrates, animals with backbones. Humans are vertebrates, too. Our bodies have much in common with those of other vertebrates. In evolutionary terms, we’re more closely related to these animals than to invertebrates, the animals without backbones, such as worms, jellyfish, and insects.

We vertebrates are some of the largest and fastest animals on Earth. But let’s keep our perspective. The first animals were invertebrates. Today, there are far more species of invertebrates than vertebrates, and in terms of individual organisms, we vertebrates are far outnumbered.

The climb through the tree of life from invertebrates to vertebrates is long and winding. As we’ve already seen, the invertebrates are not a homogeneous group of organisms. The relationship between vertebrates and invertebrates is not simply a division into two categories.

In this activity, we’ll explore that relationship, and find the place of humans in the tree of life. We’ll begin with a look at two diverse groups of protostomes, mollusks and arthropods. Then, we’ll begin our exploration of the deuterostomes, the other important clade of bilateral animals, and we’ll make our way to the root of the vertebrate branch in the tree of life. Finally, we’ll survey vertebrate diversity in light of the evolutionary history of organisms in this group.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education