Undeniable (20 page)

Flying fish jump out of the water and fly, apparently to escape predators. I mean if you're a self-respecting tuna or mackerel, you eat fish. That's your business. You're swimming along and you see a fish that seems perfect for a meal. You approach at high speed and attack—jaws agape. And then, your prey does a few strokes with its fins, breaks the surface, and disappears. How frustrating for the tuna? Flying fish have been observed to link consecutive glides up to four hundred meters. If you're a mackerel, or a human out in a boat fishing, having the fish you're trying to catch move from where you are to a spot suddenly four football fields away in just a few seconds—well, it can be discouraging. Flying fish are found around the world at tropical latitudes. Why not? Who could catch them? Their regular fish fins are shaped to enable them to impart momentum to both fluids: seawater and air. They can slip through the sea and produce lift enough to glide through the air. Their fins are analogs of bird wings and homologous to the fishy predators they fly to avoid.

Analog structures in living things develop as different organisms make their way in their environment generation to generation. This tendency applies to plants as well as to animals. The leaves of trees and the leaves of sea plants (seaweeds) are an example: similar forms, arrived at completely independently. It's a universal aspect of evolution's adaptive imperative: Fit in or die.

As one embraces the processes that enable, indeed compel, one structure to be ever so slightly changed as its descendant bearer fits in with her or his environment just a little better than his ancestors, one can go again to my thought problem about the engineer trying to transform a bicycle into roller luggage. Each has two wheels. Each has a place to grip. If you are the craftsperson constrained by the rules of evolution, you'll have to come up with homologous structures from one design iteration to the next. The wheels will have to move from a side-by-side setup to a fore-and-aft configuration. And in each intermediate step, the whole thing has to function well enough to make it to its next generation. This pattern is revealed in the fossil record.

Life in all its forms must contend with the straightforward, inexorable rules of classical physics—of energy and motion. Sure, all of the organisms in our world are a result of the subtle and surprising chemistry that is ultimately a consequence of quantum mechanics and the interaction of particles smaller than atoms. Nevertheless, swimming, flying, pushing roots down through soil, drifting in the sea, etc., are all classical physics effects that are nevertheless every bit as astonishing as wondrous modern discoveries like the Higgs boson or the accelerating expansion of the universe. The laws of classical physics are sufficient to drive all of the convergent evolution and the analogous and homologous structures I've been discussing here.

What I find so compelling about evolution, convergent evolution especially, is that it is clearly a fundamental law of nature, like the laws of gravitation, electromagnetism, and heat transfer that shape our world. And yet it is much more personal than those other laws, because we are a direct consequence of it. Stranger still, we can understand it: nature comprehending itself from within.

 

20

WHAT GOOD IS HALF A WING?

A recurring theme among the skeptics of Darwinian evolution is that creatures are so perfectly suited to their tasks that they could not have become that way through blind natural processes. I hear it all the time. The exquisite wings of birds and bees, to take but two examples, are such marvels of engineering that they must have been designed by a deliberate creator. This line of thinking carries with it the mistaken idea that every biological structure makes sense only in its current form. If a hawk's wing is sheer perfection, then it stands to reason that it could not have evolved this way through incremental steps; otherwise, history would be littered with deeply flawed, incomplete versions of what the hawk is today. Often, skeptical creationists put it this way: What good is half a wing?

Like many popular criticisms of evolutionary theory, this one makes some intuitive sense, but only until you start looking at how the natural world really works. I am happy to address the question of what good is half a wing—or half an eye, or half a heart. Now that we've thought about good-enough design and convergent evolution and beneficial adaptions, we are ready to address this question, too. Please join me and take a look at Exhibit A, more specifically, exhibit
Archaeopteryx
, the amazing fossilized animal that resembles a bird and land-crawling reptile at the same time. The first specimen was discovered in 1860, just a year and half after Darwin published
On the Origin of Species
.

The most striking thing about
Archaeopteryx
is that he or she has feathers. The fossil is so beautifully preserved that you can clearly see their outlines. What did this animal do with them? Well, following the half a wing argument, the feathers must have been there so that
Archaeopteryx
could fly. Sure enough, when researchers look closely at
Archaeopteryx
's fossil feather features they see the same kind of feather sockets or quill knobs that we see in modern birds.

In science, a hypothesis should not only explain the evidence we have found, it should also make predictions about things not yet discovered. Knowing that
Archaeopteryx
had feathers, evolutionary biologists predicted that there should be other transitional forms between bird and reptile, and there should be transitional forms of feathers and wings. Something extraordinary happened over the past two decades: People digging in previously unstudied fossil fields in China found the remains of feathered dinosaurs. Not just one or two dinosaurs, but many different species. Furthermore, the evidence now shows that a lot of other, familiar dinosaurs had feathers as well. Perhaps they all did; the feathers just weren't preserved well enough to show it. This is or was true even for land-dwelling predators like the velociraptors that starred in the movie
Jurassic Park
. (What's that you say? You don't remember seeing feathers in the movie? That's because it was made before paleontologists discovered those feathered fossils. Science is inherently a work in progress.)

Velociraptors clearly did not fly. These dinosaurs had big thick leg muscles and arms too small for winging it. Nevertheless they had feathers; as with the
Archaeopteryx
fossils, we can see their feather anchor points, the quill knobs. You have to figure velociraptors must have had feathers for reasons other than flying.

It seems to me the most likely purpose of the feathers was either to keep the animal warm or to make it hot … and by hot, I mean sexually attractive, to show that it was well suited to reproduction. In fact, those are the two leading theories among experts in the field. Hey, mulling it over some more, maybe their feathers performed both functions. They kept these animals warm and they helped a velociraptor show off, like a stylish winter coat.

But what about our scientifically beloved
Archaeopteryx
? What of her or his feathers? Were they airworthy? The answer seems to be clearly: maybe. It's intriguing to say the least. If you've ever recovered a bird feather, you can see that there is a central quill, and on either side is what we call the “barbs.” Those are the feathery parts of a feather. Furthermore the barbs are held to each other by other structures we call barbules (little beards) and hooklets (little hooks). The whole thing becomes quite rigid considering how crazy lightweight it is.

Among the remarkable things about a feather is that its shaft is hollow yet sturdily structured at the same time. If you've ever seen a newborn baby chick, you may have noticed that he or she has thin feathers; they look almost like individual long hairs. They also have scales, not too different from an alligator's or snake's scales around their cute little happy chick feet. Feathers, scales, and hair all start from the same kinds of cells. Each of us, snakes, birds, and people, has the ability to produce structures made of keratin, the natural plastic that is a snake's scales, a bird's feathers, and your hair and fingernails.

Researchers have looked ever so closely at the fossilized remains of
Archaeopteryx
. The way their bones are arranged makes most scientists think that these animals could not raise their wings above their heads. All the birds that you and I know can. Big upstrokes help modern birds push enough air down and back quickly enough to produce lift. It's not clear that
Archaeopteryx
could fly with this kind of modern motion. However, and this for me is huge, the feathers found with and near the
Archaeopteryx
fossils were not symmetrical, which implies that these animals could fly, at least to some extent.

If you fly airplanes, this next bit is obvious. If not, next time you're around airplanes, notice that the wing is thicker in the front than the back. As a very good first approximation, wings are thickest at a point that's about a quarter of the distance from front to back, from leading edge to trailing edge. The leading-to-trailing dimension on a wing is called its chord (like a line segment in a circle). The thickest part of the wing is at the quarter-chord point. That's where we humans run the biggest support beam or spar. It goes lengthwise through the wing (crosswise to the body of the plane). Well, the feathers birds use for flying are about the same. The shaft or quill of a feather used for flight runs along a feather's quarter-chord point. The feathers of the
Archaeopteryx
run the same way.

On modern birds, and on ancient fossil birds, the tail feathers are
not
asymmetrical. Instead, they are nearly the same left to right. Those feathers are not subject to the forces or wing loading of a bird's flight feathers. And, wait … wait … there's a little more. Modern birds have extra feathers called coverts on top of their primary feathers. On modern flying birds, the coverts are used to smooth the airflow over a bird's wings. They work exactly like the fairings we put on our human-designed airplanes. Look under the wing of a modern jetliner, you'll see the long so-called canoe fairings that keep the air going smoothly around the mechanisms that control high-lift surfaces like flaps. The fairings add weight to the airplane, but they're worth it in cutting drag. The covert feathers on a bird add a little weight as well. They also take energy for the bird to grow and replace as feathers get beaten up in use. It's akin to your fingernails growing continuously, because you wear them down. (Try putting adhesive tape over your nails for a couple hours. You'll see how much we use our nails. It's surprising.)

Empennage refers to the tail feathers or fletching of an arrow, the tail of a bird, or the tail of an airplane. Note the Latin root
penna
, which means “feather.” For centuries, humans used quills for pens. While we're on the subject of empennages, note well that a peacock's main tail feathers are much like other birds' tail feathers. The wild and striking plumage that we see displayed in courtship is made entirely of elongated, dressed-up coverts, big little feathers. Just like all other birds that fly, peacocks don't subject their coverts to high lift loads, even though their coverts are huge.

In a remarkable investigation, scientists analyzed the fossils of
Archaeopteryx
feathers and the fossil feathers of related species, with an exquisite X-ray system. They determined that there is over an 80 percent chance that the feathers were dark or black, like a raven or crow. If you've ever thrown a Frisbee, you may have noticed that darker-colored disks are stiffer. The same is true for food storage containers, the more opaque, the harder and stiffer. The plastic's so-called pigment loading affects its stiffness. So it is generally with keratin, the stuff of feathers. Dark feathers would be stiffer and perhaps better suited for flying.

In 2013, an eleventh specimen of
Archaeopteryx
was unearthed. Apparently,
Archaeopteryx
evolved around the same time as several other feathered reptiles. Analysis of
Archaeopteryx
“trouser feathers” around his or her ankles indicates that he or she probably could fly at least to a limited extent. Even if the feathers weren't suitable for full-time flying, other contemporary species did have airworthy feathers. The crime scene is ancient, and the jury is still out.

I admit it; I'm fascinated with living things that can fly. Perhaps I'm just envious. It strikes me as quite a clever strategy. If you can get airborne you can really cover some ground, or some ocean. You can avoid predators, find food, and shop for a place to roost all without getting your feet wet or dusty. But flight for any organism or human-built machine is a complicated business. In engineering we say that if you have a big enough engine, you can make anything fly. In many ways, the more difficult problem is steering. Imagine a car that you couldn't steer. You wouldn't need a warranty, because it would be guaranteed to crash. Flying takes continual, accurate steering inputs in three axes: rolling, turning, and up and down-ing (roll, yaw, and pitch). Without control in flight, a bird would be a dead duck. So did
Archaeopteryx
have the wits to do it?

Careful study of the skull of an
Archaeopteryx
, specifically her or his brain case, indicates that she or he not only had feathers and wings sufficient for flight, she had a brain big enough to fly. Very nearly all of our planes have horizontal stabilizers or tail-planes and vertical tails, the part that sticks straight up and has the moveable rudder incorporated. But now consider the B-2 bomber airplane; it has no vertical tail, no tail at all. Military tacticians wanted to eliminate the vertical tail, because radar would bounce off of it easily. Any plane with a vertical tail is a great deal easier to detect than planes that would somehow not need one. Keep in mind that these tailpieces keep the plane going where we want it to go, exactly like the fletching on the bowstring end of an arrow, the arrow's empennage.