Abstract
<jats:title>Abstract</jats:title> <jats:p>This book asks why animals appear abruptly in the fossil record about 540 million years ago. It first explains what an animal is and why many animals, although not all, are distinct individuals. It looks at how the different kinds of animal are related, and the order in which they appeared. The hard evidence is the fossil record. The book first surveys the animals that had evolved by the early Cambrian, exploring the order in which they diverged from one another. There was a great radiation of large complex organisms during the last 25 million years of the previous period, the Ediacaran. According to the book, these fossils are not animals and have little to do with animal evolution, although this is not a point of view that specialists will agree with. These biological events happened during a time of environmental upheaval, a sea change, including worldwide glaciation and increased oxygenation. How animals first appeared and later responded to these tumultuous times is organized into eight scenarios, each reflecting a different opinion about early animal evolution. It concludes that the most likely scenario is that the first animals were minute creatures that lived in the mud. The most likely sequence of events is laid out as a series of twelve crucial innovations, beginning with a colonial choanoflagellate and ending with a large worm. This evolved into a variety of bilaterally symmetrical animals during the Cambrian.</jats:p>