From Moby Dick to Shamu, whales have long fascinated humans. Their remarkable status as ocean-dwelling mammals, along with dolphins and porpoises, makes them at once related to us and yet inconceivably different from us. Thus their evolution--the developmental steps required to leave solid ground for a life in the water--has long fascinated scientists, including Kenneth Rose of the Program for Functional Anatomy and Evolution at the School of Medicine.
In the Sept. 21 issue of Science, Rose provides outside perspective on a report in the same issue that fills an important gap in scientists' understanding of whale evolution. The report by others documents new fossils of very primitive whales that were found in Pakistan.
The new fossils, Rose says, have well-developed limbs and are the first to have well-enough preserved ankle bones to allow scientists to change their conclusions about which animals are the closest relatives of these 50-million-year-old primitive whales. The importance of this finding, and a similar report about even older fossils that appears in the Sept. 20 issue of Nature, is that it unifies the scientific perspective on whale evolution, Rose says.
"What's most important about these new fossils is that they contain well-preserved ankles, the most important region morphologically for establishing evolutionary relationships as early whales were in the process of becoming aquatic," Rose says. "Previously, only fragments have been known of these species, and the few ankle bones that some had identified were not widely accepted among scientists because of the condition of the fossils and their lack of direct association with whale skulls."
There are two main ways scientists determine evolutionary relationships among various species, Rose says. Some scientists consider physical characteristics of specimens--the morphology--and look for similarities or differences, the presence and relative complexity of various systems and other measures that help create connections in the evolutionary tree. In the last two decades, other scientists have focused on molecular examinations of species, determining the extent to which two species' genetic information shares common features, through studying both nuclear DNA and DNA found in tiny cellular structures called mitochondria.
Until the new fossils were found, morphology and genetics gave different answers to the question of which are whales' closest land-dwelling relatives, thanks in part to the limited information about the primitive whales' ankles, Rose says.
Morphologic analysis of the fossil ankles, which Rose conducted independently for his perspective article, shows that these primitive whales (referred to scientifically as the cetaceans) are closely related to "even-toed ungulates" (an evolutionary order known as the artiodactyls) such as sheep, pigs, cows, camels, deer and hippos.
"There has been no question about whales' relationship to hoofed mammals; the question was, to which group of hoofed mammals?" Rose says. "Prior to having the ankle information, morphologically early whales appeared to be the sister group of mesonychians, an extinct group of land-dwelling carnivores. The new fossils provide compelling evidence that whales are not just related to but descended from artiodactyls rather than mesonychians, bringing the morphological evidence into accord with the molecular data."
While the agreement of morphology with molecular analysis is welcome and affirmative of both methodologies, Rose cautions that there are problems created by the conclusions. Among the outstanding conundrums, he says: The close relationship between cetaceans and hippos appears to require the breakup of groups that have been thought for years to be closely related.
Rose has published in the past on comparative anatomy of both artiodactyls and mesonychians, but his current work focuses on comparative and functional anatomy of fossils and existing animals to help interpret the probable behavior of extinct mammals. He was invited by the editors of Science to write the perspective on the new fossil discoveries.