Whale of a change for cetacean history.

A new study of DNA sequences has badly shaken the standard family tree of cetaceans, suggesting intriguing new twists in whales' evolutionary history.

Taxonomists divide living whales and dolphins into two suborders: the Odontocetes, which have teeth, and the Mysticeres, whose mouths have banks of comb-like baleen used to filter small fish and crustaceans from the water. Odontocetes, which include the sperm whale, can track prey through the remarkable evolutionary adaptation of "echolocation" -- a sonar system that emits sounds and senses the echoes bouncing off objects. Mysticetes, such as the blue whale, lack the ability to echolocate.

Paleontologists have long thought that Mysticetes and Odontocetes represent separate evolutionary lines that split from a group of ancestral toothed whales some 40 million years ago. But Michel C. Milinkovitch of Yale University and his colleagues uncovered a different story by studying the genes of 16 species of living whales, a cow, a sloth, and a human. Because natural mutations over the millennia change DNA sequences, scientists can use these sequences to sort out which animals are most closely related to which.

Unexpectedly, Milinkovitch and his co-workers found that sperm whales did not group with the other Odontocetes. Rather, this particular toothed whale seems more closely related to the baleen whales, despite noticeable outward differences, they report in the Jan. 28 NATURE.

The new findings suggest that Odontocetes and Mysticetes are not true evolutionary lines. They also indicate that baleen whales lost the ability to echolocate, whereas their close cousin, the sperm whale, retained it. Milinkovitch explains that baleen whales may have dropped their sonar because the evolutionary changes involved in developing the huge baleen structures radically changed the shape of their skulls.