Bird Flight Is Easy to Lose But Hard to Gain
Is it easier for a plane to lose its wings and roll like a car
or for a car to sprout wings and fly like an airplane?
Current Biology has a special issue this week on birds. Many of the articles are informative and illuminating, but when they attribute bird flight to evolution, serious questions arise. How can a blind, purposeless, unconcerned process bring about powered flight by a series of lucky accidents? The answer you read repeatedly in the series goes like this: It evolved because it evolved.
The following links go to open-access papers in the series so that our readers can check them out.
Biomechanics of avian flight (Andrew A. Biewener, Current Biology, 24 Oct 2022). Here is the key paper on powered flight in birds. Let’s see if Dr Biewener explains how evolution achieved this marvel of engineering, with all its component parts working together to achieve the function of travel through the atmosphere against the force of gravity by a heavier-than-air craft. He begins:
Bird flight performance and behavior continue to captivate large numbers of recreational and professional ornithologists. In many locales, annual bird counts are carried out to assess how avian biodiversity is being affected by climate change and human impact. Consideration of how birds have evolved to achieve key traits for flying of their feathers, wing and body form, as well as the underlying neuromuscular and skeletal machinery, provides an opportunity to ‘look under the hood’ to appreciate how they power their impressive acrobatic flight abilities. I will begin this primer by briefly summarizing the key morphological changes that led to the evolution of flapping flight, a remarkable evolutionary achievement that resulted in the explosive diversification and success of modern birds.
Powered flight requires numerous systems working together. Credit: Illustra Media.
To see how flight evolved, he says, we need to see how the parts evolved that led to flight evolution. It evolved because it evolved. Does it sound like he has an open mind to consider all causes that might bear on this “remarkable achievement” in birds? Why does it have to be a “remarkable evolutionary achievement”? After all, he just spoke of the advances made by Leonardo da Vinci and the Wright Brothers who applied engineering principles, using their goal-directing intelligent minds.
Biewener continues his evolutionary story by lining up potential fossil ancestors that he thinks were progressing toward powered flight. We will mark in red the words that act as synonyms for Darwinian evolution:
The capacity for powered flapping flight exhibited by modern birds arose in the theropod dinosaurs within the Paraves. Although debate continues regarding a gliding (‘trees down’) versus wing-assisted terrestrial (‘ground up’) origin for flapping flight, the fact that early birds evolved from small predatory bipedal (‘two-legged’) theropod ancestors strongly skews the evidence in favor of a terrestrial origin. Based on estimates of wing loading — the ratio of wing area to body weight — and flight muscle power capability, a variety of early fossil birds (Avialae), including the iconic Archeopteryx, show evidence for the potential of powered flight. Several early birds were therefore likely employing an incipient forelimb wing flight stroke that could assist in a variety of ground-running behaviors by generating aerodynamic force, in addition to the forces that their hind limbs exerted on the ground.
He latches on to Ken Dial’s WAIR hypothesis, which had already been falsified (19 July 2016, 25 June 2014). He pictures a young dinosaur running along the ground with its forelimbs outstretched. It feels some aerodynamic force in its forelimbs, and its hind limbs can run and jump. Over time, its leaps and jumps let it take off into the air, and its forelimbs turn into wings. Does this count as a scientific hypothesis? It sounds Lamarckian. Unless an accidental mutation landed on a gamete, any lucky break would have been lost (see video about bird development).
The lucky mutation, furthermore, would have had to be sufficiently beneficial to make a difference in the next generation: say, to help the offspring leap a few inches farther with each step to outrun a predator or to reach a prey. Meanwhile, though, the predator (in the just-so story) is also evolving to run faster, and the prey is evolving to stay out of reach. The theropod dinosaur’s advantage, therefore, gets washed out immediately in the next generation. One evolutionist called this “slippage on the treadmill” in natural selection theory (9 Sept 2017; 13 May 2020; the technical term is “decompensatory epistasis” ).
Miracle Talk
Biewener continues his story:
The high-power requirements for effective flight subsequently resulted in the conserved evolution of several key anatomical features. Among these are asymmetric, stiff flight feathers and aerodynamic wings (Figure 1A,B); a stiff, lightweight skeleton, with air-filled bones (Figure 2A) linked to respiratory air sacs, substantially reduced distal wing bone elements, and forelimb joints facilitating elevation–depression movements of the wing as a whole while allowing for changes in wing planform shape; substantial enlargement of the power-generating flight muscles (pectoralis and supracoracoideus) with a concomitant expansion of the sternum by means of an extensive ventral keel (Figure 2B); and modification of the avian shoulder joint with passive structural support provided by means of robust coracoids, a furcula (‘wishbone’) and joint ligaments to resist and transmit the aerodynamic forces generated by the wings to the bird’s trunk.
Whoa! Where did all those things come from? That’s miracle talk! SuperDarwinMan just leapt over continents in a single bound! Look at some of the flight requirements illustrated in this video.
The supracoracoideus muscle—an engineering marvel—is like a pulley. It allows the strong flight muscles to be under the wings, keeping the center of gravity low, and yet attach on top of the wings through an opening. It’s an amazing design illustrated in Illustra Media’s film Flight: The Genius of Birds. That muscle, and all the other things in Biewener’s list, he says came about by evolution. Talk about begging the question!
Question-begging continues throughout the rest of the paper. Feathers evolved. Flight feathers evolved, complete with asymmetric shapes and interlocking barbules. Hollow bones evolved. The muscles evolved. Birdsong evolved. Navigation evolved. Biewener marvels at the functional excellence of each feature, but thinks they arose by the Stuff Happens Law. They evolved because they evolved. What other explanation is there? The only thing humans designed, in his view, is a threat to birds by habitat destruction and climate change.
Birds show a broad suite of musculoskeletal, neural and feather adaptations that enable them to fly with remarkable agility, to navigate complex environments, and sustain robust flight performance in challenging wind conditions. By evolving the remarkable ability to fly, birds greatly expanded their geographic foraging and mating ranges, improved their ability to avoid ground-based predators, and achieved seasonal migratory routes over inhospitable regions (the Saharan desert, for example). This resulted in the successful and explosive diversification of modern birds that we know today, which number in the range of 11,000 extant species, even though the future success of many species is threatened by human impacts and climate change. Hopefully, the robust and spectacular flight abilities that birds have evolved will enable most species to adapt to changes in climate and resulting changes in the timing and locations of valued food sources.
One pictures a flock of birds having a conference, thinking about the coming crisis. Maybe they use twitter. The chairbird tweets, ‘Well, we evolved how to fly. How can we evolve how to beat the humans who are threatening us with climate change?’ One committee of swallows swallows hard to think about how to use intelligent design to solve the problem. Another committee of swifts prays to the Darwin gods to send the lucky mutations swiftly so that they can evolve adaptations before they go extinct.
More Storytelling for the Birds
The avisphere (Florian Maderspacher, Current Biology, 24 Oct 2022). Instead of an atmosphere, Dr Maderspacher pictures a bird-sphere: an avisphere. Does her vivid prose rise aloft over the just-so story It evolved because it evolved, or does it fly over the cuckoo’s nest?
Birds’ breakout into three dimensions was ultimately enabled by that most crucial evolutionary innovation — the feather. Sometime during the Jurassic, complex feathers with interlocking barbules evolved in a few lineages of theropod dinosaur. In some of them, feathers grew big, creating an airfoil for gliding and ultimately powered flight. Tiled feathers form the outer hull of the bird, supporting aerodynamic shape. But feathers also double as formidable billboards in all sorts of colors and shapes, advertising the evolutionary prowess of their bearer. The swift-moving, elevated life off-the-ground allowed birds to become more daring in their visual displays. Some of them use their feathers in protean ways, assuming entirely new forms, like some birds of paradise do. The stunning visual beauty of birds may have tuned our own sense of aesthetics, if only in simple appropriation whereby humans from Bavaria to Papua New Guinea adorn themselves with (stolen) feathers. The same liberated exuberance evolved in the acoustic domain in the myriad songs and sounds of birds that dominate Earth’s soundscapes.
Primer: Flightless Birds (Florian Maderspacher, Current Biology, 24 Oct 2022). Florian is much better at describing how many lineages of birds lost the ability to fly. She describes ground birds, like ratites and rails, and swimming birds, like penguins, in some detail. Readers will learn surprising things about giant “terror birds” and species on islands that apparently lost the need to fly. But even the loss of flight exemplifies evolution, she says, using the word evolved and related words over 50 times. For inspiration, she quotes Father Darwin, who plagiarized Priest Lamarck.
The evolution and diversification of birds, a 150-million-year success story, has been sparked and driven by the many selective advantages offered by powered flight: escape from predators or unpleasant conditions, colonization of new habitats, or the ability to track food sources. The anatomy, physiology and life history of birds are shaped indelibly by flight: from their feathers to their muscles and skeleton, all the way down to their cellular physiology, and possibly even their genome size and lifespan. Why would natural selection favor the abandonment of such a precious skill? Darwin had encountered flightless birds in South America, such as flightless steamer ducks or rheas he saw being hunted by gauchos; and of course he came up with an answer in The Origin: it was either disuse of wings for flight, for instance on “oceanic islands, tenanted by no beast of prey” or in the case of the ostrich, “as natural selection increased in successive generations the size and weight of its body, its legs were used more, and its wings less, until they became incapable of flight”.
Adelie penguins (R. Harrington). Penguins use their flippers so efficiently, they are better explained as designed for swimming than as flight wings co-opted by evolution. All the other adaptations for feeding underwater would have also required too many lucky mutations to be considered probable.
The loss of a function is much easier to explain than the gain of a function, as Michael Behe explained in Darwin Devolves. Cave fish don’t need eyes in the dark, so vision devolved. Metabolism was no longer needed for some obligate parasites, so their metabolic machinery was discarded. And wings were a detriment on the Galapagos Islands for some species of birds and insects, so they degenerated into the stubby wings of the flightless cormorants and flightless grasshoppers. Even creationists believe that.
But could powered flight re-evolve in a flightless bird? Even Maderspacher’s faith in evolution is not that strong!
Molecular phylogenies have yielded several surprising relationships among paleognaths, although some uncertainty remains (Figure 2). Kiwis and the extinct moa, though both New Zealand natives, are not closely related; instead, kiwis are the sister group to the giant elephant birds of Madagascar (amazingly, elephant bird and moa bones yielded DNA for such phylogenetic studies). Moa, in turn, are closely related to the flighted tinamous, which, rather than being an outgroup to all flightless paleognaths, are thus nested among them. This placement would mean either that tinamous had to re-evolve flight from a flightless state or that flightlessness evolved several times in the ratites. The former seems nigh impossible, as prolonged flightlessness often leads to a reduction or loss of important components of the flight apparatus, such as flight feathers (remiges), wing bones, flight muscles and their characteristic attachment keel (carina) on the breastbone (sternum). All of this would have to re-evolve. Rather, flight may have been lost many times over in different ratite lineages, which, given how rampant flight loss is in the bird tree of life, seems far more parsimonious a notion.
Let’s try to understand her belief. It was easy for evolution to evolve powered flight in a dinosaur for the first time, when it had none of the genes or equipment for it, but “nigh impossible” for a bird that already possessed the genes for these things, once it stopped flying, to get it back. If it is “far more parsimonious” to accept a “notion” that flight was lost multiple times rather than regained, why is it not parsimonious to accept an explanation that powered flight is best explained by intelligent causes?
We know you have been holding it in, but now you can laugh out loud (LOL) and shout to the Darwin Party, “Lots of luck!” (LOL).