Why can’t humans fly?

While visiting the Canadian Museum of Flight in Langley, I saw extensive displays of aircraft, some from the First World War, some from the Second World War, and even more from recent times, including modern fighter jets and their engines.  

Thinking about the exhibits and all the lives lost in wars over the decades, my mind flew back to the start of the Second World War in 1939 when German forces invaded Poland. We heard the news that the war had begun on our radio, but we felt safe, our town was far away from any war activity. On that day, I was 9 days shy of being 10 years old and my parents and I went for a brief walk in a neighboring park. It was a quiet sunny Sunday. Suddenly we heard the loud noise of a low-flying small airplane. After a few moments, a second small airplane appeared behind the first one. We heard the very sharp tat-tat-tat-tat of a machine gun as we stopped, amazed at this evolving scene. The first airplane blew up and large pieces of it fell toward the ground. A Hungarian Air Force plane had shot down a Serbian plane; on that day the plane and its pilot were considered to be the enemy. 

With these memories coming back, and thinking of the human misery of war, I asked myself: why can’t humans fly without all that machinery? 

There are two main reasons why humans cannot fly, and both are tied up with evolutionary factors. One reason is the physics of the body. A bird can fly because it has a lightweight skeleton with hollow bones, air sacs connected to huge lungs, and its wingspan and wing muscle strength are in balance with its body size. Calculations of the ratio between a human’s size and strength reveal that an adult male would require a wingspan of 6.7 metres to be able to fly, never mind the weight of those wings!

The other reason has to do with natural history: birds evolved from dinosaurs. Their development into flying creatures evolved through formation of the wings from the forelimbs and changes in the alternating gait of forelimbs’ characteristic of most terrestrial animals to the necessary synchronous flapping of wings. 

Recently studies revealed a fundamental genetically related neurological reason for this change. Investigations uncovered that mammals and reptiles are able to walk due to a genetically determined arrangement in the spinal cord. Birds’ ability to fly is determined by a mutation of or absence of those genetic arrangements. For example, in rodents the genetically related molecule ephrin-B3 is involved in neurological arrangements in the spinal cord that dictate a stepping motion from left to right with their front and back limbs. Interestingly, a mutation of the molecule ephrin-B3 in mice demonstrates a simultaneous jumping motion of both sides, somewhat similar to that of a bird. It is presumed that in the course of evolution, mutations in, or complete absence of genetic coding of the molecule ephrin-B3, supported birds to develop the alternate neural networks that made it possible for the simultaneous flapping of wings. The human is a bit like the rodents, one step at a time. 

Feeling a bit more informed if not any wiser, it occurred to me how incredibly complex biology is, and how the neural evolution of the human brain basically came to a point to say: forget about ephrin-B3 whatever that is or does, we will develop jet engines instead, beat all the birds, and think of how to drop bombs on other human beings from the air.
—George Szasz, CM MD

Suggested reading
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.

This post has not been peer reviewed by the BCMJ Editorial Board.

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