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Thursday, Sept. 20, 2001
Almost like a hippo
By ROWAN HOOPER
In "The Origin of Species," Darwin describes how black bears in North America often swim "for hours with widely open mouth, thus catching, almost like a whale, insects in the water." Darwin was making a hypothetical point about how evolution might work -- the swimming bear, he suggested, might be the first step in the evolution of whales.
As it happens, although the idea that whales evolved from land animals was a typically farsighted one, it was one of the surprisingly few things that Darwin was wrong about -- whales didn't evolve from bears. This week the story of whale evolution, one of the most remarkable and informative in all of biology, has been definitively told.
After millions of years of gradual adaptation, vertebrates left the oceans. In the millions of years that followed, they lost fishlike features and became specialized for terrestrial life. Some of the most radical changes occurred in mammals, which evolved to become hairy and warmblooded, delivering live young, feeding them with milk and often living in social groups.
But then a group of mammals returned to the oceans. These creatures -- cetaceans -- had to shed their terrestrial adaptations and regain fishlike features: become streamlined and hairless, develop flippers and so on.
Despite the huge transition, cetaceans retained some features of their terrestrial lives: certain bones in the ears, and cartilaginous stumps of hind limbs that appear and disappear during embryological development.
Such features are examples of what Darwin called "descent with modification": evidence of evolution from a common ancestor. They pose a question that greatly disturbed Darwin's contemporaries (and still troubles Christians today): If whales can evolve from terrestrial animals, then why can't humans have evolved from apes?
Paleontologists once thought that, because of the similarities in their teeth, whales evolved from a group of now-extinct animals called mesonychids -- carnivorous beasts that looked like hyenas with hooves. But molecular evidence suggested that whales are more closely related to artiodactyls -- hoofed animals that include sheep, cows, pigs and hippos.
The key to proving this were the ankle bones. Artiodactyls have ankles unlike any other living or extinct group of animals, so finding an ancient whalelike fossil that still had its legs would answer the question. Unfortunately, all fossils uncovered were missing the hand and foot bones, perhaps because ancient sharks scavenged those parts before the carcass was fossilized.
However, two independent groups of researchers recently found what they were looking for in Pakistan, where, along with north India, whales are thought to have evolved. Both today published studies based on their findings, proving at last that cetaceans did evolve from artiodactyls.
Paleontologist Philip Gingerich of the University of Michigan headed one of the groups. After a 10-year search, he and colleagues from the Geological Survey of Pakistan found a 47-million-year-old fossilized whale ankle bone. The finding showed that the early cetaceans, named Rodhocetus, were indeed semi-aquatic, with sheeplike ankle bones and whalelike skulls.
"These animals could hitch their way out of water and back in like sea lions do today," said Gingerich, whose work is published today in Science, "but they were more aquatic than I realized."
Hans Thewissen of Northeastern Ohio Universities College of Medicine and his team of paleontologists found some even older cetaceans, and their work is reported in the rival science journal Nature. The new mammalian fossils -- a fox-size and a wolf-size species -- date from the early Eocene Era, some 54 million years ago. Back then, cetaceans were fully terrestrial. Yet despite having features that are clearly terrestrial, such as long, spindly legs, the animals have ear bones that are unique to cetaceans.
"The two papers agree that whales are much more closely related to artiodactyls than to mesonychians," said Thewissen.
Furthermore, reconstructions of Gingerich's fossils suggest they had webbed hands and feet and probably also used their tails to propel themselves through the water: true "missing links" between terrestrial grazers and aquatic whales.
"This is a major discovery," said Christian de Muizon, at the Museum National d'Histoire Naturelle, Paris, in a telephone interview. "The fossils will take their place with Archeopteryx as famous 'missing links.' For the first time, morphological data has demonstrated that cetaceans are closely related to artiodactyls."
Gingerich was, for a long time, opposed to the artiodactyl hypothesis, but the new fossils changed his mind. "Now I even admit the possibility that hippos are a side line of artiodactyls that might be closer to the whales than any other living animals," he said.
Gingerich might have been more sympathetic to the idea that whales are related to hippos if he had seen the creature found in 1919, off Vancouver Island. Whalers caught a humpback whale that showed the vestiges of evolution: It had two, 1.2-meter protrusions on either side of its tail. Due to some freak of development, ancient genes, remnants from ancestors that grazed on dry land, had got turned on, and the whale had grown hind limbs.
Like the new fossils, it was compelling evidence for Darwinian evolution: almost like a whale . . . if not quite like a hippo.
E-mail Rowan Hooper at firstname.lastname@example.org