To the best of our knowledge, true flight has only evolved a few times on Earth. Bats, birds, now-extinct pterosaurs, and flying insects have all filled similar evolutionary niches, and it's likely they all got there in similar ways.
We understand the evolution of birds the best of those four groups (or at least think we do), due to well-documented fossil evidence uncovered in the 20th century. Tiny cousins of Velociraptor started climbing trees and ambushing whatever walked underneath. The ones who could jump farther from their hiding spots were less likely to starve and more likely to reproduce, so scales gradually grew into longer and softer feathers that produced a gliding effect. All the climbing made forelimbs longer and stronger, until they became capable of making the transition to true wings.
Bats followed a similar evolutionary pattern, starting with a tiny tree-dwelling mammal, and making the transition from climber, to glider, to flier. A minority of biologists believes, based on similarities in the brain structure of Megachiroptera (flying foxes) to that found in primates (and no other mammals), that these giant fruit bats might be more closely related to us than to the brown bats we have throughout North America. It's a bit of an odd hypothesis, but if it turned out to be true, it would mean that the traits necessary for flight evolved twice within the class mammalia, through an identical series of adaptations, arriving at close to the same result. These separate lineages would also explain why Megachiroptera are the only bats that don't use echolocation.
Flying squirrels aren't closely related to bats, but could they be on their way to becoming bats?
Pterosaurs presumably evolved from tiny lizards under similar environmental pressures, but paleontologists can't come to an agreement on which tiny lizards they evolved from.
The first animals to master flight on this planet were the arthropods that would later become flying insects. Insect wings were once thought to have developed from gills like those found in mayflies, but a general consensus is forming that the first flying insects descended from the trees in a similar progression to those of birds and bats.
Flying squirrels aren't the only animals currently at the gliding stage of this process. Flying snakes are well documented in the rainforests of Southeast Asia, (where the tallest trees are less densely packed than the ones in South America, an environment which seems to favor prehensile tails and other climbing adaptations over gliding adaptations) and have the ability to flatten themselves by sucking in their abdomens and spreading out their ribs, allowing them to slither absurd distances through the sky like flexible, elongated frisbees. They're able to produce lift and even change direction slightly, in a way that the Department of Defense is spending money to try and understand. If their descendants ever work out how to turn around in mid-air, actual flying vipers will suddenly be a thing. Their scales may even soften and elongate like theropod scales did, eventually becoming feathers.
I want to know how obvious the ability to fly would be to a biologist looking at a flying snake's bones. I want to know if this adaptation once existed elsewhere, among other snake species in similar environments, and paleontologists just haven't caught on yet. I want to know if there was a time when the tree canopy in South and Central America was much thinner than it is today. I want to know if ancient Olmecs were used to looking up to watch feathered serpents trace strange paths through the sky.
Archaeologists don't agree on how important the feathered serpent concept was to the Olmecs. The Mayan version, a deity known as Kukulkan to the Yucatec Maya and as Q'uq'umatz to the K'iche Maya, doesn't appear to have reached its eventual level of importance until the Post-Classic era. The Aztecs called it Quetzalcoatl, this time one of the three or four most important beings in their religion.
Think about a telephone game that goes on for more than two thousand years, starting with sightings of what looks a lot like a mysterious kingdom of magic flying snakes (which are eventually outcompeted by their much larger climbing cousins and go extinct). Written language exists for at least part of that history, but knowledge is tightly controlled and mystified by the religious class.
At some point, other serpent deities showed up. All of them could fly. Vision serpents were said to dwell in the topmost branches of the Maya version of the world tree, and acted as conduits between the physical world and the spirit realms. Lots of Maya rituals involved getting the attention of vision serpents by bleeding all over specially prepared paper, which was then burnt. The bleeding man or woman (probably experiencing drug and endorphin induced hallucinations) would then watch the smoke from the offering form a serpent, and either a god or an ancestor would pop his or her head out of its mouth to tell the bleeder something crucially important.
There's also the fire serpents, or Xiuhcouatl. The Aztecs saw them partly as weapons of Huitzilopochtli, their patron god of war and the sun. Fire serpents were also revered by Post-Classic Maya warrior cults, who also depicted them as weapons of the sun. Two really obvious ways to use fire serpents in a D&D game would be to reskin salamanders as Xiuhcouatl and/or the spell meteor swarm as rain of serpents or something like that.
My JOESKY TAX is a pair of spells or rituals related to vision serpents:
Call Vision Serpent (Level 1)
This ritual is performed whenever some needs to contact the dead or divine for advice in some dilemma or other. The spellcaster prepares a pile of papers with arcane diagrams and secret names, as well as the name of the spirit or deity being summoned. Whoever wishes to commune with the spirits must offer up the majority of their blood, allowing themselves to be drugged, and then piercing and cutting themselves in the most painful ways availabl, allowing their blood to pour onto the papers. The visionary must save or die from blood loss during this process (okay if you wanna be a big softy about it, drop 1d20 from their maximum HP instead; this still might kill them or nerf them forever, but this ritual isn't meant to be performed lightly). Even if this save succeeds, the bloodletting process reduces the character's current HP to 1.
The papers are gathered into a bowl when it looks like the visionary is getting ready to pass out from blood loss (and drugs), and lit. The plume of smoke takes the shape of a serpent, visible only to the recipient. The serpent, having fed on the blood of the recipient, calls forth the spirit or deity that was specifically asked for. This entity will patiently listen to the dilemma of the vision's recipient, then respond in one of the following ways (referee's choice or random):
>the entity will share the location of an object or NPC that will be crucial in solving the problem.
>the entity will reveal hidden information about what's going on behind the scenes.
>the entity will directly advise the recipient to make a particular decision (usually good advice).
>the entity will reveal one or more secret weaknesses in the recipient's enemy.
>the entity will angrily accuse the recipient of wasting their time, and vanish without helping at all. This only happens if the entity feels it is being called upon to address something below its station (a king to settle a gambling debt) or outside of its divine portfolio.
Serpent Road (Level 1)
Spellcasters are also able to convince the vision serpent to act as a planar gateway for the subject of a nearly identical ritual, though a fresh human heart from a willing donor must be added to the sacrifice. Assuming the aspiring planar traveller makes their save during the bloodletting, they are devoured by the serpent, which appears to them as a vast smoke-walled tunnel. To observers, a plume of smoke appears to completely envelop the traveller and then disappear, taking the traveler with it. The serpent then spits out the traveler (along with any possessions being carried) in whichever supernatural realm was named in the blood-soaked ritual papers. The subject will be weakened (1 HP) and somewhat disoriented from the ritual, but the experience is sobering enough that the character won't be too high to function.
This ritual is a two-way ticket, and the vision serpent will facilitate the subject's return as soon as some condition, predetermined by the spellcaster preparing the ritual papers, is met. It's worth noting that the diagrams and names on the papers are incredibly complicated, and that it would generally be pretty easy for the the spellcaster to lie to the traveller about either the destination or the return condition.
The spell can also be used to provide simultaneous travel for a group, by providing one sacrificial heart for each traveller. All group members must perform the bloodletting portion of the ritual, including the saving throw and HP loss.
The spell can also be used to provide simultaneous travel for a group, by providing one sacrificial heart for each traveller. All group members must perform the bloodletting portion of the ritual, including the saving throw and HP loss.
