Comparative anatomy. This is where scientists compare what we see in different species and assume the more similar they are, the more closely related they are. Now look at the following few images. What do they have in common? A cat's paw. A whale's flipper. It's not a fan. A horse's hoof. And finally human hands. What do these have in common? Now here is a clue. It's all about what is inside. All of these have the bones for five limbs. Scientists call this the pentadactyl limb system. Now scientists look for related factors in unrelated organisms and try to juice common ancestors and work out evolutionary relationships. There is however a huge problem. Is the feature we look at homologists due to a common ancestor or anologist simply due to natural selection. The problem is scientists cannot tell which is which. So they try to choose features that do not confer any evolutionary advantage per year like there are any. Camouflage or colour would be a terrible example. It is believed that bone structure is a better example. All of these are thought to come from a common ancestor is they have the same overall bone structure. The differences are thought to be due to natural selection causing the sizes of individual bones to change. Some scientists dismiss this work as we cannot prove similarities due to a common ancestor and not just due to natural selection. I take the view that comparative anatomy is simply another jigsaw piece in the puzzle that is evolution. Embryos are another thing we can compare. Scientists have noticed that embryos all share a common pattern of development. Along with the embryos they are similar, the more recently scientists think they were related. This image if you look at it carefully suggests there are three distinct groups of more closely related organisms. The first group would be fish and salamanders. They have a very similar pattern of development. The second group would be tortoise and chicken. They again have a similar pattern of development. The final group would be the pigs, cows, rabbits, humans or what we call mammals. This suggests they have a common pattern of development and are more closely related. A good scientist would not trust this evidence alone. This evidence does support other pieces of evidence that we learn about in this course for the sequence of evolution on our planet.