Fossil Links Whales Back to the Land

It was a strange decision for a mammal. To turn back, 300 million years after vertebrates first colonized land, and crawl back into the sea. Yet it’s exactly what the ancestors of whales did.

There’s never been any question that whales are mammals--they breathe air, are warmblooded and give live birth to young they nurse with milk. They even sport some hair, in the stiff whiskers around their mouths.

But just what was the whale’s last ancestor on land? It’s been one of the most vexing questions facing paleontologists--one they have been trying hard to answer for decades.

Now they think they have. In independent reports published late last week, two groups of paleontologists describe newly discovered fossil skeletons indicating whales are most closely related to the group of plant-eating animals that includes sheep, pigs, camels, cows, giraffes and hippos--the cloven-hoofed ungulates known as artiodactyls.

Some of the oldest of these fossil animals couldn’t look less like whales. They are wolf-like, hoofed creatures that lived on land and may have excelled at running.

One set of a large number of fossils called pakicetids, 50 million-year-old fully terrestrial whales, was unearthed by a group led by J.G.M. “Hans” Thewissen, an associate professor of anatomy at Northeastern Ohio Universities College of Medicine.

The other set, which includes the relatively complete skeletons of two 47 million-year-old amphibious whales called protocetids, was dug up by a group led by Philip D. Gingerich, a professor of paleontology at the University of Michigan, and several scientists from the Geological Survey of Pakistan.

Both samples were found in the low hills of northern Pakistan, in hard rocks that underlie what once was the warm, shallow Tethys Sea. The sea is thought to be the place where whales evolved from land animals 50 million years ago.

The finds are extraordinary because they capture a fleeting moment in geological time. The transition of whales from land to sea mammal occurred in a geological instant, lasting just 8 million years. The fossils capture a crucial point when the animals were undergoing an astounding physiological transformation.

The fossils discovered by Thewissen “superbly document the link between modern whales and their land-based forebears and should take their place among other famous ‘intermediates,’ such as the most primitive bird, Archaeopteryx , and the early hominid Australopithecus ,” according to Christian de Muizon, an expert on fossil whales and research director of the National Center for Scientific Research in Paris, who reviewed Thewissen’s work.

The steady stream of whale fossils that has been uncovered in the last two decades has greatly improved scientific understanding of these marine leviathans, said Lawrence G. Barnes, a whale fossil expert and curator of vertebrate paleontology at the Natural History Museum of Los Angeles County.

“We’re getting the big pieces here,” he said. “We can stack them up and tell the story pretty well.”

A famous comparative anatomist, William Henry Flower, reported in 1883 that modern whales might be related to artiodactyls because of similarities in blood vessels and nerves. Laboratory biologists, starting in the 1950s, used immunological and then genetic tests to suggest that whales descended from artiodactyls and were most closely related to modern hippos.

But those who studied bones found the concept difficult to accept. Instead, many paleontologists believed that whales had descended from a now-extinct group of carnivorous hoofed mammals called mesonychians that ranged widely in size but looked something like modern hyenas.

The basis for that theory was that the broad, triangular teeth of the mesonychians looked so similar to those of ancient whales.

Artiodactyls have one ankle bone, the astralagus, which is different from that found in other mammals. Artiodactyls’ astralagus has a trademark “double-pulley” which connects the bone to both the tibia and more distant parts of the ankle and makes it more flexible for running. It is so distinct that a paleontologist can identify it “in a second in the dark,” said Gingerich.

If the whales’ early ancestors were related to pigs, hippos and other artiodactyls, they would have similar ankle bones, paleontologists reasoned.

Gingerich devoted years of work to trying to locate whale fossils that would show what sort of ankle bones they and their ancestors possessed.

In 1991, he found a 40 million-year-old skeleton of a creature called Basilosaurus, an ancient whale that resembled a sea snake. But he was disappointed: the hind limbs of the Basilosaurus already had evolved to a point where they were useless for walking or for understanding the terrestrial origin of whales.

Until now, Gingerich had been hard pressed to find any limb bones in the pakicetids, the oldest known whales to live comfortably on land.

“I worked like a mad person in Pakistan for 10 years to find these hands and feet,” Gingerich said. “I say mad person because it was frustrating. We found many, many skeletons, but no hands or feet.”

Suspecting that sharks had carried off the hands and feet of dead whales while feeding, Gingerich this year began fossil hunting in an area that once contained shallow water.

“It was eureka. Like a candy store,” he said. “Four skeletons, all with hands and feet.”

The ankle bones had the trademark double-pulleys. They looked like bones from artiodactyls. And they were found with skulls that had ear bones known to be found in whales. It was enough to instantly rule out the idea that whales came from carnivorous mesonychians, an idea Gingerich had believed for years.

“No one was more surprised than me,” he said. “No one.”

Thewissen agreed that many paleontologists had been “barking up the wrong tree.”

A graduate student of Gingerich’s, Thewissen started his own laboratory in 1992. In 1994, he made a major discovery in Pakistan, unearthing a 47 million-year-old fossil called Ambulocetus natans , meaning the whale that walks and swims. This was a transitional creature that looked something like a crocodile, lived in shallow water and may have swum somewhat clumsily, he said.

His team started excavating bones in 1992 but did not bring back the last crucial bones until last year. When he found the ankle bones, he thought they came from artiodactyls.

But when he found only whale teeth in the jumbled fossil bed, he knew they had come from whales and that whales were closely related to artiodactyls.

“It’s highly lucky that this is documented by fossils,” Thewissen said. “This happened in only one place in the world.”

In addition to describing his fossils, Thewissen conducted a computer analysis on the samples to generate information on the larger picture of how whales are related to artiodactyls. His analysis suggests that whales are closely related to all artiodactyls and not to any specific animal in the group, such as the hippo.

“It’s as if they share a grandmother but not a mother,” Thewissen said.

His conclusions differ from the findings of some molecular biologists who say hippos are the closest relative to whales. They also differ from Gingerich, who finds evidence in the foot bones of hippos to suggest they are close kin to whales.

Many paleontologists who are aware of the new research, including Muizon and Kenneth Rose of Johns Hopkins School of Medicine, are skeptical of Gingerich’s conclusion that whales and hippos are so closely related.

The fossils that could answer the question could be lying in Pakistan. But neither scientist will get to them soon because of the threat of warfare in the region.

Thewissen says he may postpone a trip scheduled for late winter. A more fruitful place to search for missing links, he said, might be small museums in Europe where fossils from the region have been stored but not well studied. “It involves going to lots and lots of little museums and going through dusty drawers and sorting through fossils,” he said.

Gingerich, for his part, was scheduled to fly to Pakistan last Thursday but has postponed his trip. This is a familiar drill for him; he left the area the day before the Persian Gulf War started. “It’s just part of doing business in that part of the world. You just have to be flexible.”