Upon arriving at the zoo, we immediately hit the concession stand for some popcorn, or maybe one of those giant roasted turkey drumsticks, because like any great form of entertainment, anatomy is best enjoyed with a snack. We proceed immediately to the primate enclosure, where some New World monkeys jealously eye what's left of our food. We notice that they share a lot in common with us—their faces, their little hands, the shape of their skulls—compared to a dog or a flamingo or a snake, we have a LOT in common with monkeys, and this is immediately evident when we see them.
Next, we move on to the reptile house, where we check out a giant tortoise. Unlike the monkey, the tortoise has a lot of features that we just can't relate to. Its skull is pretty small compared to the rest of its body; it has thick, scaly skin, and what's up with that shell?
Still holding the remains of our turkey drumstick (mostly just the bone now), an idea suddenly occurs to us: despite the differences between humans, monkeys, tortoises, and the unlucky turkey, there are some features we all have in common. For example, we all have two arms and two legs, skulls, vertebrae, and ribs. These features are called homologous structures.
We say that structures are homologous in two or more organisms when those structures came from a common ancestor. They might look really different because they have changed over time to be useful in different environments, but originally, they came from an ancestor we all shared. The bones that make up our arms make up a turkey's wings. The bones that we call ribs in humans, monkeys, and turkeys actually make up the tortoise's shell. That's right—from our last common ancestor, the same bones went to make up the rib cage in the human species and changed over time to provide shelter and protection to tortoises and their relatives.
Image from here. 1) Salamander, 2) Frog, 3) Turtle, 4) Aetosaurus, 5) Pleisiosaurus, 6) Ichthyosaurus, 7) Mesosaurus, 8) Duck.
If these structures all look so different due to millions of years of evolution in wildly different environments, how do we know they're homologous to each other? How can we be sure they came from a common ancestor? Let's use one of the lower leg bones (tibia) as an example. We can tell tibiae are homologous in humans, monkeys, tortoises, and turkeys because they always connect with the upper leg bone (femur) and with the bones of foot, and similar muscles connect them to other bones.
There are certain bumps and grooves on all the tibiae, and although those features might be a little different from tibia to tibia, they're there, and in roughly the same places. Finally, scientists who study development know that the tibia originates in the same way in all these different animals. All these lines of evidence support the conclusion that tibiae are homologous in humans, monkeys, tortoises, and turkeys—that is, a long time ago, a common ancestor had a tibia, and over time, evolution has modified the tibia for different animals in different environments.
Vestigial structures are another class of anatomical features that provide evidence for common ancestry and evolution. Vestigial structures serve no apparent purpose in species that possess them at present, but may have been important in their ancestors. For example, whales have pelvis (hip) bones, but they don't need them, since they don't walk; in other words, these hip bones are completely useless to modern whales. However, the fact that whales have hip bones even though they don't need them suggests that their ancestors might have walked on land. Fossil evidence shows this to be correct.
In sum, comparative anatomy shows how seemingly disparate kinds of organisms actually share many fundamental similarities, and strongly supports the notion that those similarities are derived from a common ancestor. The differences we see in modern organisms are the result of changes over time, as organisms adapt to their environment; in other words, evolution.