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August 31, 1999

Long Dismissed as Piglike, Hippo Gains a Nobler Cousin, the Whale

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    Imagine having always been told that your closest relative is a pig, then waking up one morning to find that no, your nearest cousin is a whale.

    This decisive change in social standing has recently befallen the hippopotamus, which will doubtless receive new respect everywhere but in Kansas, where the Board of Education discourages any thought that different species might be related to one another through evolution.

    Outside of Kansas, the whale is obviously a mammal that returned to the sea, but determining its closest terrestrial relatives is hard because its form has been so extensively modified. It has lost its hind limbs altogether and its forelimbs have morphed into flippers.

    One might suppose that the easiest way for a land animal to become a whale would be to turn first into a hippopotamus. Hippos are almost halfway to whales, as they are hairless, nurse their infants underwater and communicate by underwater sound.

    But mammalogists long thought otherwise. The hippo's nearest relatives were the pig and the peccary, they declared. Enormous controversy greeted the suggestion in 1985 by Vincent Sarich, a pioneer of molecular-based evolution, that the hippo's closest cousin was the whale.

    Now that DNA is so easy to sequence, molecular evolutionists have been refining the family trees drawn up the old-fashioned way, on the basis of an animal's outward appearance. Several recent DNA-based reconstructions of the even-toed ungulates, the order that includes camels, giraffes, pigs, hippos and whales, have confirmed Dr. Sarich's original suggestion.

    The latest of these studies, published in today's issue of Proceedings of the National Academy of Sciences, is based on the viruslike elements that have copied themselves throughout the genomes of many species. The DNA of these extinct viruses help mark forks on a family tree because they are either present or absent at a given position in a species' DNA.

    The authors, led by Dr. Norihiro Okada of the Tokyo Institute of Technology, believe the extinct virus method is less ambiguous than other DNA-based methods of reconstructing family trees. It is so good, Dr. Okada has written, that one can dispense with the usual statistical analysis.

    In a commentary, Dr. David M. Hillis, a molecular evolutionist at the University of Texas at Austin, said the extinct viruses, known to biologists as SINE and LINE insertions, were good but not perfect markers of evolution, and that dispensing with statistical analysis was "highly inadvisable."

    Despite disagreements over methods, molecular evolutionists are now at one on the whale's family tree. The story goes like this. First there was an ancestral even-toed ungulate. Then the family tree split between camels and all the rest. Next the pigs and peccaries split off, followed by giraffes and deer, leaving just the ancestor of all hippos. Romping in the water, some hippos ventured into the ocean. These seafaring hippos then branched into the two superfamilies of the baleen whales (finbacks, blue whales) and the toothed whales (dolphins, porpoises).

    But don't tell it in Topeka.

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