Final answer:
The first whale ancestor with tail flukes, vestigial hind legs, and an altered nasal structure was likely through forms such as Pakicetus and Rodhocetus. These early ancestors are part of a fossil lineage showing gradual aquatic adaptations in cetaceans from terrestrial origins.
Step-by-step explanation:
The first whale ancestor that exhibited traits such as tail flukes, very small hind legs, and a nasally shifted blowhole was likely a member of early cetacean forms such as Pakicetus and Rodhocetus. Evidence suggests that modern whales are descendants of terrestrial mammals, based on various vestigial structures, notably the pelvic bones that were once attached to hind legs. These rudimentary legs no longer serve their original purpose, much like the flippers of penguins have evolved to adapt to an aquatic environment rather than flying. The upshot is that there is a rich fossil record, which shows a transition from land-dwelling creatures to aquatic life. The morphology of cetaceans gradually changed over time to adapt to life in the water. For instance, Rodhocetus, which was highly aquatic, had a type of ankle bone unique to terrestrial mammals, implying a land-based origin. The transition of the nasal openings in these ancestors towards the top of the head to form a blowhole is another characteristic adaptation to aquatic life.