Duane Gish, an American creationist, is renowned for his lively and popular (if wildly misguided) lectures attacking evolution. I once attended one, during which Gish made fun of biologists' theory that whales descended from land animals related to cows. How, he asked, could such a transition occur, since the intermediate form would have been poorly adapted to both land and water, and thus couldn't be built by natural selection? (This resembles the half-a-wing argument against the evolution of birds.) To illustrate his point, Gish showed a slide of a mermaidlike cartoon animal whose front half was a spotted cow and whose rear half was a fish. Apparently puzzled over its own evolutionary fate, this clearly maladapted beast was standing at the water's edge, a large question mark hovering over its head. The cartoon had the intended effect: the audience burst into laughter. How stupid, they thought, could evolutionists be?
Indeed, a "mer-cow" is a ludicrous example of a transitional form between terrestrial and aquatic mammals—an "udder failure," as Gish called it. But let's forget the jokes and rhetoric, and look to nature. Can we find any mammals that live on both land and water, the kind of creature that supposedly could not have evolved?
Easily. A good candidate is the hippopotamus, which, although closely related to terrestrial mammals, is about as aquatic as a land mammal can get. (There are two species, the pygmy hippo and the "regular" hippo, whose scientific name is, appropriately, Hippopotamus amphibius.) Hippos spend most of their time submerged in tropical rivers and swamps, surveying their domain with eyes, noses, and ears that sit atop their head, all of which can be tightly closed underwater. Hippos mate in the water, and their babies, who can swim before they can walk, are born and suckle underwater. Because they are mostly aquatic, hippos have special adaptations for coming ashore to graze: they usually feed at night, and, because they're prone to sunburn, secrete an oily red fluid that contains a pigment—hipposudoric acid—that acts as a sunscreen and possibly an antibiotic. This has given rise to the myth that hippos sweat blood. Hippos are obviously well adapted to their environment, and it's not hard to see that if they could find enough food in the water, they might eventually evolve into totally aquatic, whale-like creatures.
But we don't just have to imagine how whales evolved by extrapolating from living species. Whales happen to have an excellent fossil record, courtesy of their aquatic habits and robust, easily fossilized bones. And how they evolved has emerged within only the last twenty years. This is one of our best examples of an evolutionary transition, since we have a chronologically ordered series of fossils, perhaps a lineage of ancestors and descendants, showing their movement from land to water.
It's been recognized since the seventeenth century that whales and their relatives, the dolphins and porpoises, are mammals. They are warm-blooded, produce live young whom they feed with milk, and have hair around their blowholes. And evidence from whale DNA, as well as vestigial traits like their rudimentary pelvis and hind legs, show that their ancestors lived on land. Whales almost certainly evolved from a species of the artiodactyls: the group of mammals that have an even number of toes, such as camels and pigs.12 Biologists now believe that the closest living relative of whales is—you guessed it—the hippopotamus, so maybe the hippo-to-whale scenario is not so far-fetched after all.
But whales have their own unique features that set them apart from their terrestrial relatives. These include the absence of rear legs, front limbs that are shaped like paddles, a flattened fluke-like tail, a blowhole (a nostril atop the head), a short neck, simple conical teeth (different from the complex, multicusped teeth of land animals), special features of the ear that allow them to hear underwater, and robust projections on top of the vertebrae to anchor the strong swimming muscles of the tail. Thanks to an amazing series of fossil finds in the Middle East, we can trace the evolution of each of these traits—except for the boneless tail, which doesn't fossilize—from a terrestrial to an aquatic form.
Sixty million years ago there were plenty of fossil mammals, but no fossil whales. Creatures that resemble modern whales show up thirty million years later. We should be able, then, to find the transitional forms within this gap. And once again, that's exactly where they are. Figure 12 shows, in chronological order, some of the fossils involved in this transition, spanning the period between fifty-two and forty million years ago.
There is no need to describe this transition in detail, as the drawings clearly speak—if not shout—of how a land-living animal took to the water. The sequence begins with a recently discovered fossil of a close relative of whales, a raccoon-sized animal called Indohyus. Living forty-eight million years ago, Indohyus was, as predicted, an artiodactyl. It is clearly closely related to whales because it has special features of the ears and teeth seen only in modern whales and their aquatic ancestors. Although Indohyus appears slightly later than largely aquatic ancestors of whales, it is probably very close to what the whale ancestor looked like. And it was at least partially aquatic. We know this because its bones were denser than those of fully terrestrial mammals, which kept the creature from bobbing about in the water, and because the isotopes extracted from its teeth show that it absorbed a lot of oxygen from water. It probably waded in shallow streams or lakes to graze on vegetation or escape from its enemies, much like a similar animal, the African water chevrotain, does today.13 This part-time life in water probably put the ancestor of whales on the road to becoming fully aquatic.
Indohyus was not the ancestor of whales, but was almost certainly its cousin. But if we go back four million more years, to fifty-two million years ago, we see what might well be that ancestor. It is a fossil skull from a wolf-sized creature called Pakicetus, which is a bit more whale-like than Indohyus, having simpler teeth and more whale-like ears. Pakicetus still looked nothing like a modern whale, so if you had been around to see it, you wouldn't have guessed that it or its close relatives would give rise to a dramatic evolutionary radiation. Then follows, in rapid order, a series of fossils that become more and more aquatic with time. At fifty million years ago there is the remarkable Ambulocetus (literally, "walking whale"), with an elongated skull and reduced but still robust limbs, limbs that still ended in hooves that reveal its ancestry. It probably spent most
figure 12. Transitional forms in the evolution of modern whales from the ancient artiodactyl Indohyus (Balaena is the modern baleen whale, with a vestigial pelvis and hindlimb, while the other forms are transitional fossils). Relative sizes of the animals are shown in gray shading.
of its time in shallow water, and would have waddled awkwardly on land, much like a seal. Rodhocetus (forty-seven million years ago) is even more aquatic. Its nostrils have moved somewhat backward, and it has a more elongated skull. With stout extensions on the backbone to anchor its tail muscles, Rodhocetus must have been a good swimmer, but was handicapped on land by its small pelvis and hindlimbs. The creature certainly spent most if not all of its time at sea. Finally, at forty million years ago, we find the fossils Basilosaurus and Dorudon—clearly fully aquatic mammals, with short necks and blowholes atop the skull. They could not have spent any time on land, for their pelvis and hindlimbs were reduced (the 50-foot Dorudon had legs only 2 feet long) and were unconnected to the rest of the skeleton.
The evolution of whales from land animals was remarkably fast: most of the action took place within only ten million years. That's not much longer than the time it took us to diverge from our common ancestor with chimpanzees, a transition that involved far less modification of the body. Still, adapting to life at sea did not require the evolution of any brand-new features—only modifications of old ones.
But why did some animals go back to the water at all? After all, millions of years earlier their ancestors had invaded the land. We're not sure why there was a reverse migration, but there are several ideas. One possibility involves the disappearance of the dinosaurs along with their fierce marine cousins, the fish-eating mosasaurs, ichthyosaurs, and plesiosaurs. These creatures would not only have competed with aquatic mammals for food, but probably made a meal of them. With their reptilian competitors extinct, the ancestors of whales may have found an open niche, free from predators and loaded with food. The sea was ripe for invasion. All of its benefits were only a few mutations away.
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