Biology 2e is designed to cover the scope and sequence requirements of a typical two-semester biology course for science majors. The text provides comprehensive coverage of foundational research and core biology concepts through an evolutionary lens. Biology includes rich features that engage students in scientific inquiry, highlight careers in the biological sciences, and offer everyday applications. The book also includes various types of practice and homework questions that help students understand—and apply—key concepts. The 2nd edition has been revised to incorporate clearer, more current, and more dynamic explanations, while maintaining the same organization as the first edition. Art and illustrations have been substantially improved, and the textbook features additional assessments and related resources.

By the end of this section, you will be able to do the following:

Describe the features that characterized the earliest animals and approximately when they appeared on earth
Explain the significance of the Cambrian period for animal evolution and the changes in animal diversity that took place during that time
Describe some of the unresolved questions surrounding the Cambrian explosion
Discuss the implications of mass animal extinctions that have occurred in evolutionary history

Des Collins studies the fossils of the Burgess Shale that represent the Cambrian Explosion of animal life. An animation of this event shows the transition from the simple animals that existed before the explosion to the many complex, weird forms that evolved in a geological blink of the eye. Rudy Raff presents several theories to explain what caused the explosion. By the end of the explosion, all 35 of the present groups or phyla of animals existed and no new ones have evolved since. Nature had evolved life's essential designs in a single evolutionary leap. Every new shape of life that has followed has been a variation on one of these themes or phyla. [13:07]