A team of international scientists, including Hans Thewissen, an anatomist and paleontologist at the Northeastern Ohio Universities College of Medicine (NEOUCOM), has discovered that the inner ear of whales evolved much more quickly than expected, allowing the animals to become fully aquatic early in their evolution. The team’s research, funded by the National Science Foundation (NSF), shows that the semicircular canals, the organ responsible for balance and located in the inner ear, was adapted to aquatic life approximately 45 million years ago. The discovery will be published in the May 9 issue of the journal Nature.
Cetaceans (whales, dolphins and porpoises) have unique semicircular canals that allow them to be highly acrobatic swimmers without becoming dizzy. By investigating this organ in ancient fossils, the researchers found that early whales acquired this special trait quickly and early on in their evolution. This was a defining event that likely resulted in their total independence of life on land.
“The early evolutionary development of small semicircular canals by cetaceans opened an entirely new mammalian niche for habitation and contributed to the broad diversity of marine living habits that we see in whales today,” said Rich Lane, director of NSF’s paleontology program, which funded the research. “The evolutionary acquisition of such specialized organs or abilities (like the brain and upright walking habit of man) provide mechanisms by which highly evolved organisms dominate in certain environments.”
The semicircular canals sense head movements and this vital information is used to coordinate the body during locomotion. This happens subconsciously, and humans only become aware of an organ of balance when things go wrong, such as during sea sickness, drunkenness and wild roller coaster rides.
The researchers found that in living cetaceans the semicircular canals are much smaller than in any other mammal of the same body size. In fact, the semicircular canals of the huge blue whale are smaller than those of humans. In general, cetaceans are more acrobatic than similarly sized land animals (imagine an elephant making the jumps of a similar-sized whale). This could be the result of the small canals, because the small size makes the canals less sensitive, preventing the animal from becoming dizzy (i.e. experiencing vertigo).
Between 40 and 50 million years ago, early cetaceans evolved from land mammals into swimmers. Over the last decade many new whale fossils have been found which show how this dramatic change happened. Skeletons of the transitional species (“whales with limbs”) demonstrate that they were adept, otter-like swimmers.
Just last year, Thewissen’s discovery of two early whale ancestors showed that the earliest whales were mainly land animals, and resolved a controversy over the relationships of whales, showing that they are most closely related to modern even-toed ungulates (such as pigs, hippos, camels, deer, and sheep) than to an extinct group of meat-eating mammals.
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