Up in the Air

“Taking Wing” book cover

Anthropologist Pat Shipman's new book seeks to answer the complex riddle of how birds came to fly.

"Flying seems such an impossibly difficult task," Pat Shipman writes in her newest book, Taking Wing: Archaeopteryx and the Evolution of Bird Flight. We humans, despite long centuries of frequently disastrous obsession with getting airborne, have only recently mastered the feat, and then only with a big boost from technology.

Nature, on the other hand, has solved the complex riddle of flying three separate times: in birds, in bats, and in the now-extinct creatures known as pterodactyls—and that's just among the vertebrates. Shipman calls flight "one of the most stunning adaptations evolved by any creature anywhere." How and why it took off are the questions her book strives to answer.

Shipman, an adjunct associate professor of anthropology at Penn State, centers her tale on Archaeopteryx, the so-called First Bird, which flourished during the late Mezozoic period, some 150 million years ago. Seven fossil specimens of this avian missing link have been unearthed from the limestone sediment of an ancient lagoon in Bavaria: all that remains of a creature with both feathered wings and the toothy jaw and long, bony tail of a reptile. The best-preserved of these, known as the Berlin Archaeopteryx, is, in Shipman's eyes, "perhaps the most beautiful fossil in the world."

Beauty aside, since 1861, when the first Archaeopteryx was discovered, its significance has been controversial. Was this creature—half-avian, half-reptile—"a truly transitional form?" Only two years after publication of the Origin of Species, the question was hotly debated, its answer seen as either proving or disproving Darwin's new theory of evolution. Some researchers, even as late as the 1980s, dismissed Archaeopteryx as a fraud.

It is certainly genuine, Shipman affirms, and truly transitional. But what can this prototype tell us about how birds came to fly?

She begins by considering the basic challenges involved. Getting aloft—taking off into the "distinctly unforgiving medium" that is air—is merely the first problem. Remaining airborne for any length of time requires both adequate propulsion and the ability to steer. A third problem, sadly overlooked by many early human would-be aviators, is the necessity of a controlled landing.

To surmount these obstacles, birds have developed a suite of highly specific adaptations that provide a combination of lightness, power, and grace: wings and feathers, yes, but also huge pectoral muscles; sturdy, intricate shoulder joints; and hollow bones that can hold air, part of what Shipman calls a "turbo-charged" respiratory system. In a chapter on flight dynamics she shows how all these components function together in the refined flying creatures of today. But to understand how far birds have come, she argues, we have to know where they started from. Here Archaeopteryx, not the beginning but the mid-point, is both uniquely helpful and maddeningly inconclusive.

From Archaeopteryx alone comes the general agreement that birds evolved from some type of reptile. Beyond that, however, the experts break down roughly into two schools: the paleo-ornithologists, who assert that birds began as crocodilians, of the same lineage that later produced crocodiles; and the dinosaurologists, who see Archaeopteryx and subsequent birds evolving from warm-blooded dinosaurs. The disagreement runs deep, Shipman notes: The researchers' very different frames of reference make for fundamental conflicts in the interpretation of evidence. And what remains of Archaeopteryx simply does not speak clearly enough, after all these millenia, to resolve these disputes.

Seemingly minor differences—regarding the wrist bones, the toes, and other small features—add up to opposing visions of just what manner of creature these bird ancestors were, where they lived, how they got around. Did they perch themselves in trees? Amble earth-bound? Did flying originate "trees-down," or "ground-up?"

The "trees-down" hypothesis neatly solves the vexatious question of how the first flyers achieved the near miracle of takeoff. In this scenario, flight begins with a perched lizard's simple hop to the next branch toward food or safety, progressing gradually via natural selection to parachuting, gliding, and eventually flapping, with not a few instances of deadly plummeting along the way. As Shipman deadpans, "The problem is that, once takeoff is solved, landing becomes an acute and serious issue."

The "ground-up" theory defuses the latter issue by conjuring a small, bipedal dinosaur, feathered for warmth, that runs rapidly along the ground, occasionally leaping after its prey. But, then, how did such a creature ever manage to take off? Extending its feathered arms, this thinking goes, would have provided balance and steering as the creature skittered along. Those arms, with selection, would improve as airfoils until they began to create lift, "prolonging the leaps into short-duration, low-altitude flights that improve predatory success without running the risk of catastrophic crashes."

Each camp, Shipman notes, has had more success over the years in pointing out flaws in the other side's theory than in proving the merit of its own. To "trees-down" partisans, Archaeopteryx clearly lacked the strong pectoral muscles needed to power takeoff. To "ground-up" supporters, its feet and claws were extremely ill-suited for perching in trees.

New evidence is needed, and indeed new evidence continues to emerge, most recently from the 1996 discovery in China of a small nonflying bipedal dinosaur whose back appears to have been covered head-to-tail with "some sort of hair or downy fibers." If these fibers turn out on close inspection to be feathers, Shipman states, this Sinosauropteryx will be a big boost for the "ground-up" crowd. But to date the question remains open, the two sides firmly resolved in opposition. "Like more fossils of Archaeopteryx," Shipman concludes, "the truth is waiting to be discovered."

Pat Shipman, Ph.D., is adjunct associate professor of anthropology in the College of the Liberal Arts, 315 Carpenter Bldg., University Park, PA 16802; 814-865-2509, or pls10@psu.edu. Taking Wing: Archaeopteryx and the Evolution of Bird Flight, published by Simon and Schuster in 1998, was a finalist for the 1999 Los Angeles Times Book Prize in the science and technology category.

Last Updated September 01, 1999