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As part II of anatomically correct myths here we have the Angel. Angels appear in many cultures and are most commonly seen as simply winged humans. Is there a realistic way that angels could evolve? Using earth or near earth biology how close could I get to the classic Angel? Is there a reason that a Angel couldn't evolve? I imagine that this one will be very hard to justify, so any primate with wings can count as an Angel not just humans.

A list of all of the Anatomically Correct questions can be found here:

http://meta.worldbuilding.stackexchange.com/questions/2797/anatomically-correct-series/2798#2798

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    $\begingroup$ I think a lot of the arguments against a griffin evolving will apply here. How and why would a primate evolve an extra pair of limbs (wings) and how would their body support the reduction in bone density and increase in muscle mass? $\endgroup$ – KillingTime Sep 10 '15 at 21:21
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    $\begingroup$ If you want believable evolution, you have to always keep in mind that evolution is incremental. A species doesn't just suddenly sprout wings; each step toward those (somewhat) fully developed wings must, in itself, provide an advantage or at the very least not a disadvantage to the individual, in terms of ability to produce offspring. If you can come up with a way to explain that, then and only then in biology you can explain nearly any crazy end state that you can think of. IIRC the currently established theory is that wings started out helping with jumping; flying came much later. $\endgroup$ – a CVn Sep 10 '15 at 21:29
  • $\begingroup$ (And of course, the offspring must itself survive long enough to reach reproductive age, and reproduce. It's no use producing a lot of offspring if they all die at an early age.) $\endgroup$ – a CVn Sep 10 '15 at 21:36
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    $\begingroup$ Wings were also very useful for keeping eggs warm in smaller animals which couldn't do the job just by sitting on them. Of course, there are still the big problems with six-limbed animals and feathered mammals and large flying things. (You could pretty much just copy-paste my griffon answer, find and replace griffon with angel, and have a good answer. $\endgroup$ – Saidoro Sep 10 '15 at 23:07
  • $\begingroup$ I like the Diablo 2 (video game) version of an angel. The things on the back of the angel are not wings at all, people drew them as wings in ancient pictures, maybe because they didn't have another thing to associate to it, but the real things were something more practical (not less fantastical). $\endgroup$ – Hatoru Hansou Feb 8 '16 at 23:55
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If you mean how you can create anatomically correct models of flying humans, then this question can be scientifically and rationally answered. However if you are asking the anatomy of an angel, you are asking an absurd question. You are trying to judge something which is not scientific, under scientific rules. Here I will go on to explain how can you create (in a novel of course) flying humans.

A - Skeletal Structure

You are going to need a very very very light skeleton. That's a prerequisite. We are talking about extremely hollow bones here. At least bird-grade bones or better yet, if you can get pterosaur-grade bones. Pterosaurs were those huge flying monsters that ruled the skies in the times of dinosaurs. Their bones were extremely lightweight. A huge pterosaur (3 times the size of a human) would have bones completely hollow and no more than 1 mm thick. Yeah, now we are talking.

B - Wings

If you examine any flying creature you will find out that their width is greater than their length. That is, you open up their wings, the wingspan will be greater than their length. So a human with 6 ft height would have a wingspan of at least 11 feet when the wings are fully open. Of course the human will fold the wings and keep them on their back just like birds, when not in use.

The next issue is the wing surface area. Are you going to opt for membranous wings (like pterosaurs or bats), crustacean wings (membranous, but different type, ones you see on insects) or feather wings (self explanatory, all bird wings are these types)? I think crustacean wings are not an option as they cannot take high air pressure and would tear open quickly.

Bird feathers are very strong and offer the best air-thrust method, but they are also heavy. Yes! For the scale of human flight, bird feather WOULD be heavy. If you want to use bird feathers for human flight, the shoulder muscles would have to be really really powerful. In case of membranous wings (bat/pterosaur grade), these are relatively light and you can make do with lesser shoulder muscles.

C - Flight Muscle Attachment Points

If you want your flying humans to have wings AND arms separately, then things could get slightly complicated. You are going to have to form muscle attachment points for the wing muscles. These have to have an anchor point on the chest region, like all birds and pterosaurs. So your flying humans are going to have really really stocky and strong chests.

D - Flight Posture

How are your flying humans going to look like, in mid-flight? Are they going to fly in horizontal body posture (like most birds) or vertical posture (the way they stand, looking somewhat like a hovering humming bird)? I cannot answer this myself as it involves a lot of complex aerodynamics, but I think a horizontal flight posture is more likely.

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    $\begingroup$ Also note that, for horizontal body posture, the wings must attach just above the pelvis, not at the shoulder blades. This will complicate the question of attachment points and muscle locations. $\endgroup$ – WhatRoughBeast Oct 17 '15 at 21:44
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    $\begingroup$ Not necessarily. Birds have their wings attached to their shoulders and they fly pretty horizontally while keeping a vertical posture upon landing (crane, stork etc). $\endgroup$ – Youstay Igo Oct 18 '15 at 3:29
  • $\begingroup$ Birds also have 1)long necks which provide a counterbalance, 2) much shorter torsos, and 3) much lighter legs which fold forward to bring the mass under the center of lift. A human with wings attaching at the shoulder blades would never be able to fly with the torso horizontal. $\endgroup$ – WhatRoughBeast Oct 18 '15 at 17:49
  • $\begingroup$ Not completely horizontal, true. But ... 1- the muscles would be attached on a large surface area, not small and would probably go half the way on the back. 2- the power of thrust is the determining factor of the posture. With a powerful enough thrust, a horizontal posture can be maintained even with balance discrepancies. $\endgroup$ – Youstay Igo Oct 18 '15 at 18:41
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    $\begingroup$ A "plausible angel" with leathery wings and a hugely muscled physique sounds rather more like a demon, doesn't it? $\endgroup$ – Justin Jul 29 '16 at 13:44
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I think the question is could a six-limbed (2 arms, 2 legs, 2 feathered wings) flying humanoid evolve naturally on an Earthlike planet. The answer, I think, is yes*. I don't think the end result would look much like a human though.

Environment

Environmentally, I think that you'd probably be looking at a near earth-sized moon orbiting a gas giant, heated by relatively mild tidal forces. Lots of volcanoes, quakes, new landmasses erupting out of the ocean, tsunamis, etc. etc. I would also expect that gravity is a bit weaker, and the atmosphere a bit thicker with a noticeably higher oxygen content. That would mean you've got a little more room to play with in body size, but they wouldn't be able to fly on Earth.

The key is that I would expect six-limbed life to be common in charismatic megafauna. Basically, I'd expect stuff analogous to centaurs, lammasu, sphinxes, and dragons (not large ones, obviously) to be fairly common on that world. The obvious advantage to flight on this world is that it allows them to escape the disasters much easier, and colonize new land masses very quickly.

Physicality

As I understand it, the main body engineering issue with powered flight is the ratio of surface area to volume/mass to propulsion. To get moving you need propulsion, to stay aloft you need a huge surface area with a narrow profile compared to your volume/mass, but the bigger your surface area, the more mass you have. The more mass you have, the more propulsion you need and the more surface area you need, which costs more mass... and basically the bigger you get, the greater the difficulty in powered flight.

I would expect that each wing is about as long as their body. So if they're about 5 feet tall, I would expect a wingspan of about 10 feet, before any feathers. Feathers would probably add another 2 feet in length on each wing, giving them a functional wing span of 14 feet. I would expect that their manipulating limbs, the ones that look like our arms, to be quite short. I'd guess their fingertips would touch their hip bone. Their resting position for the arms would probably be curled up to their chest, like bird legs. I would also expect that their legs would perform much the same function as a bird's tail, and thus have a rather impressive amount of plumage. Their feet would almost certainly be more like a hawk or an owl's feet, with a larger central "ankle" area and stubbier toes- no talons though. I imagine that they'd stand primarily on their toes. Their face would probably look a lot like a canine's, but with larger eyes, smaller ears, and a much larger brain pan. I'd also imagine that their entire body has some form of feathers covering it.

Biology

They'd need a huge heart, massive wing muscles, hollow bones, and very very efficient lungs, even with the higher oxygen percentage in the atmosphere. They'd fly like vultures or geese, which would help them save energy. They'd need to be primarily carnivorous, to fuel their metabolism, but I could also see them going after seeds and other high-calorie forms of sustenance.

I suspect that you'd see a lot of similar adaptations to birds on earth, with their red blood cells having nuclei and able to see into the ultraviolet. Heck, these 'angels' would either be like birds, and lay eggs, or be like marsupials, and have a stretchy pouch for their partially developed babies. I doubt they'd give birth the way a human does.

If they're tool users, I'd imagine nets to be their primary tool for hunting. In the air, I'd expect lances, bolas, or javelins. They may also have something like a knuckle or a spike on their wings, which would allow them to use their enormous wing strength to defend themselves on the ground.

So basically, you'd end up with something more akin to nightmare fuel than angels if you tried to engineer a biologically successful hexapodal flying creature.

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Couple thoughts on this...

  1. Angels aren't bred. Meaning, they don't have to mate, have baby angels, evolve, etc. Instead angels are designed and created, like robots, genetically engineered clones, or something, to be servants for God and to perform tasks; be messengers, be protectors, etc.
    This doesn't mean they are unthinking machines, and can still have sentience and will to a point. Sort of a weak AI.
  2. Angels aren't confined to 3 dimensions. By existing outside of the 3 dimensions our senses can perceive, angels are able to move unseen, pass through (around, over) physical objects and barriers, and do the other things that angels do, while still being able to interact with our perceived dimension as they need to in order to fulfill duties.
    The best way to picture this is with Flatland. If our world was Flatland, we would see a wall as a barrier that can't be passed through without going through a doorway. But an angel existing in 3 dimensions could "rise above" the flat plain and go "over" the wall. Any angels above the two dimensional plain would be undetectable by any flat instruments since they are unable to look outside of their dimension, but any angel wanting to interact with Flatland would only need to descend or reach down into the plain, or even hover just above it and whisper into your ear (or subconscious).
    The book Spaceland by Rudy Rucker has a lot of similar ideas, where a being living in a fourth dimension interacts with our 3D world.
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    $\begingroup$ Make the angels be designed is, I think, the only way to avoid the challenge of having to find a credible evolutionary path that results in winged humans. More than coming from a higher dimensional space I prefer to say that their known body is like a 3D avatar that they control so I don't have to imagine how a 4D/5D being may look. The wings then may be to impress people (note that we are dodging the question, not trying to answer it). Any mythical impossible creature can be an avatar of a 4D being playing with us. $\endgroup$ – Hatoru Hansou Feb 9 '16 at 0:19
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    $\begingroup$ @HatoruHansou I see the other dimensional part as a way to for them to get around do their duties, influence people and events, and all the other stuff that angels are tasked to do, but be completely apart from our reality where they could be detected unless they want to. And they would be able to control which parts of their aspects interact with us. The wings could be functional both for traveling in space, but also possibly between layers of reality. I think it's possible to think of them as 3 dimensional but with the option of not being in the same layers of reality, instead of avatars. $\endgroup$ – AndyD273 Feb 9 '16 at 15:46
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A biggest problem here is that a classical angel cannot fly with classical angelic wings. They are just too small. A hand glider gives a rough idea what wings are required to have a human airborne. And, obviously, nobody can really flap such wings.

If you are OK with flyless angels, the sexual selection can go long way developing purely decorative appendages.

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The first question to ask is how do they get wings in the first place? There are three ways i can think of this occurring from a realistic standpoint.

1) All land animals evolved from a being with 6 limbs, Two of which became wings

2) An evolutionary ancestor had an adaptation which later became wings

3) Wings are technologically grafted onto the creature/added to their DNA and over time adaptations occur which help support them.

Well, 1 is in my opinion the boring solution so i won't cover that, and 3 requires them to be both technologically advanced enough to graph wings (biological or mechanical) and have a valid enough reason to want wings in the first place. So i'll be exploring 2.

Now, for my own ideas i decided to look into the mammalian evoltionary tree. Initially i planned on having a mammalian ancestor gain a neutral mutation (one that doesn't positively or negatively affect their chance of survival) but then i came across Pelycosaurs (early, mammal-like reptiles). More specifically, Sail-Backed Pelycosaurs. and i though to myself 'Now If i just added a second sail, these would be an ideal base to give mammals wings'. So i looked into other animals (most extinct) who had two fin-like or sail-like structures going down their back, and hey presto i found one (i cant remember its name at the moment however). So, having two sail-like wing stuctures are possible in reality.

From here the question becomes Why do they have these structures? and why keep them? Well, when in doubt about creating a certain adaptation or stucture, unless it gives them severe physical issues, have 'Mating Displays' be the reason it exists. A way to showoff that X creature is tougher and more powerful, and therefore able to produce offspring that is more likely to survive.

Ok, we have the How and the Why. Now to decide how they become used for flight and what appearance they will take, Will they be used to showcase how high X can jump? Will they be useful in catching prey? Will X use them to increase the distance they can jump/glide?. What will they look like? Fully fused, Feathered and bird-like? Made of a thin membrane like a bats? a little hairy like a pterosaur? or structured like an insect's wings?

Now other things to consider with this approach: what adaptations do you want them to have? Now, if you want them to be basically like a human, just with wings, then the creature they descend from needs to have been a climber at some point -> This is so dexterous, grasping 'hands' can form, allowing for complex tool use and creation. If they are bipedal, then there likewise needs to be a reason to which bipedalism is necessary and practical. Lots of little things (if you want to get hugely in depth about it) that are mainly covered by the topics above.

Most of this came from my own exploration into the topic of 'humanoid beings with wings' but there are likely other possible answers as well.

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As an addition to @Youstay Igo because a comment wouldn't work:

Evolution of an Angel:

One of the few ways I can see an Angel-like creature starting to appear, is in a type of polynesian archipelago with lots of vulcanic features as well as hard to traverse terrain on the islands.

At first the proto-angels that evolve will start using simple tools. If most of the islands lack treewood capable of sustaining good boats, for example because the wood is heavier than water (not sure if possible in that climate), then a large portion would need to swim to colonize other islands and perhaps survive off the fish and sea-fruit they eat there. Recently scientists discovered that mankind is still evolving, with one "species" of human living so exclusively in (shallow) waters that they have trouble walking on land and adapt to the water well with genetic mutations that support their lifestyle. This evolution isn't unheard of, as Dolphins and Whales are thought to have been land mammals that returned to the water. Now take this evolution further: They will retain their arms and hands to keep using the simple tools they have access to, but need to develop easier ways to swim. They won't just evolve flaps between their fingers, but their shoulderblades could create extrusions to form a crude type of fins, which will later evolve to have musculature for easier steering in water and the basics of "flight" under water like Pinguins who "fly" under water. These fins would become the precursor to wings.

As the evolution goes on and more and more of the islands in the area become inhabited, some of these proto-angels will need to start moving inland for food and sustenance. Whether it be predators they try to escape from by climbing trees ASAP or having to scale and jump across dangerous terrain, those fins could start to evolve to support jumping into the tree's and over the terrain, evolving the proto-angels to create a lighter skeleton and more musculature for their proto-wings. Because they are still tool-using creatures and the islands will eventually start evolving wildlife and plantlife that is suitable for intelligent life to start using, intelligence will start becoming a larger part and their arms will also start evolving, without devolving the wings necessary to remain capable across the islands. In fact, the first truly flying angels would be able to scout out islands, settle them and benefit from any foodsources far better than any other proto-angel that came before. The intelligence could also be the defense mechanism to the predators that stalk the thinner and lighter skeleton Angels. This could be the method to Angel evolution.

How would an Angel be build up?

Birds have wings that are attached to the torso with truly massive muscles compared to the rest of the bird. These wings would need to use a kinematic chain similar to humans do when jumping. This kinematic chain would connect the arm musculature to the wings and allow strength from the chest muscles to be passed through to the wings. This would mean that the arms would have a lower movement range to compensate.

Materials wise, the Angel is likely in need of some special stuff. Besides a bird-based lung system and a ridgid chest, it would need some special stuff to keep light and strong. For example, by making all ligaments spidersilk-based you can reduce the amount required and thus the mass, and bones would need to be some carbon-enhanced material such as incorporating Carbon Nanotubes into the normal bones for extra lightweight strength. Spidersilk based products might also be the option for the surface area of the wings, if you like the feather aestetic the feathers themselves could use spidersilk to remain light, thin and still strong.

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Considering that a canonical angel of most religions is not as much a winged human as an energy being capable of manifesting to human witnesses, the issue of their anatomy and physiology becomes a moot point. I would propose that what human eye perceives as wings could be the two overlapping cocoon-like energy fields with a humanoid silhouette in the middle, thus, an illusion of a winged human. As far as evolution of such beings, it's up to the author, since we're talking about creatures evolved far beyond their material body.

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