Before you read on, I'd like to first apologize for the length and any glossing over of subjects in my answer. Not sleeping well will do that ... and I doubt I'll be sleeping after having written this. I will however, be using this as a template in my conworld.
First, a bit of an explanation.
Medusa is a member of the Gorgons. While depictions of Gorgons vary across Greek literature, the term commonly refers to any one of the three sisters, having hair made of living, venomous snakes and a visage that turned those that looked at them to stone. In her book, Language of the Goddess, Marija Gimbutas argues that the “Gorgon extends back to at least 6000 BC, as a ceramic mask from the Sesklo culture…” In the same book, she also identifies the Promethean archetype of the Gorgon in Neolithic motifs, and especially so in anthropomorphic vases and terracotta masks.
Some of the reptilian attributes associated with the Gorgon are a belt made of snakes, and snakes either emanating from her head, or entangled in her hair. Some believe that these traits are derived from early Greek religious concepts, such as the dragon Delphyne, whose skin was believed to be made of impenetrable scales. There are also some similarities to Humbaba in the epic of Gilgamesh.
Traditionally, Gorgons have been depicted as having wings, brazen claws, the tusks of boars and scaly skin. Lionesses or sphinxes are also frequently associated with Gorgons. The Gorgons were said to be the daughters of sea deities, Ceto and her brother-husband, Phorcys. For this, I’ll assume you are referring to the more general depiction of Medusa as most people recognize her: the lower body of a snake, the upper body of a woman and of course, snakes for hair.
Evolution and myth: What are we looking for?
Based on the traditional depictions of Gorgons, we first need to
start looking at an animal that is comfortable with water.
Secondly, we’re looking for some explanation for the upper body
resembling a woman.
Thirdly, we want some kind of trait that would explain the snakes in
Finally, we want to explain how this creature would turn a person
into stone … or something like stone.
Building a Nightmare: Step One
Following the traditional depictions and the generally accepted “vision” of Medusa people tend to jump to, a snake is the obvious choice. In this case, a spitting cobra, but I’ll get into why a bit later.
A little bit about the spitting cobra.
All venomous snakes transmit their venom through tiny holes in their fangs. In spitting cobras, these holes are larger, allowing them to project their venom at a distance of 6 to 8 feet, and studies have shown that they hit their target at least 8 out of 10 times. They of course aim for the eyes. Once the venom is in your eyes, you will feel quite a bit of pain, and if left untreated, can cause blindness.
The venom of a spitting cobra commonly contains a combination of neurotoxins and cytotoxins, which can damage nerve tissue and shut down individual cells. It’s generally not harmful to human skin, but can cause serious damage if it gets into the eyes, inside your nostrils, or into an exposed wound.
Spitting however is only a defense mechanism. Spitting cobras will still hunt. Their diet is pretty much the same as any other snake – whatever it can kill and get its mouth around.
Spitting cobras tend to be found in southern Africa and Southeast Asia. In Asia, they are found in forests, fields, grasslands and sometimes near human settlements, while in Africa, they tend to be found in dry savanna and semi-desert areas.
Generally speaking, all snakes can swim.
The black-necked spitting cobra can be nocturnal or diurnal, depending on the time of year, geographic location, and average daytime temperature. Additionally, it is one of the most adaptable of the sub-Saharan spitting cobras, spanning across the center of Africa from coast to coast, and can be found at altitudes up to 1,800 meters. Moist savannas, cleared former forest regions, rivers and streams, coastal scrubs and dry grassland are all possible habitats. Tree trunks however, seem to be preferred. As with many snakes, they are excellent climbers and can be arboreal at times.
Finally, the black-necked spitting cobra has been known to spit venom with only the slightest provocation, and is less prone to actually bite as other related species.
Building a Nightmare: Step Two
Having an upper body that resembles a human being is quite an evolutionary feat, but not one that can’t be explained. Many species have taken to mimicking their environment, their prey and even their predators, as can be seen with the orchid mantis, the zone-tailed hawk, the owl butterfly, the South American leaf fish and many, many more. Evolution and nature have provided us with a way. So, let us take a Darwinian approach to this. Our proto-gorgon has gone down an evolutionary path in which mimicking the appearance of humans has further assisted the survivability of an already very adaptable species. Like all snakes, our proto-gorgon is a carnivore, and will actively hunt. Through this, it’s generally accepted that intelligence develops, as a carnivore must be a step ahead of its prey in order to secure its next meal. Because it’s taken to mimicking the appearance of a human, we’re looking at a case of convergent evolution with homo sapiens, and divergent evolution with its parent species as it follows the movements of its new favored prey.
Ancillary appendages appearing as arms and hands could therefore develop, and whether their original purpose had been as a sensory organ of some sort, could potentially be applied to using tools, furthering the development of the brain, and very likely, a more refined appendage. They would retain their serpentine tails, as being arboreal has a distinct advantage in stalking its prey as humanity spreads into more pleasing territories. Simply stalking an individual and waiting until they were separated, would be followed by some noise to draw their attention, and then a blinding spray of venom to the eyes. Stunned, the proto-gorgon need only incapacitate its prey … but as intelligence develops, hunting a communal species solitarily, isn't always the most efficient tactic – especially one that has begun hunting as well. Both predator and prey, the two species form a cyclical relationship.
Early human become fearful, and hunt their new predator, as the proto-gorgon hunts them. The nature of their relationship, and the developing intelligence of a tool-wielding carnivore species, causes a shift in how the proto-gorgons interact with each other. Once converging only to mate, they now begin to develop small communities and hunt together … but that doesn't answer why these proto-gorgons would hunt humans.
Food is the answer to that question, and a lot of it. By successfully stalking a human hunting party, these proto-gorgons could benefit from not only the humans they stalk, but the fruits of their labor by feeding off their recent kill.
Creating a Nightmare: Part Three
What about the snake-hair? As a community develops among the proto-gorgon species, so to do customs and social norms ... and this really isn't all that abnormal. Some snakes, like garters are communal after all, and sharp-tailed snakes, as well as ring-neck snakes are known to utilize communal egg laying sites. Of course, community and snakes isn't always a perfect match, sometimes resulting in cannibalism … but more on that later. With customs and social norms, we can look to several varieties of snakes that defend their nests, as well as protect their young. In fact, the female African rock python has been known to defend their young for as long as four months. Perhaps, as a matter of some custom that has developed within their communities, it’s in the hair of our proto-gorgons that they carry their young … and this would in fact provide some small advantages, as hair would help insulate the young, and having descended from an aggressive breed of spitting cobra, they are likely to pelt any potential threat with their venom.
Creating a Monster: Part Four
Turning to stone? Not likely, but we can have some fun with the venom that may yield a satisfying result – calcification. Turning something into bone would be much easier than turning it into stone, and requires far less of a stretch of the imagination. Those rare few who survive an encounter, would see the affected portions of skin hardening over time … along with some other very nasty side effects. Corneal calcification causing blindness, after being sprayed in the eyes?
The venom, as mentioned earlier, is a mix of neurotoxins and cytotoxins. In cobras, Alpha-neurotoxins are common and ACh flow, causing the feeling of numbness and paralysis. So a bite, or getting sprayed in the eyes, can cause come serious problems … Now, what if the attached cytotoxin caused an immediate response in the body, promoting an elevated calcium level in the blood. This hypercalcaemia can be caused by excessive skeletal calcium release, increased calcium absorption, or decreased renal calcium excretion. Let’s say that once introduced into the bloodstream, the venom affects the renal gland, effectively shutting it down, and sets off skeletal calcium release. Along with paralysis, the victim would also experience bone pain, abdominal pain, nausea and vomiting. If they manage to survive the encounter, they would also experience depression, anxiety, cognitive dysfunction, insomnia and coma, with the possibility of also suffering from fatigue, anorexia and pancreatitis. Other negative symptoms for survivors would include peptic ulcers as a result of increased gastrin production, and the high levels of calcium ions would decrease the neuron membrane permeability to sodium ions, leading to hypotonicity of smooth and striated muscle, explaining the fatigue, muscle weakness, low tone, and sluggish reflexes in muscle groups while sluggish nerves would explain drowsiness, confusion, hallucinations, stupor and coma. When reaching the gut, it would also cause constipation.
This alone would not explain a rapid onset of generalized Dystrophic Calcinosis Cutis, which would result in multiple firm, whitish dermal papules, plaques, nodules or subcutaneous nodules forming. Occasionally, these lesions ulcerate, extruding a chalky, white material. When severe, vascular calcification can cause diminished pulse and cutaneous gangrene. A better (and more terrifying) option, would be for this to occur as a slow and drawn out process, and occur along with the formation of heterotopic bone in a process similar to fibrodysplasia ossificans progressive, in which the muscle, ligaments and other connective tissue are converted into bone.
This is why it’s terrifying. Because of the effect on survivors, it would then allow for a culture within the proto-gorgons to develop that involves hunting their prey long before there is need, holding them captive, and allowing nature to takes its course as they become hollow bone statues. Their flesh, or whatever was left of it, would rot away, and their internals would decay. Speed this process up by whatever mechanism you deem appropriate, and you would have a nutrient rich meal, naturally sealed away in a bone sculpture and ready to be cracked into when needed. Survivors who escape capture, would face the same fate, much to the horror of their friends and families. The remaining bone could then be repurposed as necessary.
A bit more on culture
This variation of the gorgon as a species has developed into quite a nightmare … but how much worse could it get? Two things immediately spring to mind. A culture revolving around hunting, and the potential of cannibalism … having developed a more communal nature as a matter of mutual necessity, we can’t ignore that snakes are by their very nature, carnivorous. Of the very few communal and semi-communal snake species, cannibalism does occur, and there is the potential for it to occur in our proto-gorgon species. With community comes responsibility and safety. While potentially destructive to a community, cannibalism doesn't have to have a negative social impact within the species itself. As a matter of fact, several human tribes have practised ritual cannibalism … and some would claim that there are isolated indigenous tribes that still do. As a community and intelligence develops, so to can spirituality as a means to rationalize the otherwise unexplainable. It would be conceivable then, that among our communal hunters, cannibalism takes on a spiritual note and becomes ritualized.
A race of gorgons, who have developed into a hunting community, have a cyclical hunter/prey relationship with anatomically modern humans, who can calcify their prey to preserve their fluids as a staple, and as such, harvest their prey like livestock. To make matters worse, inter-species warfare resulting from ritualized cannibalism adds to their terrifying image, while also helping to keep the population low. A glorification of hunting and hunters as the providers of the community, leads to sport … and what better creature to hunt, than one that can reason?
If anything, I hope this spurs on more ideas. There's a lot of potential in the subject, and can make for some terrifying legends.