One piece of evidence for evolution is the fossil records. Fossils are the remains of life preserved in the rocks. And since different layers of rock are formed during different times, scientists can compare the fossils from one time period to other time periods and to life today. By studying the fossils, they can document how life has changed over the millions of years that Earth has had life on it. Over time, the number of discovered fossils has grown and will continue to grow. Scientists can use these fossils to document the intermediate stages of evolution as one species evolves to another. And by discovering new fossils, the gaps in evolution continue to close. This provides support to the theory of evolution because the fossils show how organisms have evolved over time. Another piece of evidence is the geographic distribution of living species. Species that are living on different continents, had each descended from different ancestors. But because many of the animals were living under similar conditions, on separate continents, they experienced the same pressures of natural selection, so they evolved to have very similar features. This supports evolution because it proves that animals that are not related, will evolve to be similar based on their environment. Homologous body structures is another piece of evidence for evolution. Scientists have discovered very similar body structures on vertebrates. For example, the wings, legs, hands and flippers of animals all have very different functions, but they are all made up of the same bone structure. Researchers are able to group animals based on their similarities and differences in homologous structures and then determine how recently they shared a common ancestor. Some structures have evolved so they are just vestigial organs. They have been reduced in size and no longer serve a purpose. An example is the appendix of a human. It has been reduced in size and no longer has a purpose in the digestive system. This supports evolution because it shows that vertebrates once shared a common ancestor because they have very similar body structures. There are also many similarities in embryology. The early stages of development for many vertebrates are very similar. The same groups of cells develop in similar patterns, to produce the organs and tissues of all animals with backbones. These cells grow in similar ways and produce the homologous structures. It helps prove evolution because it shows how organisms start out very similar, then evolve from there. Also very different animals share very similar genes. For example, humans and yeasts share a gene that codes for the protein myosin. DNA evidence also shows evidence of evolution. Studying the DNA of species shows the relationships of evolution and how species have changed. The more similar the DNA, the more recently they shared a common ancestor. The more different the DNA, the more they have diverged from one another. The last piece of evidence for evolution is molecular clocks. A molecular clock uses the comparisons of DNA to estimate the amount of time that species have been evolving independently. Mutations build up in the DNA of organisms at similar rates, so the extent of difference between the DNA is how long ago they shared a common ancestor.