Closely related organisms often share similar structures in their bodies, despite their different lifestyles. The simplest explanation for such similarities is that both species descended from a common ancestor with similar features. This is sometimes verified by the fossil record.
Anatomical similarities between different groups of organisms, some of which live in different environments from the others, strongly support the theory of common descent. Dolphins, for example, live like large predatory fish, but their resemblance to fish is superficial. Like all placental mammals, dolphins have lungs and simple jawbones and gestate their young internally. The simplest explanation for these similarities with dogs, cats and primates is that dolphins are more closely related to other mammals than they are to the fish they live among. Another example of analogous anatomy across species is the tetrapod limb. The basic form of this structure is a single long bone attached to a shoulder at one end and an elbow at the other. Below the elbow, two smaller bones extend to a wrist, which terminates in small bones for fingers and toes. This structure is universal among mammals, birds, reptiles and amphibians, but unknown among invertebrates. The most parsimonious explanation of this fact is that all tetrapods descended from animals with similar limbs but that insects did not.