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

This comprehensive study guide covers the main terms and concepts needed for a unit on the evolution of multicellular life.

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This 10-minute video lesson continues to discuss the beginnings of life on Earth. It looks at the Cambrian explosion and biodiversity in the Phanerozoic Eon. [Cosmology and Astronomy playlist: Lesson 41 of 85]

Khan Academy learning modules include a Community space where users can ask questions and seek help from community members. Educators should consult with their Technology administrators to determine the use of Khan Academy learning modules in their classroom. Please review materials from external sites before sharing with students.

For life to evolve from simple single-celled organisms to many millions of species of prokaryotic species to simple eukaryotic species to all the protists, fungi, plants, and animals, took some time. Well over 3 billion years. Learn more about the timeline of evolution in this learning module produced by CK-12.

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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]