Shubin has walked through a few of the shared traits that support the tree that flows from fish to humans, including limbs in Chapter 2, odor genes in Chapter 8, and eyes in Chapter 9. The family tree and animal observation itself form a circular feedback loop, as scientists use and observation to build the most likely tree based on available evidence, then use that tree to make predictions, check to see if those predictions hold true in nature, and tweak the tree as new observations come out of these predictions.