In Your Inner Fish: A Journey into the 3.5 Billion Year History of the Human Body, Neil Shubin writes about the importance of Hox genes in his research. Hox genes are like an instruction sheet for an embryo. They help organize the basic body layout of many organisms. In an effort to find genetic evolutionary evidence of human beings' ancestors, Shubin and his cohorts discovered that the genetic capability to build limbs was actually present in primitive fish. Shubin looked at Hox genes in paddlefish by inserting molecular markers and was able to show the same activity was present in tetrapod limbs. Tetrapods have a second stage of Hox gene expression where hands and feet form. This is significant because it links fish and tetrapods in the line of evolution as common ancestors.