Getting Off the Ground
Splayed between layers of limestone like a blossom pressed flat in the book of time, the fossilized remains of archaeopteryx--the best-known primitive ancestor of modern birds--are a question cast in stone.
The fossils reveal a primordial changeling--a feathered, lizard-like creature the size of a crow that combines distinctly avian wings with a reptilian tail and knitting-needle teeth.
In the 138 years since Bavarian quarry men unearthed the first specimens of archaeopteryx, scientists have argued furiously over its place in the evolution of flight. In the process, they turned this controversial creature into perhaps the most prominent scientific symbol of Darwin’s theory of evolution.
Everything known today about archaeopteryx comes from just seven skeletons and a single, isolated fossil feather. And for all the conflicting arguments, no one is sure how these ancestral cousins of birds evolved the ability to fly.
Were they arboreal creatures that, over the eons, turned an ability to glide from trees into swooping aerobatics? Or did they begin as ground-dwelling dinosaurs that became progressively airborne in stumbling hops, leaps and ever-higher bounds?
Indeed, could archaeopteryx fly at all? If researchers could ever understand how this one primordial shore bird first took wing, they would have the key to the origin of flight for all birds.
Certainly, anything--even Los Angeles City Hall--can fly if it can move fast enough.
But a biomechanical analysis of archaeopteryx reveals that its estimated running speed is only a third of what would be required to get an animal of its weight aloft. That so-called “velocity gap” would have kept it permanently grounded. At the same time, its wing muscles were not powerful enough to generate the necessary lift to become airborne from a standing start.
If it could not fly, why would it have evolved such sophisticated feathers? Moreover, why would any creature unable to fly develop wings or the distinctive arrangement of muscles needed to flap them?
These are the evolutionary conundrums embodied by archaeopteryx and the origin of flight. The experts who pose them acknowledge that it is all but impossible to answer them directly, for an animal’s behavior is not preserved along with its bones.
One thing is certain. So irresistible is the urge to fly that evolution repeatedly invented new ways to take wing, making flight almost as old as terrestrial life itself.
Insects, no doubt, were first to rule the empire of air. Giant dragonflies, darting in a buzz of iridescent wings almost 3 feet long, were the frequent fliers of 280 million years ago. Not long after, the first vertebrates took flight. Gaunt and ungainly pterosaurs soared on wings up to 35 feet across. Eventually, humankind took evolution into its own hands and invented wide-bodied jets, replete with air-sickness bags and overhead luggage compartments, to carry the species aloft.
Archaeopteryx dates from about 150 million years ago.
“Understanding the origin of flight is not the same as the origin of birds,” says Luis Chiappe, an international authority on avian evolution at the Los Angeles County Museum of Natural History.
“To study the origin of birds, you have bones. You have fossils of feathered dinosaurs. You have things you can look at, things you can touch,” Chiappe said. “For the origin of flight, the evidence is intangible. A function rarely gets fossilized.”
Even so, Chiappe, working with aerodynamicist Phillip Burgers at the San Diego Natural History Museum, now feels certain he has a theory of flight that fits the few known facts about archaeopteryx.
As detailed in research published recently in Nature, the theory is drawn not so much from a study of the ancient bones as from a new appreciation of biomechanics and the principles of aerodynamics.
The key, they deduced, is a better understanding of how such creatures used their feathered forelimbs when running. By flapping its wings as it ran, archaeopteryx could generate considerable extra thrust, their analysis showed. That would have enabled it to run much faster than the power of its legs alone.
“Archaeopteryx may well have been flapping its wings while running. You would have to add the thrust from that flapping of its wings to the speed of its running. Then archaeopteryx can easily reach the minimum flying speed,” Chiappe said.
By their calculations, archaeopteryx could have reached its minimum flying speed of 7.8 meters per second--about 18 mph--in as little as three seconds.
Their theory not only eliminates a key biomechanical obstacle to flight, it also offers a compelling evolutionary reason why wings would have developed well in advance of the ability to fly.
“The gain is speed,” Chiappe said. “And speed is good for anyone catching prey or avoiding predators. Flight evolved as a byproduct of running fast.”
At the same time, dinosaur fossils recently discovered in China prove that feathers also evolved independently of flight--as protective insulation for many species of dinosaurs, including those that resemble ancestors of archaeopteryx.
Therefore, equipped already by nature with feathers and wings, it may have only been a relatively minor evolutionary step forward to develop the ability to fly.
“Our hypothesis is not really testable,” Chiappe acknowledges. But “the calculations are sound. It provides a very likely explanation of how bird flight may--and may is an important word--may have evolved.”
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A New Theory Takes Flight
For generations, experts have been stumped when it comes to explaining how the earliest known ancestor of modern birds--a creature called archaeopteryx--might have been able to fly. Based on the scant fossil evidence, the creature was too slow and too heavy to reach air speed. But Luis Chiappe, an authority on avian evolution at the Los Angeles County Museum of Natural History, has a new theory of flight that may explain how this most famous ancient bird got off the ground. By flapping its wings, the new analysis shows the archaeopteryx could generate considerable extra forward thrust that could have enabled it to run much faster than with the power of its legs alone. Within seconds, it could easily have become airborne.
