The skies seemed to be the last frontier for our human race. While it was finally conquered by man over a century ago, other specimens of the animal kingdom conquered it over 150 million years ago (Rayner, 1988). Thus, giving way to one of the most remarkable traits in the animal kingdom, flight. Flight has evolved four times in the animal kingdom, in the pterosaurs, insects, birds, and bats (Rayner, 1988). For each of these groups, they evolved the ability to fly from separate ancestors that couldn’t fly. The evolution of phenotypic similarities between species is known as convergent evolution. It easily illustrates that populations can respond to ecological challenges in a predictable way (Stern, 2013). Perhaps the most remarkable case of convergent evolution is the origin of flight. The evolution of powered flight in vertebrates is marked by two major hypotheses. The oldest model is known as the ‘arboreal’ model or the trees-down theory. It was first proposed by Darwin who argued that bats evolved flight from a gliding stage, and that bats were not all that different from contemporary gliding squirrels (Rayner, 1988). The arboreal theory states that flight evolved when species living in trees or elevated areas developed the ability to glide. The second theory is known as the ‘cursorial’ model or the ground-up theory. This theory is marked by the belief that a ground-running biped evolved flight as an active-flier by running along the ground and jumping into the air. Over time, wings and feathers then developed to aid them in propulsion and flight eventually evolved. (Norberg, 1985). While an engineer is capable of designing a wing almost completely by scratch based upon theory, animal evolution is constrained by two rather restrictive rules. First, any new structure, such as wings or feathers, must be developed by modifying preexisting structures Secondly. change is often very slow and proceeds over infinitely small steps (Pennycuick, 2008). Archaeopteryx: A Controversial BeginningFlight continues to exist as a remarkable testament to evolution. With the advent of our own means of flying we have a deeper appreciation for this astonishing trait. The evolution of birds is a particularly remarkable story, as their evolution was much more complex than simply increasing the span of a patagial gliding wing. The development of flight feathers and their mechanical integration with the arm skeleton was a completely novel development. With the help of paleontology and quite a bit of luck, the beginning of the timeline for birds was discovered in 1861 in southern Germany (Young, 2007). The discovery was that of
Running Head: Evolution of Flight in Birds: Archaeopteryx3Archaeopteryx lithographica, one of the most important fossils ever to be discovered, which is shown below in Figure 1A-B (Mayr, Pohl, and Peters, 2005). Figure 1: (A) Ventral view of the Archaeopteryx skeleton with wing and tail feather impressions. (B) Ultraviolet-induced imaging of the preserved bone substance. (Mayr et al., 2005). Archaeopteryx was the earliest fossil found with flight feathers, thanks mainly to the astonishing preservation of its 150 million-year-old fossils in fine-grained lithographic limestone (Pennycuick, 2008). Archaeopteryx has been largely accepted as the transitional species between the dinosaurs of the Jurassic age and the birds we see today (Young, 2007). However, in recent decades with the discovery of new fossils and analysis with new technologies, the controversy surrounding Archaeopteryx’s claim to be the first known bird has grown to an all-time high. Many scientists are beginning to ask, did bird flight really evolve with Archaeopteryx? With growing support to knock it from its prestigious spot, an analysis of Archaeopteryx’s paleontology, anatomy, biomechanics, phylogenetics, and physiology is needed to exemplify its adaptations for flight and show how flight originated with this remarkable species. Physical Structure and Characteristics Archaeopteryx shared many primitive characteristics with theropod dinosaurs of the Late Jurassic period, such as a full set of teeth, a flat sternum, gastralia (belly) ribs, claws, a long bony tail and pinnate feathers (Alonso, Milner, Ketcham, Cookson, and Rowe, 2004). However, Archaeopteryx shared many novel characteristics with modern birds such as a furcula
Running Head: Evolution of Flight in Birds: Archaeopteryx4(wishbone), significantly reduced fingers, asymmetrical flight feathers on its tail and wings, and a wing feather arrangement that is similarly seen with modern birds giving it a high degree of powered flight capability (Alonso et al., 2004). One of the more striking charactistics of modern birds relates to their brain. Brain adaptations have a significant importance when it comes to flightas it is only accomplished in thanks to complex neural control networks and other specialized adaptations (Jerison, 1968). Through the three-dimensional reconstruction of the braincase of Archaeopteryx with computed tomography (CT) scanning, it was determined that Archaeopteryx possessed several neurological and structural adaptations that are necessary for flight (Alonso et al., 2004). For example, structural adaptations in the brain and inner ear supported a more dominant sense of vision, as well as enhanced auditory and spatial sensory perception (Alonso et al., 2004). The forebrain of Archaeopteryx was also shown to be enlarged, suggesting a more significant somatosensory integration with these newly enhanced senses that are required for a lifestyle of flying (Alonso et al., 2004). Figure 2: Encephalic volumes (ml) of Archaeopteryx, birds, and reptiles in relation to body mass (g). (Alonso et al., 2004)Paleontological observations of the brain were also significant in suggesting Archaeopteryx as a transitional species. Archaeopteryx’s brain was found to be clearly avian in its external form, but also similar in type and structure to that of reptiles. The similarities in
Running Head: Evolution of Flight in Birds: Archaeopteryx5regard to living birds were most striking, as there were many. In Archaeopteryx, although the cerebral hemispheres were slightly narrower, they were similar in general contour to living birds (Jerison, 1968). The optic lobes of Archaeopteryx were medial and below the cerebrum as they are positioned in living birds. Also, the cerebellum was in contact with the posterior edge of the cerebrum and dorsal to the optic lobes as is similarly observed in living birds (Jerison, 1968). An endocranial cast gave further evidence for Archaeopteryx’s avian brain adaptations as the cleavage between forebrain and midbrain was clearly shown. The endocranial cast of reptiles would not show this cleavage since reptilian brains do not fill the entire cranial cavity (Jersion, 1968). Furthermore, in terms of the size of its brain in relation to its body mass, Archaeopteryx was found to be intermediate between bird and reptilian brains as is shown in Figure 2 above (Alonso et al., 2004). Another in-depth study using high-resolution computed tomography was able to estimate and compare cranial volumes of living birds, Archaeopteryx, and a number of non-avian dinosaurs that were phylogenetically close to the origins of flight in birds (Balanoff, Bever, Rowe, and Norell, 2013). This study was rather revolutionary as they were able to volumetrically partition different regions of the brain in order to further examine how Archaeopteryx’s brain adaptations played a role in the origins of the ability to fly in birds (Balanoff et al., 2013). This method also allowed the detection of previously unknown evolutionary complexity when it came to this topic. Biomechanics and Aerodynamic Capabilities Although Archaeopteryx may have had the correct characteristics giving it the capability to fly, was it really able to fly? Through close examination of the biomechanics and aerodynamics involved in avian flight, we can uncover the true flight capabilities behind Archaeopteryx. The evolution of gliding occurred many more times than flight, especially when it comes to powered flight. The evolution of wings, and feathers as well, allow flying animals to generate both thrust and lift by moving their wings relative to their body. Recent research using innovative technology has uncovered a critical feature of Archaeopteryx’s mosaic anatomy unifying non-avian dinosaurs and flying birds in the context of locomotion (Voeten, Cubo, Margerie, Röper, Beyrand, Bureš, Tafforeau, and Sanchez, 2018). Synchotron microtomography allowed virtual 3D reconstructions of the bones of Archaeopteryx to be created with extrodarinary quality. From the images, it was clear that the walls of the wing
Running Head: Evolution of Flight in Birds: Archaeopteryx6bones of Archaeopteryx were much thinner than non-avian dinosaurs and were instead remarkably similar to conventional bird bones (Voeten et al., 2018). The cross-sectional geometry of Archaeopteryx’s humerus and ulnar bones shared more biomechanical and physiological flight-related adaptations with modern birds than was previously known (Voeten et al., 2018). Furthermore, with the primitive shoulder structure of Archaeopteryx in mind, data analysis showed that the bones of Archaeopteryx plotted closer to modern birds like pheasants, which occasionally use active flight to cross barriers or evade predators (Voeten et al., 2018). The vanes seen in the primary flight feathers of Archaeopteryx were also observed to conform to the same asymmetric pattern seen in living birds (Feduccia and Tordoff, 1979). Vanes are the flat parts of each side of the primary shaft of a feather. The asymmetric pattern was shown to have critical aerodynamic functions that can be interpreted as having selectively evolved in the context of flight (Feduccia and Tordoff, 1979). Further evidence showing Archaeopteryx’s ability to fly was presented in a study calculating the thrust generated by the wing of Archaeopteryx from an aerodynamic perspective(Burgers and Chiappe, 1999). The limited volume of its pectoral muscles led to Archaeopteryx only being able to flap its wings at a low amplitude, as it was lacking the pulley-like action of the supracoracoid muscle seen with modern birds (Burgers and Chiappe, 1999). Nonetheless, its wings gave it enough thrust to enable flight after first running to takeoff (Burgers and Chiappe, 1999). Once running, Archaeopteryx continued flapping and the increased residual lift relieves the hindlimbs of its body weight, enabling Archaeopteryx to run even more faster, which creates even more lift (Burgers and Chiappe, 1999). At a certain point, residual lift overcomes its total weight and take off is achieved, and wing thrust is now capable of generating enough velocity for sustained lift (Burgers and Chiappe, 1999). This is shown below in Figure 3. These results also provide solid evidence supporting the cursorial model of flight for Archaeopteryx (Burgers and Chiappe, 1999).
Running Head: Evolution of Flight in Birds: Archaeopteryx7Figure 3: Progression of residual lift and net thrust throughout the take-off run of Archaeopteryx (Burgers and Chiappe, 1999).Phylogenetic Tree IssuesThe position of Archaeopteryx as the earliest known bird in the phylogenetic tree is another subject of debate in the controversy following the species. While it remains as the first known bird in numerous phylogenetic trees, recent discovery of other closely-related non-avian feathered species has led to some scientists changing its phylogenetic positioning. The recent discovery of an advanced feathered theropod dinosaur, Xiaotingia, in China sparked renewed debate as phylogenetic analyses using this new species held some shocking results (Xu, You, Du, and Han, 2011). The study removed Archaeopteryx from its placement with birds and grouped it instead with a family of feathered theropod dinosaurs known as deinonychosaurs (Xu et al., 2011). This new phylogeny had significant implications for the scientific community and many called for further investigation into this new phylogeny, with some pointing out that the study was weakly supported with only parsimony methods being employed (Lee and Worthy, 2012). A new study was quickly performed using maximum-likelihood and Bayesian methods applied to the same data set from the original study. These methods are not as frequently used in
Running Head: Evolution of Flight in Birds: Archaeopteryx8paleontology and morphology, but are widely used and accepted in molecular biology (Lee and Worthy, 2012). With these methods, instead of considering all anatomical traits to be equally informative, greater weight was placed on slow-evolving characteristics. This was done to minimize the effect of biological traits that may have evolved independently (Lee and Worthy, 2012). The phylogeny analysis using these new methods robustly restored Archaeopteryx to its position as a basal bird as shown in Figure 4 below (Lee and Worthy, 2012). While Archaeopteryx may have gained back its spot as the earliest known bird in this case, future discoveries are likely to question its position in the phylogenetic tree once again. Figure 4: Archaeopteryx united with other birds, with a posterior probability of 1.0 in Bayesian analysis (Lee and Worthy, 2012). Gaps in the Literature and Strategies for the FutureThe analysis above shows how Archaeopteryx presents many lines of evidence suggesting how it is the earliest known species of feathered dinosaur capable of flight, and thus was the earliest bird. However, evidence is quickly mounting to potentially knock Archaeopteryx from its perch as the earliest known bird. How and why Archaeopteryx’s feathers evolved deserves further investigation as it is one of the primary features attributed to modern birds and likely gives it significant capabilities enabling flight as previously discussed (Feduccia and Tordoff, 1979). While evidence may show aerodynamic features of their feather, their use by Archaeopteryx in terms of whether they evolved strictly for flight is less understood. Their feathers may have instead been used for regulating body temperature or were co-opted from this use into flight (Cowen and Lipps, 1982). Another topic that had very little research dedicated to it were paleoenvironmental studies. One of the most significant principles of evolution is that genes can be shaped by the environment organisms live in. This is often known as epigenetics and this interaction may be one of the most powerful and influential evolutionary forces in evolution. Understanding the surroun