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On Science: Fossil record reveals a whale of a tale

This article first appeared in the St. Louis Beacon, Feb. 18, 2009 - Moby Dick, the white whale hunted by Capt. Ahab in Melville's novel, was a sperm whale. One of the ocean's great predators, a large sperm whale is a voracious meat-eater that may span more than 60 feet and weigh 50 tons.

A sperm whale is not a fish. Unlike the great white shark in Jaws, a whale has hairs (not many), and a female whale has milk-producing mammary glands with which it feeds its young. A sperm whale is a mammal, just as you are!

This raises an interesting question. If Darwin is right about the fossil record reflecting life's evolutionary past, then fossils tell us mammals evolved from reptiles on land at about the time of the dinosaurs. How did they end up back in the water? As Darwin's 200th birthday passes, it seems fitting to revisit this question.

The evolutionary history of whales has long fascinated biologists, but only in recent years have fossils been discovered that have revealed the answer to this intriguing question. A series of discoveries now allows biologists to trace the evolutionary history of these colossal animals back to their beginnings at the dawn of the Age of Mammals.

Whales, it turns out, are the descendants of four-legged land mammals that reinvaded the sea about 50 million years ago, much as seals and walruses are doing today. It's pretty startling to realize that Moby Dick's evolutionary ancestor lived on the steppes of Asia and looked like a modest-sized pig a few feet long and weighing perhaps 50 pounds, with four toes on each foot.

From what land mammal did whales arise? Researchers had long speculated that it might have been a hoofed meat-eater with three toes known as a mesonychid, related to rhinoceroses. Subtle clues suggest this: the arrangement of ridges on the molar teeth, the positioning of the ear bones in the skull. But findings reported over the last few years reveal these subtle clues to have been misleading.

Ankle bones from two newly described 50 million-year-old whale species discovered in 2001 by Philip Gingerich of the University of Michigan are those of an artiodactyl, a four-toed mammal related to hippos, cattle and pigs. More recently, Japanese researchers studying DNA have discovered unique genetic markers shared today only by whales and hippos.

Biologists now conclude that whales, like hippos, are descended from a group of early four-hoofed mammals called anthracotheres, modest-sized grazing animals with a piggish appearance abundant in Europe and Asia 50 million years ago.

In Pakistan in 1994, biologists discovered its descendant, the oldest known whale. The fossil was 49 million years old, had four legs, each with four-toed feet and a little hoof at the tip of each toe. Dubbed Ambulocetus (walking whale), it was sharp-toothed and about the size of a large sea lion. Analysis of the minerals in its teeth reveal it drank fresh water, so like a seal it was not yet completely a marine animal. Its nostrils were on the end of the snout, like a dog's.

Appearing in the fossil record a few million years later is Rodhocetus, also seal-like but with smaller hind limbs and the teeth of an ocean water drinker. Its nostrils are shifted higher on the skull, halfway toward the top of the head.

Almost 10 million years later, about 37 million years ago, we see the first representatives of Basilosaurus, a giant 60-foot-long serpent-like whale with shrunken hind legs still complete down to jointed knees and toes.

The earliest modern whales appear in the fossil record 15 million years ago. The nostrils are now in the top of the head, a "blowhole" that allows it to break the surface, inhale and resubmerge without having to stop or tilt the head up. The hind legs are gone, with vestigial tiny bones remaining that are unattached to the pelvis. Still, today's whales retain all the genes used to code for legs -- occasionally a whale is born having sprouted a leg or two.

So it seems to have taken 35 million years to evolve a whale from the piglike ancestor of a hippopotamus -- intermediate steps preserved in the fossil record for us to see. Darwin, who always believed that gaps in the vertebrate fossil record would eventually be filled in, would have been delighted.

George B. Johnson's "On Science" column looks at scientific issues and explains them in an accessible manner. 

Johnson, Ph.D., professor emeritus of Biology at Washington University, has taught biology and genetics to undergraduates for more than 30 years. Also professor of genetics at Washington University’s School of Medicine, Johnson is a student of population genetics and evolution, renowned for his pioneering studies of genetic variability. He has authored more than 50 scientific publications and seven texts.

As the founding director of The Living World, the education center at the St Louis Zoo, from 1987 to 1990, he was responsible for developing innovative high-tech exhibits and new educational programs.

Copyright George Johnson

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