There are four major categories of evidence for evolution fossil evidence, anatomical evidence, embryological evidence and biochemical evidence Let's look at fossil evidence first Fossils are often used as evidence for evolution. They're the traces or remains of dead organisms that lived long ago Sedimentary rock contains the most fossils and is formed from mud, sand, silt and other fine particles To make a fossil, first the animal needs to die and be quickly buried under the mud or silt so that no other critters will be able to break down the dead animal Over time, the minerals in the mud and silt will replace the tissue of the animal and will later reveal a mold or cast of the long-dead organism The law of superposition is that older layers are on the bottom and younger layers of rock are on top This allows us to relatively figure out the age of different organisms or fossils that we find in layers of rock So an organism found below another organism would be older than the one on top So letter z is older than letter x in this case Absolute dating places a numerical age range using radiometric dating, luminescence dating, dendrochronology and amino acid dating Some scientists prefer not to call it absolute but chronometric or calendar dating instead because the word absolute makes it sound much more certain and precise than it really is It's still an estimation, like the relative dating of rocks based on the law of superposition But it has a number to it For example, in dendrochronology scientists count the rings in a tree to estimate the number of years that have gone by Differences in growth through the seasons create a ring, and it generally takes one year, but there can be some variation Transitional species are intermediate species which may be in the fossil record or could be missing from the fossil record It's proposed that there would be evidence of species that were in the middle evolution steps from one species to another Archaeopteryx pictured here is used to provide a link between the theory that birds evolved from reptiles Now we're going to start looking at the anatomical evidence for evolution Homologous structures help scientists find relatedness among species There are anatomical structures in different species that originated by heredity from a structure in the most recent common ancestor of the species For example, if you look at the bone structure of a pterodactyl, a bat and a bird you'll see some basic similarities This suggests they had a common ancestor with a similar forearm structure perhaps very long ago but if you look at how they use their forearms for flight they're very different The Pterodactyl has one long finger that spans a large section of the wing. The bat has a wing between its fingers and the bird has feathers all along the forearm. These are analogous structures because even though they have closely related functions, they do not have a common ancestor for that function. Each of these species evolved their ability to fly from a different ancestral line Vestigial structures are another piece of evidence because the structures seem to serve no function, but resemble structures with functional roles in other organisms. An example that's commonly used is the human appendix The appendix was thought to be a shrunken remain of the cecum that can be found in many kinds of herbivores The idea is that it's a remnant of evolution yet to be truly removed In 2013 however, scientists refuted the relationship between cecum size and appendix presence The appendix is actually a housing complex for mutualistic bacteria. That aid in digestion for many species including humans Embryology is the study of the development of embryos. The more closely related species would be similar in their earliest stages of development These exaggerated drawings by Ernst Haeckel were used to show similarities between early stages of an embryo among different species including salamanders, pigs and humans. And while they're not truly accurate they were some of the first attempts at using embryology as evidence for evolution Today the stages are studied at the same point in their evolution unlike these images Lastly when we read DNA in amino acids to find a quantitative way to organize evolutionary ancestry, we're using a molecular clock Mutations happen at a steady rate and the more mutations there are mean a more distant relationship between the species Fewer mutations means a more recent common ancestor. So based on the amino acid differences between human hemoglobin and gorilla gemoglobin, gorillas are the closest relative to humans on this list And lampreys are the farthest. Thanks for watching this episode of Teacher's Pet. Don't forget to like and subscribe and follow me on Twitter @sciencepet