Evolution is a theory that holds that all organisms living on earth today share a common ancestor. It is believed that specific changes or adaptations are established in species across generations to help them survive, reproduce and raise offspring. But how are we sure that these changes have occurred? Today, there is an abundant amount of scientific evidence suggesting that anatomical and physiological alterations have occurred in species that have led them to transform from aquatic to terrestrial animals, allowing them to survive different environmental conditions. Below are some gradual changes that an aquatic organism must go through to move on land and adapt to the new terrestrial habitat: the circulatory and especially the respiratory system must be improved, structural adaptations must be modified, the development of a skeletal system to protect the body and organs, and an adaptation of the senses must also take place. The evolution from water to land was a major event in vertebrates. The respiratory, circulatory and reproductive systems of species show how they evolved and adapted to the environment. Vertebrates that belong to the phylum Chordata had, at least at some point in their lives, a hollow dorsal nerve cord, a notochord, pharyngeal pouches, and a tail that extends beyond the anus. Fish are said to have been among the first vertebrates. The first fish to appear in the fossil record were jawless, covered with bony plates, gills, scales, but no fins. According to Arreola, they later evolved to adapt to their environment and began to form paired jaws and fins. The jaws were serviceable with muscles and teeth, which helped them eat a wider variety of food, and they were even able to defend themselves bi...... middle of paper ...... is a form of evidence showing how life has changed over the years and how species today are linked to a common ancestor. Paleontologists look for characteristics from one lifestyle to another. They create a tree of life that separates species according to their common structure. Fossils show how tetrapod legs evolved from fins, and gene studies show how mutation and natural selection led to the long limb bones of fins (Hoff). In this case, they discovered how a species that lives on water transitioned to a terrestrial lifestyle. Perhaps the descendants found the land an abundant food source and a refuge from competition. Over millions of years, through variation and selection, fins became legs, hind legs appeared, and bodies created fur and took on the familiar shape of the animals we have on earth. These species had to evolve and change the way