Final answer:
The first whale ancestors with traits like a large powerful tail, reduced legs, and specialized hearing structures in the jaw are believed to be species such as Rodhocetus, which indicate transition from life on land to aquatic environments. The evolution of these traits is part of whales' adaptation to life in the water, and they share a common ancestry with even-toed ungulates, closely related to the hippopotamus.
Step-by-step explanation:
The first whale ancestor to exhibit the trait of a powerful tail, shortened legs, and a fat pad in the jaw for hearing is believed to be Rodhocetus, which had features compatible with a largely aquatic lifestyle. Fossil evidence suggests that modern whales evolved from land-dwelling ancestors; these early cetaceans underwent significant adaptations over time, transitioning from terrestrial to aquatic environments. For example, the fossil record indicates that Rodhocetus and similar ancestors possessed a type of ankle bone that linked them with land mammals. Over millions of years, whales have become more adapted to aquatic life, losing their hindlimbs and developing specialized anatomical features such as a large tail fluke and modifications in the jaw for hearing underwater.
It is important to note that cetaceans, which include both toothed and baleen whales, share a common ancestry with even-toed ungulates, and among these, the hippopotamus is the closest living relative to whales. This common ancestry is evident in phylogenetic trees that illustrate evolutionary relationships. These evolutionary changes, such as the development of a powerful tail and loss of legs, have enabled modern whales to thrive in aquatic environments. Understanding the evolution of these traits helps scientists trace the ancestry of whales back to their terrestrial origins.