>>96204130You can easily see the lineage of a mammal by looking at their skeleton. Their external body shape may evolve over time, but the structures that make up their skeleton as well as the presence of many organs are the same as ours. An easy example is the hands, mammals will generally have 5 finger bones even if they have lost their fingers and contorted them into fins. Humans have a tailbone for the same reason. The major indicator of a mammal is the presence of hair, which dolphins have a little bit of on their beaks, although adults go bald and just have hair follicles. Whether they have hair isn't really the point, but rather that their skin is still structured the same as ours, suggesting it evolved to its current form from a mammalian ancestor that we also share.
