Why Evolution Is True (10 page)

In either the “trees down” or “ground up” scenario, natural selection could begin to favor individuals who could fly farther instead of merely gliding, leaping, or flying for short bursts. Then would come the other innovations shared by modern birds, including hollow bones for lightness and that large breastbone.

While we may speculate about the details, the existence of transitional fossils—and the evolution of birds from reptiles—is fact. Fossils like Archae
opteryx
and its later relatives show a mixture of birdlike and early reptilian traits, and they occur at the right time in the fossil record. Scientists predicted that birds evolved from theropod dinosaurs, and, sure enough, we find theropod dinosaurs with feathers. We see a progression in time from early theropods having thin, filamentous body coverings to later ones with distinct feathers, probably adept gliders. What we see in bird evolution is the refashioning of old features (forelimbs with fingers and thin filaments on the skin) into new ones (fingerless wings and feathers)—just as evolutionary theory predicts.

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, whalelike 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 flukelike 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 30 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 52 and 40 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 48 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 the 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.

FIGURE 12
. Transitional forms in the evolution of modern whales.
(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 shading to the right. The “tree” shows the evolutionary relationships of these species.

 

Indohyus
was not the ancestor of whales, but was almost certainly its cousin. But if we go back 4 million more years, to 52 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 whalelike than
Indohyus,
having simpler teeth and whalelike 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 50 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 of its time in shallow water, and would have waddled awkwardly on land, much like a seal.
Rodhocetus
(47 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 40 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 fifty-foot
Dorudon
had legs only two 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 10 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.

IF AT THIS POINT you’re feeling overwhelmed with fossils, be consoled that I’ve omitted hundreds of others that also show evolution. There is the transition between reptiles and mammals, so amply documented with intermediate “mammal-like reptiles” that they are the subjects of many books. Then there are the horses, a branching evolutionary bush leading from a small, five-toed ancestor to the proud hoofed species of today. And of course there is the human fossil record, described in chapter 8—surely the best example of an evolutionary prediction fulfilled.

At the risk of overkill, I’ll briefly mention a few more important transitional forms. The first is an insect. From anatomical similarities, entomologists had long supposed that ants evolved from nonsocial wasps. In 1967, E. O. Wilson and his colleagues found a “transitional” ant, preserved in amber, bearing almost exactly the combination of antlike and wasplike features that entomologists had predicted (figure 13).

FIGURE 13
. Transitional insect: an early ant showing primitive features of wasps—the predicted ancestral group—and derived features of ants. A single specimen of this species,
Sphecomyrma freyd,
was found preserved in amber dating from 92 million years ago.

 

Similarly, snakes have long been supposed to have evolved from lizard-like reptiles that lost their legs, since reptiles with legs appear in the fossil record well before snakes. In 2006, paleontologists digging in Patagonia found a fossil of the earliest known snake, 90 million years old. Just as predicted, it had a small pelvic girdle and reduced hind legs. And perhaps the most thrilling find of all is a 530-million-year-old fossil from China called
Haikouella lanceolata
, resembling a small eel with a frilly dorsal fin. But it also had a head, a brain, a heart, and a cartilaginous bar along the back—the notochord. This marks it as perhaps the earliest chordate, the group that gave rise to all vertebrates, including ourselves. In this complex, inch-long creature may lie the roots of our own evolution.

The fossil record teaches us three things. First, it speaks loudly and eloquently of evolution. The record in the rocks confirms several predictions of evolutionary theory: gradual change within lineages, splitting of lineages, and the existence of transitional forms between very different kinds of organisms. There is no getting around this evidence, no waving it away. Evolution happened, and in many cases we see how.