Whales, dolphins and porpoises are the rulers of the open ocean. But it wasn't always so. Some 50 million years ago their ancestors were terrestrial beasts, scarcely more aquatic than dogs. When did cetaceans evolve the features necessary to take the plunge? Fossils discovered over the past two decades have documented the emergence of a number of their skeletal shifts toward seaworthiness. Now researchers have determined when these creatures acquired another key trait--an organ of balance (formed in part by the inner ear's so-called semicircular canals) suited to their active aquatic lifestyle. The findings, detailed today in the journal Nature, reveal that the transition was speedier than previously thought.

The semicircular canal system senses head movements and helps to coordinate the body during locomotion. As it turns out, living cetaceans have semicircular canals that are three times smaller, relative to body size, than those of land-dwelling mammals. The canals' small size, say Fred Spoor of University College London, Hans Thewissen of the Northeastern Ohio Universities College of Medicine and their colleagues, presumably reduces their sensitivity to the animals' acrobatic swimming and hence prevents vertigo. To figure out when the cetacean canals attained their diminutive stature, the team examined the inner ears of fossil whales using computed tomography. It determined that the small semicircular canals characteristic of extant cetaceans evolved a mere five million years after the first whales walked the earth. [In the image above, from left to right, inner ears of an early land-dwelling whale, an early marine whale, and a modern whale are shown.]

That early, sudden change contrasts sharply with the gradual morphing seen in the postcranial skeleton, which continued to take shape some eight million years after the canals shrank. Yet once the canals became small, they would have been "incompatible with terrestrial competence," the authors assert. Thus, although cetaceans retained well-developed legs for millions of years, which previously seemed indicative of a prolonged amphibious evolutionary stage, terrestrial locomotion beyond a cautious crawl along the beach would have been out of the question once this highly specialized canal system evolved. Indeed, the researchers conclude, that sensory change marked a "key 'point of no return' event in early cetacean evolution, leading to full independence from life on land."