As a graduate student in Boston, I was enlisted to help a senior scientist who had written a paper about whether it was more efficient for warm-blooded animals to run on two legs or four. He planned to submit the paper to Nature, one of the most prestigious scientific journals, and asked me to help him take a photograph striking enough to land on the journal cover and call attention to his work. Eager to get out of the laboratory, I spent an entire afternoon chasing a horse and an ostrich around a corral, hoping to get them to run side by side, demonstrating both types of running in a single frame. Needless to say, the animals refused to cooperate, and, all species being exhausted, we finally gave up. Although we never got the picture,14 the experience did teach me a biology lesson: ostriches can't fly, but they can still use their wings. When they're running, they use their wings for balance, extending them to the sides to keep from toppling over. And when an ostrich becomes agitated—as it tends to do when you chase it around a corral—it runs straight at you, extending its wings in a threat display. That's a sign to get out of the way, for a miffed ostrich can easily disembowel you with one swift kick. They also use their wings in mating displays,15 and spread them out to shade their chicks from the harsh African sun.

The lesson, though, goes deeper. The wings of the ostrich are a vestigial trait: a feature of a species that was an adaptation in its ancestors, but that has either lost its usefulness completely or, as in the ostrich, has been co-opted for new uses. Like all flightless birds, ostriches are descended from flying ancestors. We know this from both fossil evidence and from the pattern of ancestry that flightless birds carry in their DNA. But the wings, though still present, can no longer help the birds take flight to forage or escape predators and bothersome graduate students. Yet the wings are not useless—they've evolved new functions. They help the bird maintain balance, mate, and threaten its enemies.

The African ostrich isn't the only flightless bird. Besides the ratites— the large flightless birds that include the South American rhea, the Australian emu, and the New Zealand kiwi—dozens of other bird species have independently lost the ability to fly. These include flightless rails, grebes, ducks, and, of course, penguins. Perhaps the most bizarre is the New Zealand kakapo, a tubby flightless parrot that lives mainly on the ground but can also climb trees and "parachute" gently to the forest floor. Kakapos are critically endangered: fewer than 100 still exist in the wild. Because they can't fly, they are easy prey for introduced predators like cats and rats.

All flightless birds have wings. In some, like the kiwi, the wings are so small—only a few inches long and buried beneath their feathers—that they don't seem to have any function. They're just remnants. In others, as we saw with the ostrich, the wings have new uses. In penguins, the ancestral wings have evolved into flippers, allowing the bird to swim underwater with amazing speed. Yet they all have exactly the same bones that we see in the wings of species that can fly. That's because the wings of flightless birds weren't the product of deliberate design (why would a creator use exactly the same bones in flying and flightless wings, including the wings of swimming penguins?), but of evolution from flying ancestors.

Opponents of evolution always raise the same argument when vestigial traits are cited as evidence for evolution. "The features are not useless," they say. "They are either useful for something, or we haven't yet discovered what they're for." They claim, in other words, that a trait can't be vestigial if it still has a function, or a function yet to be found.

But this rejoinder misses the point. Evolutionary theory doesn't say that vestigial characters have no function. A trait can be vestigial and functional at the same time. It is vestigial not because it's functionless, but because it no longer performs the function for which it evolved. The wings of an ostrich are useful, but that doesn't mean that they tell us nothing about evolution. Wouldn't it be odd if a creator helped an ostrich balance itself by giving it appendages that just happen to look exactly like reduced wings, and which are constructed in exactly the same way as wings used for flying?

Indeed, we expect that ancestral features will evolve new uses: that's just what happens when evolution builds new traits from old ones. Darwin himself noted that "an organ rendered, during changed habits of life, useless or injurious for one purpose, might easily be modified and used for another purpose."

But even when we've established that a trait is vestigial, the questions don't end. In which ancestors was it functional? What was it used for? Why did it lose function? Why is it still there instead of having disappeared completely? And which new functions, if any, has it evolved?

Let's take wings again. Obviously, there are many advantages to having wings, advantages shared by the flying ancestors of flightless birds. So why did some species lose their ability to fly? We're not absolutely sure, but we do have some powerful clues. Most of the birds that evolved flightlessness did so on islands—the extinct dodo on Mauritius, the Hawaiian rail, the kakapo and kiwi in New Zealand, and the many flightless birds named after the islands they inhabit (the Samoan wood rail, the Gough Island moorhen, the Auckland Island teal, and so on). As we'll see in the next chapter, one of the notable features of remote islands is their lack of mammals and reptiles—species that prey on birds. But what about ratites that live on continents, like ostriches? All of these evolved in the Southern Hemisphere, where there were far fewer mammalian predators than in the north.

The long and short of it is this: flight is metabolically expensive, using up a lot of energy that could otherwise be diverted to reproduction. If you're flying mainly to stay away from predators, but predators are often missing on islands, or if food is readily obtained on the ground, as it can be on islands (which often lack many trees), then why do you need fully functioning wings? In such a situation, birds with reduced wings would have a reproductive advantage, and natural selection could favor flightlessness. Also, wings are large appendages that are easily injured. If they're unnecessary, you can avoid injury by reducing them. In both situations, selection would directly favor mutations that led to progressively smaller wings, resulting in an inability to fly.

So why haven't they disappeared completely? In some cases they nearly have: the wings of the kiwi are functionless nubs. But when the wings have assumed new uses, as in the ostrich, they will be maintained by natural selection, though in a form that doesn't allow flight. In other species, wings may be in the process of disappearing, and we're simply seeing them in the middle of this process.

Vestigial eyes are also common. Many animals, including burrow-ers and cave-dwellers, live in complete darkness, but we know from constructing evolutionary trees that they descended from species that lived above ground and had functioning eyes. Like wings, eyes are a burden when you don't need them. They take energy to build, and can be easily injured. So any mutations that favored their loss would clearly be advantageous when it's just too dark to see. Alternatively, mutations that reduced vision could simply accumulate over time if they neither helped nor hurt the animal.

Just such an evolutionary loss of eyes occurred in the ancestor of the eastern Mediterranean blind mole rat. This is a long, cylindrical rodent with stubby legs, resembling a fur-covered salami with a tiny mouth. This creature spends its entire life underground. Yet it still retains a vestige of an eye—a tiny organ only 1 millimeter across and completely hidden beneath a protective layer of skin. The remnant eye can't form images. Molecular evidence tells us that, around twenty-five million years ago, blind mole rats evolved from sighted rodents, and their withered eyes attest to this ancestry. But why do these remnants remain at all? Recent studies show that they contain a photopigment that is sensitive to low levels of light, and helps regulate the animal's daily rhythm of activity. This residual function, driven by small amounts of light that penetrate underground, could explain the persistence of vestigial eyes.

True moles, which are not rodents but insectivores, have independently lost their eyes, retaining only a vestigial, skin-covered organ that you can see by pushing aside the fur on its head. Similarly, in some burrowing snakes the eyes are completely hidden beneath the scales. Many cave animals also have eyes that are reduced or missing. These include fish (like the blind cave fish you can buy at pet stores), spiders, salamanders, shrimp, and beetles. There is even a blind cave crayfish that still has eyestalks, but no eyes atop them!

Whales are treasure troves of vestigial organs. Many living species have a vestigial pelvis and leg bones, testifying, as we saw in the last chapter, to their descent from four-legged terrestrial ancestors. If you look at a complete whale skeleton in a museum, you'll often see the tiny hind limb and pelvic bones hanging from the rest of the skeleton, suspended by wires. That's because in living whales they're not connected to the rest of the bones, but are simply imbedded in tissue. They once were part of the skeleton, but became disconnected and tiny when they were no longer needed. The list of vestigial organs in animals could fill a large catalog. Darwin himself, an avid beetle collector in his youth, pointed out that some flightless beetles still have vestiges of wings beneath their fused wing covers (the beetle's "shell").

We humans have many vestigial features proving that we evolved. The most famous is the appendix. Known medically as the vermiform ("worm-shaped") appendix, it's a thin, pencil-sized cylinder of tissue that forms the end of the pouch, or caecum, that sits at the junction of our large and small intestines. Like many vestigial features, its size and degree of development are highly variable: in humans, its length ranges from about an inch to over a foot. A few people are even born without one.

In herbivorous animals like koalas, rabbits, and kangaroos, the caecum and its appendix tip are much larger than ours. This is also true of leaf-eating primates like lemurs, lorises, and spider monkeys. The enlarged pouch serves as a fermenting vessel (like the "extra stomachs" of cows), containing bacteria that help the animal break down cellulose into usable sugars. In primates whose diet includes fewer leaves, like orangutans and macaques, the caecum and appendix are reduced. In humans, who don't eat leaves and can't digest cellulose, the appendix is nearly gone. Obviously, the less herbivorous the animal, the smaller the caecum and appendix. In other words, our appendix is simply the remnant of an organ that was critically important to our leaf-eating ancestors, but of no real value to us.

Does an appendix do us any good at all? If so, it's not obvious. Removing it doesn't produce any bad side effects or increase mortality (in fact, removal seems to reduce the incidence of colitis). Discussing the appendix in his famous textbook The Vertebrate Body, the paleontologist Alfred Romer remarked dryly, "Its major importance would appear to be financial support of the surgical profession." But to be fair, it may be of some small use. The appendix contains patches of tissue that may function as part of the immune system. It has also been suggested that it provides a refuge for useful gut bacteria when an infection removes them from the rest of our digestive system.

But these minor benefits are surely outweighed by the severe problems that come with the human appendix. Its narrowness makes it easily clogged, which can lead to its infection and inflammation, otherwise known as appendicitis. If not treated, a ruptured appendix can kill you. You have about one chance in fifteen of getting appendicitis in your lifetime. Fortunately, thanks to the evolutionarily recent practice of surgery, the chance of dying when you get appendicitis is only 1 percent. But before doctors began to remove inflamed appendixes in the late nineteenth century, mortality may have exceeded 20 percent. In other words, before the days of surgical removal, more than one person in 100 died of appendicitis. That's pretty strong natural selection.

Over the vast period of human evolution—more than 99 percent of it—there were no surgeons, and we lived with a ticking time bomb in our gut. When you weigh the tiny advantages of an appendix against its huge disadvantages, it's clear that on the whole it is simply a bad thing to have. But apart from whether it's good or bad, the appendix is still vestigial, for it no longer performs the function for which it evolved.

So why do we still have one? We don't yet know the answer. It may in fact have been on its way out, but surgery has almost eliminated natural selection against people with appendixes. Another possibility is that selection simply can't shrink the appendix any more without it becoming even more harmful: a smaller appendix may run an even higher risk of being blocked. That might be an evolutionary roadblock to its complete disappearance.

Our bodies teem with other remnants of primate ancestry. We have a vestigial tail: the coccyx, or the triangular end of our spine, that's made of several fused vertebrae hanging below our pelvis. It's what remains of the long, useful tail of our ancestors (figure 14). It still has a function (some useful muscles attach to it), but remember that its vestigiality is diagnosed not by its usefulness but because it no longer has the function for which it originally evolved. Tellingly, some humans have a rudimentary tail muscle (the "extensor coccygis"), identical to the one that moves the tails of monkeys and other mammals. It still attaches to our coccyx, but since the bones can't move, the muscle is useless. You may have one and not even know it.

figure 14. Vestigial and atavistic tails. Top left: in our relatives that have tails, such as the ruffed lemur (Varecia variegata), the tail (caudal) vertebrae are unfused (the first four are labelled C1-C4). But in the human "tail", or coccyx (top right), the caudal vertebrae are fused to form a vestigial structure. Bottom: atavistic tail of a three-month old Israeli infant. X-ray of the tail (right) shows that the three caudal vertebrae are much larger and more well developed than normal, are not fused, and approach the size of the sacral vertebrae (S1-S5). The tail was later surgically removed.

Other vestigial muscles become apparent in winter, or at horror movies. These are the arrector pili, the tiny muscles that attach to the base of each body hair. When they contract, the hairs stand up, giving us "goose bumps"—so called because of their resemblance to the skin of a plucked goose. Goose bumps and the muscles that make them serve no useful function, at least in humans. In other mammals, however, they raise the fur for insulation when it's cold, and cause the animal to look larger when it's making or receiving threats. Think of a cat, whose fur bushes out when it's cold or angry. Our vestigial goose bumps are produced by exactly the same stimuli—cold or a rush of adrenaline.

And here's a final example: if you can wiggle your ears, you're demonstrating evolution. We have three muscles under our scalp that attach to our ears. In most individuals they're useless, but some people can use them to wiggle their ears. (I am one of the lucky ones, and every year I demonstrate this prowess to my evolution class, much to the students' amusement.) These are the same muscles used by other animals, like cats and horses, to move their ears around, helping them localize sounds. In those species, moving the ears helps them detect predators, locate their young, and so on. But in humans the muscles are good only for entertainment.16

To paraphrase the quote from the geneticist Theodosius Dobzhansky that begins this chapter, vestigial traits make sense only in the light of evolution. Sometimes useful, but often not, they're exactly what we'd expect to find if natural selection gradually eliminated useless features or refashioned them into new, more adaptive ones. Tiny, nonfunctional wings, a dangerous appendix, eyes that can't see, and silly ear muscles simply don't make sense if you think that species were specially created.

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