Shubin and his team noticed that, like the human limb skeletal pattern, Tiktaalik's fin has a joint at the end which connects to other bones. Tiktaalik appeared to be the first creature with a wrist bone, a vital characteristic of human anatomy. Like humans, Tiktaalik's wrist bone moved in conjunction with the elbow. Tiktaalik's bone showed that it was a primitive ancestor of the 1-2 bone limb tendency. Tiktaalik's appendage also seemed to be a cross between a fin and limb. The fin investigation further supported the idea that Tiktaalik was an intermediary animal. With a shoulder, elbow, and wrist, Tiktaalik was capable of performing many functions associated with humans today, such as a push-up. The wrist was able to lie flat against the ground in Tiktaalik. Tiktaalik's flat structure suggests that likely, in the predator waters filled with fish, Tiktaalik could easily delve deep and swim along the bottom of streams and ponds to increase its fitness. Tiktaalik's strong chest muscles also suggest that it could go onto the mudflats of the bank to avoid predators. Thus, Tiktaalik's mutation increased its fitness and was successfully passed onto its descendants such as humans. During embryological development, the hedgehog gene helps facilitate specialization, growth, and formation. In the fruit fly, the hedgehog gene ensured that one end of the fly's body segment differed completely in look from the other. Because the hedgehog gene was first discovered in flies, it was given this name because in those animals that had a mutation in this DNA sequence, bristle like structures appeared. The hedgehog gene, within every living organism, can be found in the ZPA tissue. When vitamin A is injected into the chick, the hedgehog gene becomes active on both sides of the limb. By remaining active and inactive at the appropriate times, the hedgehog gene controls the making and shaping of limbs. Thus, the hedgehog gene accounts for the different shapes and sizes of those bones such as fingers. Even in a shark, the fin, or main appendage, houses the hedgehog gene and it becomes active in both sides when injected. During development, two tissue layers interact to create teeth. The outer and inner cell layers create a fold that secretes the molecules and prompts the organ formation. A similar and nearly identical process occurs during the formation of scales, feathers, and breasts; the two tissue layers secrete to form these new organs. Because all reptiles and mammals had teeth before they had scales, feathers, or breasts, scientists discovered that the tooth formation process was modified and changed to create these new, specialized organs. The tooth formation process was repurposed and, because it increased fitness, was passed onto new populations and organisms. Thus, if we did not have teeth, these process would never have modified to create these other organs. Though an invertebrate, the amphioxus shows many similarities to vertebrate animals. Like a vertebrate, such as fish, amphibians, and mammals, an Amphioxus has a nerve cord running along the length of its back along with a notochord, or rod, that runs the length of its body, lying parallel to the nerve cord. Within the notochord, a jelly fluid provides stability and support. During human development, a notochord is also present, but it dissolves and integrates itself into the disk between our vertebrate. Also like humans, Amphioxus have gill arches supported by cartilage. Because the human head and back structure share so many similarities with the Amphioxus, the structure of the human head was likely restructured or repurposed. In the 1800s, embryonic experimenters discovered that the same structures, such as lungs and tissues, emanated from one of three layers within the body. While reptiles and mammals may be vastly different in size, shape, and diet, as embryos, all organisms experience the same stages of development. The three germ layers remain consist in all animals. The outer layer, the ectoderm, is responsible for forming the outer skin and the nervous system. The inner germ layer, the endoderm, has the inner body structure forming form it such as the digestive tract and the glands associated with these tracts. The tissue between the guts and skin of our skeleton and muscles form from the middle layer, the mesoderm. In all organisms, organs consistently form from the same germ layer. Mammals have three inner ear bones: the malleus, the incus, and the stapes. These ear bones originate from gill arches. While the stapes comes from the second arch, the malleus and incus come from the first arch. Yet, reptiles do not have these three inner ear bones; they only have one. During research, scientists discovered that two ear bones of mammals correlated to the jaw bone pieces of reptiles. The malleus and incus originate from the first gill arch, but so do parts of the reptile jaw. Also, reptiles only have one bone in their middle ear, but this is the same as the stapes of mammals as both originate form the second arch. What is called hyomandibula in a reptile is equivalent to the stapes of a mammal. Fossil evidence indicates that the malleus and incus were once part of the reptile jaw, but, undergoing shrinkage, they moved into the ear. Thus, fossil evidence suggests that the jaw bones were repurposed to create the class mammalia. Likely, this mutation occurred during the transition to land as hearing on land requires different structures than does hearing in water.