Dogs and humans both have a homologous trait, the dogs leg and a humans arm. Dogs legs are used for walking, where humans do not tend to use their arms for walking. Humans use their arms for all kinds of different things like carrying heavy things and just about anything you can think of since your arms and hands are almost one in the same, you can use them for writing, typing, art or endless possibilities. Dogs legs have the main purpose of walking. Humans can get down on all fours, but it is not as comfortable or natural as a dog. likewise for a dog, they can stand on two legs but its not very natural for them, that would be kind of cute though! however they do have almost the same structure and the same amount of bones as each other. A butterfly and a bat have analogous traits. Both of these creatures have wings and they both use them to fly. I do think a common ancestor could have had wings as well which were passed all the way down to these two very different species.