During a visit to the Canadian Museum of Flight in Langley, the sheer variety of aircraft on display – from vintage biplanes of World War I to modern fighter jets – sparked a moment of reflection. Amidst these impressive machines and the history of aerial warfare, a fundamental question arose: why, unlike birds, can’t humans simply take to the skies on their own? This question delves into the fascinating realms of physics and evolutionary biology to reveal why human flight remains a feat of engineering rather than a natural ability.
One primary constraint lies in the physics of the human body. Birds are masters of the air thanks to a suite of evolutionary adaptations perfectly tailored for flight. Their lightweight skeletons, featuring hollow bones, significantly reduce overall body mass. Furthermore, they possess air sacs connected to expansive lungs, enhancing oxygen intake and buoyancy. Crucially, the balance between a bird’s wingspan and the strength of its wing muscles is precisely calibrated to its body size, enabling lift and propulsion. When we apply similar calculations to humans, the stark reality emerges: an adult male would necessitate an astounding 6.7-meter wingspan just to achieve lift-off, even before accounting for the considerable weight of such wings. The sheer physics of our build makes natural human flight an insurmountable challenge.
Beyond physical limitations, evolutionary history provides another critical piece of the puzzle. Birds are descendants of dinosaurs, and their journey to aerial mastery involved profound transformations over millions of years. The evolution of wings from forelimbs and the shift from the alternating gait of terrestrial animals to the synchronous flapping of wings are key milestones in avian flight. Recent scientific studies have illuminated a fundamental genetic basis for this divergence. Research has revealed that mammals and reptiles are wired for walking due to specific genetically determined arrangements in their spinal cords. In contrast, a mutation or absence of these genetic arrangements is central to birds’ ability to fly. For instance, the molecule ephrin-B3 plays a role in the spinal cord neural circuits of rodents, dictating their alternating limb movements for walking. Intriguingly, when this ephrin-B3 molecule is mutated in mice, they exhibit a simultaneous jumping motion, somewhat akin to the synchronized movements of a bird’s wings. It is theorized that evolutionary changes involving the ephrin-B3 molecule, or its complete absence in birds, paved the way for the development of neural networks that enable the coordinated flapping essential for flight. Humans, much like rodents in this respect, are genetically predisposed to a step-by-step locomotion, not the simultaneous movements required for natural flight.
While nature has not equipped us with wings, human ingenuity has triumphed in the realm of flight through technology. We have bypassed biological constraints by developing jet engines and aircraft, effectively conquering the skies through innovation. However, the fundamental reasons why humans cannot fly naturally remain rooted in our physical form and evolutionary path, a testament to the intricate and specific adaptations that underpin flight in the animal kingdom.
References
Haimson B, Meir O, Sudakevitz-Merzbach R, et al. Natural loss of function of ephrin-B3 shapes spinal flight circuitry in birds. Sci Adv 2021;7(24).
Innovation News Network. Analysis of neural networks explains why humans cannot fly. Accessed 4 January 2023. www.innovationnewsnetwork.com/analysisis-of-neural-networks-explains-why-humans-cannot-fly/12535.
Marathe P. Q&A: Why can’t humans fly? Yale Scientific. Accessed 4 January 2023. www.yalescientific.org/2013/03/qa-why-cant-humans-fly.
