August 31, 1999
Long Dismissed as Piglike, Hippo Gains a Nobler Cousin, the Whale
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By NICHOLAS WADE
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.
