When you think of an animal, you will most likely think of a vertebrate. Since we are animals with a backbone ourselves, it is not strange that that which is closest to us comes to mind first. But when and how did vertebrates evolve? To answer that question, Nature editor Henry Gee takes a good hard look at invertebrates, convincing the reader that they are not all equal. More than 20 years ago, Gee wrote Before the Backbone: Views on the Origin of the Vertebrates, which took a look at historical explanations for the origins of vertebrates. Which group of invertebrates is closest to us remains a topic of active research and Across the Bridge brings readers up to date with our current thinking.