Shubin uses the development of teeth to show two things – first, that a complicated structure like a tooth can come from a relatively simple method of folding together two tissues, and secondly, that this process is the same between many different body systems across many various animals. The feathers, teeth, or mammary glands that come from this process develop in birds, mammals, and reptiles, showing that these different species must have shared an ancestor in the past from which they all inherited this process.