The Whale's Tale

Science News, Nov 6, 1999 by Richard Monastersky

Searching for the landlubbing ancestors of marine mammals

Only 24 years after Charles Darwin rewrote the book of life with his theory of natural selection, a fellow Victorian scientist named William Flower trained this powerful new idea on one of the toughest problems in zoology: the whale. Natural historians had long before recognized that whales are mammals, but that was about as far as they had come in understanding the origins of cetaceans. How evolution had managed to craft such a unique beast presented a mystery as vast as the creature itself.

In 1883, Flower offered an idea that--on the face of it--seemed positively daft. The legless leviathans, he suggested, had evolved from mammals known as ungulates, a group whose best-known characteristic is a set of hoofed feet. In other words, dolphins, porpoises, humpbacks, orcas, and ail other whales are close kin of cows, horses, pigs, and related barnyard stock.

More than a century after Flower raised his audacious hypothesis, it no longer stirs even a whiff of controversy. Dozens of scientific studies over the past 3 decades have convinced biologists that cetaceans are the progeny of hoofed mammals. Yet the whale's story is far from finished.

Two opposing groups of scientists are currently battling over the next chapter in the saga. They clash on the issue of exactly where whales fit in the tree of hoofed mammals. Far from just an arcane argument, the debate has much to say about how whales made the profound transformation from lire on land to mastery of another medium.

What's more, the skirmish documents the intense Darwinian struggle shaping the science of biology as it evolves. The war over whales pits the classical techniques of studying bones and flesh against the most modern methods of genetic analysis--two approaches that lead to different versions of the whale's origin tale.

"This is one of the fundamental questions right now," says Patrick Luckett, an embryologist at the University of Puerto Rico in San Juan and a coeditor of the JOURNAL OF MAMMALIAN EVOLUTION. "This is something that's very interesting to evolutionary biologists because there is this continuing controversy."

The most recent genetic evidence, reported in August, provides the strongest support yet for the hypothesis that whales and hippopotamuses are first cousins. If true, this would slice these marine animals from their long-standing position on the mammal family tree and graft them onto a different branch. It would also suggest that the ancestor of whales and hippos may have ventured into the water more than 55 million years ago.

From the fossil bones, however, paleontologists see no need to cut into the mammalian tree. Hippos, they say, are only distant relatives of whales, no closer than are deer, pigs, or other even-toed ungulates.

For Charles Darwin, whales represented a major case of heartburn. The famed naturalist had no trouble envisioning whales evolving from four-legged mammals, but his audience certainly did.

In his Origin of Species, Darwin notes a case of a black bear swimming for hours with its mouth agape, catching aquatic insects much as a whale might feed. "I can see no difficulty in a race of bears being rendered, by natural selection, more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale," he speculated.

The ridicule and attacks engendered by this passage grew to such a pitch that Darwin pared it down and then deleted it altogether in later editions.

Although Darwin got the particulars wrong, his swimming-bear scenario was not far off the mark. Modern molecular biologists say that they now have the unassailable evidence to track whales' origins among four-legged mammals.

In the Aug. 31 PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES, a team of Japanese and U.S. researchers used one of the newest types of genetic analysis to track the phylogeny, or relationships, among some mammals. The group focused on whales and members of the order Artiodactyla, the group that paleontologists in recent years have considered whales' closest living relatives. Artiodactyls are hoofed animals that have an even number of toes on each foot, such as pigs, giraffes, and hippos.

Norihiro Okada of the Tokyo Institute of Technology and his colleagues examined a part of the animals' genetic code that doesn't specify instructions for any genes. Some of this so-called junk DNA is made up of segments that can copy themselves and then splice the copies back into the genetic strands at various points. One group of these mobile elements consists of short interspersed elements, or SINEs; long interspersed elements, or LINEs, form another.

Such segments have been insinuating themselves into the code of life for billions of years, says Okada. "In the case of the human genome, SINEs constitute more than 10 percent of the genome, and LINEs constitute more than 15 percent," he says.

Usually harmless, these segments have an uncanny ability to co-opt the DNA copying apparatus in cells, and they may have played a role in spawning some viruses. "Retroviruses, such as the AIDS viruses, were believed to have been generated from LINEs," says Okada.