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Dr Samuel Poore, a reconstructive surgeon, has an interesting article in The Journal of Hand Surgery about how to transform a human arm into a bird wing (why that would be a popular idea, I don’t know). Read the write-up at New Scientist if you don’t have journal privileges. He ends up concluding that it is too hard (at least if the wings were intended to facilitate flight), and ends with this advice:
Despite advances in surgical technique that could theoretically lead to the ability to construct wings from arms, it is evident that humans should remain human, staying on the ground pondering and studying the intricacies of flight while letting birds be birds and angels be angels.
Unfortunately, even though I read the whole article, I do not think it is at all ‘evident that humans should remain human’ (emphasis mine). Plus, I don’t want to. Poore provides some good reasons why we can’t yet build working wings out of our arms, but that doesn’t mean we shouldn’t do if we one day can.

There are a few issues that I would raise regarding Poore’s reasons for not being able to give humans any wings:

1. Turning our arms into wings is not what most people would want.
Just look at Archangel from X-Men. Most fictional conceptions of flying humanoids have wings AND arms (except maybe harpies), because arms and hands are just so amazingly useful to human beings that I don’t see why anyone would want to give them up permanently for the mere advantage of flight.

2. What about bat wings? Or pterosaur wings, or even insect wings!
The article is titled ‘ The Morphological Basis of the Arm-to-Wing Transition’, but doesn’t look one of the three historical arm-to-wing transitions – that of the chiropterans (bats). Going down this road would overcome the problem which Poore considers the most difficult – how to give humans the ability to grow feathers. Being mammals, we have a skin structure that probably wouldn’t support feathers (though Poore did not consider the possibility of genetically modifying our skin, so that the areas supporting feathers would have avian-like skin structure). Plus, bats have clawed wings, so by increasing the number of digits on our hand, we might be able to have bat wings and still maintain some sort of ability to pick things up and manipulate objects. I don’t think people would mind looking more like gargoyles rather than angels, would they?
3. Forget about the trabeculae?
Bird, at least the big ones that fly, have rather hollow bones. Mammals have these hollows in their bones too, but not to the dramatic extent that birds do (a bird’s bone has a density of about 0.3g/cm3, whereas a human would average 1.5g/cm3). Bones are mostly air-filled, but reinforced by cross-beams known as trabeculae (Latin for ‘small beams’). This makes the bones of large flying birds extremely light. But Poore overlooks this when he states:
…for a 170-lb human to achieve any type of flight, he or she would need wings with approximately 20 square feet of surface area.
Any person seeking to achieve flight will likely seek hollow bones too, decreasing their weight (though not by much, because bones only represent ~15% of total body mass – so reducing bone weight by 80% will only reduce total body mass by 12%).
If wings are ever to be part of a future human being, the scientists of that age will probably also will look at avian lungs too, which are far more efficient for their size than mammalian lungs. This enables smaller avian lungs to do the same work as big mammalian lungs, will also reduce the weight of the body.
So, why can’t I have wings?
Basically, the reason why humans (probably) can’t ever have any functional wings is that we’re too big.

The power required to flap the wings enough to raise the body is the major roadblock in giving humans wings. Birds, bats and pterosaurs have very large pectoral muscles, (making up about 30-35% of their body mass), which powers their wings. If you think you’ve got enough muscle to flap wings, try doing a push-up and generating enough force to lift your body off the ground and imagine doing that repeatedly. Maybe you just want to glide? Well, lie on your stomach and spread your arms out as far as they can go, and push with your arms enough to raise your chest just off the ground (without bending at your elbows), and hold that position for as long as you can. To glide or fly, you have to use only your arms/wings to hold your entire bodyweight above the ground, such as in the gymnastic position known as the Maltese Cross. The human body is not adapted to this position, and only a few well-muscled gymnasts can hold it for any length of time.

The addition of the massive chest muscles that a human would require for sustained powered flight would add too much weight, which would necessitate increased wingspan and therefore increase the power needed to fly, thereby requiring more muscles…and so on. There is a point where you simply cannot get any more advantage by adding more muscle mass. But it’s not known what this upper limit is. Existing flying animals don’t get much bigger than 20kg (such as the Great Bustard), but some extinct animals might have been able to fly and were quite large, such as the ~85kg bird Argentavis and the 90-200kg pterosaur Quetzalcoatlus (and both of those animals are predicted to have been mostly soaring animals). Evolution has so far gone through at least three different pathways to sustained flight (birds, bats and pterosaurs), and proven sustained fliers all seem to have very strict constraints on body mass.

So because humans are at least three times heavier than a flying animal should be, maybe the ability for humans to fly will only be possible when we have bionic muscles of super-strength, that are lighter and stronger than biological muscle. Failing that, we could make humans two-thirds smaller. But small humans with large pectoral muscles – they’d look less like fairies and more like…well, birds.

As for wing size, Poore estimates a wing area of 20 square feet (1.85m2) for a 170lb (77kg) human. This seems surprisingly low, as Argentavis weighed about as much as a human but had a wing area of 7m2 (75 square feet) and a wingspan of up to 8m. But Argentavis had a relatively typical wing loading of 11.5kg/m2, and the maximum known for birds is 25kg/m2, so perhaps it could have done some very basic flight with wings half that size. Given his superhuman strength, Archangel’s 4-5m wingspan might be enough, but a better estimate would be almost around 7m for a winged human. But without the necessary pectoral muscles for powered flight, even 7m wings on a person would only be usable for gliding.

Anyway, I’m not saying it is impossible – a gliding human is probably possible (but far easier with a hang glider). But the important thing is, there is no reason I can see of why we shouldn’t try to give humans the ability to fly (and keep them looking mostly like humans). Many humans have wished they could fly, and I don’t see why we couldn’t allow them to grant that wish if it ever becomes possible. There’s nothing morally wrong with granting that wish.

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