Chapter 30.3, Problem 2C

Summary Introduction

To explain: When most of the major groups of animals evolved according to the current hypothesis.

Concept introduction: Animal phylogeny is the rapidly changing field. Biologists mostly depend on the fossils to compare the similarities of body plan and the pattern of development to determine the evolutionary relationships among various animal groups. Explosion in biological terms refers to the increase in size of the population and its complexity in a large scale. Explosion shows adaptive radiation as a part of evolution that occurs. The Cambrian period falls under the Paleozoic era during which major evolution of invertebrates began.