The Wieroo may be one of Burroughs's strangest creations, at least outside of Pellucidar (which presents such oddities as a gliding, aggressive stegosaurus). They resemble winged humans (with bat- or pterodactyl-like wings), emaciated to a corpselike state, to the point that the first outside-world humans to see them mistake them for ghosts. Their flying abilities are impressive for such large creatures, able to carry humans through the air while flying for miles at great heights.

OK, so what the heck are these things? And are they possible?

The Size Issue
The Wieroo are the same size as humans. This seems awfully large for a flying creature – the heaviest flying creatures alive today, the Kori and Great Bustards, are about 45 pounds (20 kilograms), and they're not very good fliers. The heaviest long-distance flyers are the condors, in the neighborhood of 22-24 pounds (10-11 kilograms).

However, in prehistoric times, flying creatures were much larger. The very largest pterosaurs were much larger than humans - some estimates put Hatzegopteryx at as much as 1000-1100 pounds (450-500 kilograms).  Argentavis, the largest known flying bird, would have been somewhere around 160 pounds (70 kilograms), in the human range. So it's not impossible. (Also, Caspak's pterodactyls seem to be significantly larger than real-world pterosaurs, "as large as a large whale".)

The Wing Problem
So … how do they have wings?

Land vertebrates only have four limbs. There are no extra limbs for the Wieroo to adapt into wings.

It's theoretically possible that through some strange mutation (probably in a homeobox gene) the Wieroo could have gained an extra pair of limbs. But the extreme unlikeliness of this (this has never happened in any group of land vertebrates), combined with the complex evolution required to turn the already specialized primate arm and hand into a fully functional wing, makes this an exceptionally implausible explanation.

Well, maybe the Wieroo's wings aren't strictly limbs. They're described as looking like pterosaur wings, which, of course, are derived from limbs: but the men marooned in Caspak were hardly anatomists.  Furthermore, pterosaur wings are supported by a single finger – perhaps these wings are supported by a rod-like structure rather than a full set of hand and arm bones.

In Out of Time's Abyss, Co-Tan reveals that, according to Galu belief, the Wieroo's wings started out as very simple structures, which the Wieroo developed into functional wings by many generations of killing any Wieroo with less-developed wings:

“There is a legend current among my people that once the Wieroo were unlike us only in that they possessed rudimentary wings. They lived in villages in the Galu country, and while the two peoples often warred, they held no hatred for one another. In those days each race came up from the beginning and there was great rivalry as to which was the higher in the scale of evolution … They were very warlike and very numerous, although they had long since adopted the policy of slaying all those among them whose wings did not show advanced development. 

"It took ages for all this to happen--very slowly came the different changes; but at last the Wieroos had wings they could use.”

- Out of Time's Abyss, Chapter 4

The Path to Flight
Perhaps the Wieroo first developed wing-like flaps as a genetic mutation that was not sufficiently detrimental to impair the ‘pseudo-winged’ hominids’ lives. It may have been something similar to the feline cutaneous asthenia that gives some cats large skin-flaps, leading to reports of “winged cats”.

 In a small village or tribe, the ‘founder effect’ could spread this disorder to most of the population within a few generations. After this, it could be taken up socially as a group identification. Once this has happened, the stage is set not only for possible killing of those proto-Wieroo without the condition (as Co-Tan states happened), or with less extreme forms, but also for treating it as an attractive trait. This could create sexual selection for larger ‘pseudo-wings’ – and runaway sexual selection can have pretty extreme effects (see the peacock's tail for an example). With sexual selection, and the killing of those with smaller or no ‘pseudo-wings’, there would be immense selective pressure to increase the size of these ‘wings’.

So, a population of ‘proto-Wieroo’ now exist with skin-flap pseudo-wings of large size. The gliding benefits provided even by very large non-rigid pseudo-wings would be marginal at best for a human-sized creature, though they might make shorter falls somewhat more survivable. Still, they would of necessity be a net handicap, only balanced out by the sexual selection in favor of them. To get functional flying wings, we need a drastic mutation.

So, some proto-Wieroo developed a mutation which produced extra bone in his ‘wing’ of skin. This might have been some form of homeobox gene mutation; these mutations can cause dramatic physical changes with only a single gene. A bone support would make the ‘wing’ appear larger and less ‘droopy’, so it might well be sexually selected for. With this new mutation – and a bony structure for the Wieroo wing – the stage is set toward actual powered flight – but it is still a long way off. It seems nearly unbelievable that this could occur in the time scale that human-like hominids have existed on Earth, though. No wonder Co-Tan said it took ‘ages’!

Another Possibility
A much wilder possibility is that the Wieroo were genetically engineered to have wings by a very ancient high-technology Caspakian civilization, old enough that no remnants of it remain visible. There is no evidence for this, admittedly, but the Wieroo are quite a stretch to be produced by natural evolution alone. And it might explain the weird reproductive cycle of other Caspakians as another artificial trait. Still, due to the total lack of evidence for any Caspakian civilization more advanced than the Wieroo (who seem to be on an advanced iron age level) this must remain a wild (but interesting) speculation…

Appearance of the Wieroo
Now, the Wieroo's wings will need very strong muscles to power their flight. In addition, the Wieroo arms and hands are stronger than one might expect from such a gaunt, corpselike creature – the one Bradley wrestles with is “possessed of enormous strength”, and Bradley only escapes its strangling grasp by clubbing it over the head with his gun. Since the Wieroo will almost certainly be lighter than humans of the same height, this is remarkable. The Wieroo's shoulder regions will be exceptionally heavily muscled, with muscles for both the strong wings and powerful arms. On the other hand, the Wieroo's legs will almost certainly be less muscular than those of humans. They probably do not walk long distances, flying anytime they are outside – every time they run, they instinctively use their wings to help them, which causes problems in dense forest. Human legs are built for long endurance; the Wieroo do not need this. Also, the Wieroo, to save weight for flight, will probably have minimal fat.

This probably explains the corpselike appearance of the Wieroo, commented on by Bradley and his party. Their legs and lower torso will have little muscle and less fat, and the shapes of the bones will probably be visible through the skin – causing them to look emaciated and near-skeletal. Even the muscular upper torso and arms will still probably have little fat; the hands will probably appear freakish to normal humans. (The Wieroo also seem to have disturbingly long fingers, heightening the bizarre appearance.)  Their faces, with little fat, will probably have sunken cheeks and a more visible bone structure.

Civilization
OK, how the heck did these guys get a civilization? They're on an island that's maybe, generously, the size of Guam. They've got writing, paper, complex architecture with multi-story buildings, metalworking… they're pretty advanced. Now admittedly Ajor's got an iron knife, so the Galu seem to be getting into the Iron Age.  But no cities, no building past huts, no writing, certainly no paper, anywhere else. Now for humans, writing only got invented independently a few times (Sumer, Egypt, the Maya civilization, China), and each time it was in a place that was quite populated for its era, the cultural heart of its whole region, with trade and the flow of information and ideas well outside its own boundaries. Egypt was relatively isolated by desert, but they still had lots of trade – with the Middle East and with Nubia (the land south of Egypt). Caspak’s got big cliffs thousands of feet high surrounding it, and it's totally unknown to the modern world. And Oo-oh is inaccessible to anyone from Caspak – Co-tan is convinced that escape from the island is totally impossible!

The Wieroo population can't be that large, only a few thousand probably, especially as their reproduction is entirely dependent on stealing Galu cos-ata-lo women. (See my Caspakian Demography essay for why I say a few thousand is the maximum.) So they simply do not have enough people to form a civilization, most likely – with so few individuals, a great genius or inventor will hardly ever appear. 

There is the possibility, though, that the Wieroo are simply more intelligent than human beings; Burroughs certainly suggests this at one point in Out of Time's Abyss (chapter 2): “Had natural selection produced during the countless ages of Caspakian life a winged monstrosity that represented the earthly pinnacle of man's evolution?” This is an unusual comment, as it appears in Bradley's speculations when he discovers the Wieroo have writing and paper-making; he also thinks of them as “the high culture of the human race within the boundaries of Caspak”, which is sensible, but there seems to be no obvious reason for Bradley to think of them as the earthly pinnacle (rather than merely Caspakian). Still, the possibility is definitely raised, and we must consider it. Co-tan seems also to think the Wieroo are of exceptional intelligence: her recited legend of Wieroo origins says that:

 “Slowly they commenced to develop certain attributes of the mind which, they considered,  placed them upon a still higher level and which gave them many  advantages over us, seeing which they thought only of mental  development” …

  “They make many wonderful things that we cannot make.  They think great thoughts, no doubt, and still dream of greatness to come, but their thoughts and their acts are regulated by ages of custom -- they are all alike -- and they are most unhappy”. 

- Out of Time's Abyss, chapter 4

This does seem to suggest that they are somewhat superhuman in intelligence (they had “many advantages” over the Galu, who are fully human, and had devoted themselves for ages to developing their minds). However, elsewhere in the book the Wieroo show no especial signs of super-intelligence, and Bradley is certainly able to outsmart them at times, so this seems unlikely. (There does remain the possibility that the Wieroo are simply poor at dealing with normal humans – Co-tan says in the same legend that “their minds became like stars and the rivers, moving always in the same manner, never varying”, which would perhaps make them easier for a human to trick.) 

It could also be suggested that the Wieroo's strong traditions allow them to retain knowledge from generation to generation more faithfully than humans, and thus that they have simply built their civilization bit by bit over a history of many thousands of years.  There is no real way to decide between these ideas, and the situation could be a combination of both.