Some groupings are so strong that, for all intents and purposes, we consider them fact. The fish–turtle–polar bear–human grouping, for example, is supported by characteristics from hundreds of genes and virtually all features of the anatomy, physiology, and cellular biology of these animals. Our fish-to-human framework is so strongly supported that we no longer try to marshal evidence for it—doing so would be like dropping a ball fifty times to test the theory of gravity.
