97. Read the following passage, then answer the question below.

The ancestors of whales did not live in the sea. About fifty million years ago, in what is now Pakistan, there lived a wolf-sized, hoofed mammal called Pakicetus. Its skeleton looks nothing like that of a modern whale, yet its skull contains a thin bone in the ear that is found otherwise only in whales. That small detail convinced paleontologists that Pakicetus belonged to the whale lineage. Within a few million years, descendants such as Ambulocetus, the 'walking whale,' had developed powerful tails and large feet suited to paddling, and they probably ambushed prey at river mouths much as crocodiles do today. By about forty million years ago, Basilosaurus, a serpentine animal more than fifteen meters long, had vestigial hind limbs too small to support its body, a sign that its life on land was long over.

Fossils tell only part of the story. Until the 1990s, many researchers assumed that whales were most closely related to an extinct group of meat-eating hoofed mammals. Genetic analysis then showed a surprising result: the closest living relatives of whales are hippopotamuses. The two groups probably shared a semi-aquatic ancestor, and later fossil finds of ankle bones that resembled those of even-toed ungulates supported the genetic evidence.

The transition also changed how whales breathe, hear, and feed. The nostrils gradually migrated to the top of the skull, becoming the blowhole, which allows a whale to breathe while most of its body remains submerged. Toothed whales evolved echolocation, whereas baleen whales replaced teeth with fringed plates that strain small animals from the water. Modern whales still carry traces of their history: tiny pelvic bones buried in the flesh, which no longer connect to any legs. Such leftover structures, biologists say, are among the most persuasive evidence that evolution proceeds by modifying existing features rather than designing new ones from scratch.

The phrase 'such leftover structures' in paragraph 3 refers to