Taking an unconventional approach, the investigators looked at tiny but telling parts of the whale ancestors of 40 million to 50 million years ago to get the big picture.
The two-year, three-dimensional analysis suggested a quick and early acquisition by the prehistoric beasts of the organ of balance -- called semicircular canal -- that permits whales, dolphins and porpoises to perform swimming acrobatics without getting dizzy.
The findings will be published Thursday in the British journal Nature.
"The semicircular canal work now suggests this (move to a wholly aquatic existence) was a rapid -- 8-million-year -- transition," Steve Webster, marine biologist with the Monterey Bay Aquarium in Monterey, Calif., told United Press International. "Faster than thought, and that's the interesting, surprising part."
In the unique study, paleontologists from England, India, Pakistan and the United States employed micro-CT -- a technique that with the aid of computers generates a 3-D picture of an object, a "slice" at a time -- to peer inside the organs preserved in the fossilized remains of four whale ancestors:
--Ichthyolestes, resembling a coyote in size and looks, dwelling in or near shallow rivers in what is now Pakistan some 50 million years ago;
--Remingtonocetus, sized like a sea lion, shaped like a snake with a long snout and large limbs, living along a swampy coast of what is now India some 43 million to 46 million years ago;
--Indocetus, a contemporary and neighbor of Remingtonocetus, though not its look-alike, sporting a short body and stout limbs reminiscent of a sea lion;
--Dorudon, a dolphin-like creature favoring shallow seas during its existence some 40 million years ago. First of the four fossils to be found, this remnant of past life was unburied in Egypt at the beginning of the century.
"While recently discovered cetacean fossils show a gradual change from a terrestrial to partly-aquatic, and later a fully-aquatic lifestyle, this study clearly shows that the change in the inner ear in relation to balance and orientation happened very quickly," said Douglas Long, acting chairman of the Department of Ornithology and Mammalogy at the California Academy of Sciences in San Francisco.
"Life on land and life in the water are so vastly different that some sort of transitional ear would not be adequate for either lifestyle. Thus, success as a fully-aquatic organism relied heavily on the necessary adaptations for such an existence, and this was a strong selective pressure," Long told United Press International.
The CT images indicate the structures were among the first whale organs to take on their modern shape.
"It happened later than 50 million years (ago), but earlier than 43 million to 46 million years," said study co-author Hans Thewissen of the Department of Anatomy at Northeastern Ohio Universities College of Medicine in Rootstown, Ohio, whose team collected three of the four specimens.
"Most other organs did not reach modern proportions until 40 million years -- locomotor organs -- or later, around 30 million years -- feeding apparatus," Thewissen added.
Researchers theorize the reason was purely practical.
"We speculate ... whales needed a stiff neck early on, because they had a long snout and water would push the snout sideways while swimming fast; a stiff neck would counteract that," Thewissen told UPI. "The semicircular canals in land mammals provide sensory feedback by acting on the neck muscles. With the neck muscles reduced and mobility limited, the semicircular canals would not be very useful."
The findings come from a rare analysis of the semicircular canal. Difficult to study, minuscule in size, encased in thick bone, the organ of balance has escaped the kind of scientific attention the ear's hearing component has long received.
"The approach of looking at the semi-circular canals of the inner ear to investigate the locomotion of animals in the past is relatively new, and has previously only been applied in primate and human evolution," lead study author Fred Spoor of the Department of Anatomy and Developmental Biology at University College London in England told UPI. "This is the first time it will be applied outside paleo-anthropology, and it will come as an eye-opener to many mammalian paleontologists."
"The modern technology of CT can help paleontologists gain crucial insight into the anatomy of the skull in ancient fossil whales!" Zhe-Xi Luo, associate curator of vertebrate paleontology at the Carnegie Museum of Natural History in Pittsburgh, Pa., told UPI.
The semicircular canal in whales is proportionally about a third the size of similar organs in other animals.
"Most acrobatic land mammals have very large canals because the canals help sensory feedback necessary for the animal to land in just the right place on a branch or the ground," Thewissen said. "This is probably less critical in an acrobatic whale/dolphin, because they are not so constrained ... in water. For them, it makes sense then to make the canals smaller, so as to not overstimulate them."
As cetaceans evolved from land-dwelling quadrupeds into swimmers, the canal system shrank quickly, the study's authors suggest.
"Dedicated agile swimming of cetaceans thus appeared to have originated as a rapid and fundamental shift in locomotion rather than the gradual transition" that fossils of larger body parts have suggested, they wrote.
At least one researcher questioned the conclusions, however.
"The data could be interpreted in several different ways, aside from the way the authors interpreted the data," John Gatesy of the University of California, Riverside, told UPI.
"I am pleasantly surprised that their anatomical models are so well produced that they leave few questions," he told UPI. "Often in paleontological studies, the fragmentary nature of fossils leaves many unanswered questions, but the completeness of their 'specimens' only underscores their argument."
The study supports basic theories of evolution, Long said.
"Here is a classic case of an organism faced with a suite of selective pressures as it exploits a new environment, and how successful -- and successive -- changes over time allow the organism to thrive," he told UPI.