Fig. 15

A straight line does not mean that no change has occurred. This tree provides a simple illustration of the fact that the outgroup lineage cannot be assumed to be “basal,” “primitive,” or “ancestral” to the other species included on the tree. In this case, humans are accurately used as the outgroup to the echinoderms, which includes sea lillies, brittle stars, sea stars, sea cucumbers, and sea urchins. Of course, humans do not resemble the common ancestor of echinoderms, and there has been an enormous amount of branching among vertebrates since the very distant split of these two lineages from their common ancestor. It is most commonly argued that the sister group of the echinoderms is the hemichordates, both of which are more closely related to vertebrates than to any other group of animals, thus making the category “invertebrate” phylogenetically invalid