I remember, as a child, seeing in a museum the skeletons of birds, bats and apes, and someone pointing out to me that they all had the same bones in their arms. It was the first time I grasped that we all had a common evolutionary ancestor, though at the time I hardly thought about it in those terms and probably just wanted to gawp at the dinosaurs.

But then someone showed me the redundant little hip bone in the skeleton of a dolphin. Why have they got hips when they don't even have hind fins? Because at one time in their evolutionary past, they did. They've just gradually lost them. At one time in their past, dolphins didn't live in the sea; they lived on the land and walked on four legs.

I remember just accepting this fact. It was only much later that I realized how powerful an idea it is. Anatomy can be used to communicate that we all — all living things — share an evolutionary past. We can use anatomy to look at when things evolved. So, for example, we can look at fossils of hands to find out when the thumb evolved (Answer: sometime in the monkey lineage).