On Earth there’s a long-held belief that some bloodlines are better than others, more noble. Today, it’s sometimes hard to appreciate why this belief captivated human society for so long. There are good reasons for the fading fascination with nobility but such reasons would not necessarily hold in a fantasy world, especially one where gods walked and ‘frolicked’ among mortals.
Much of the ready acceptance of noble lines being ‘better’ than commoners can be attributed simply to nutrition. Starve someone in childhood and they will grow up smaller and thinner. This was observed in World War II in the disparity between the average British and American soldier, it was observed in the curviness of American and post-war European women (or today, in generational differences in China as that country grows richer).
Throughout most of human history, where many ate even more poorly, it is not hard to imagine observers noting the difference between a well-fed, strapping lord-ling or lady-ling, and the scrawny peasantry, heavily taxed (by food-stuff levies) to support said lords and ladies. Layer on better training and education, social mores created, consciously or not, to distinguish the privileged from commoners and performers & artists extolling the virtues of their benefactors and for almost all of human history, there has been a manifest difference between the haves and haves-not, not just in perception but in fact.
It’s no wonder that people attributed the attributes of the nobility to “better blood.” In fact, it must have seemed obvious. And, of course, in pagan times, rulers often constructed a divine lineage. Who would dispute that gods did have better offspring, and therefore any King or Queen with such blood in their veins had to be superior.
None of this goes very far these days, with better nutrition, education and current political systems but it’s useful to remember when wondering why the vast bulk of humanity tolerated nobles for such a long time. Moreover, when crafting your own world, if you have such distinctions of rank, it wouldn’t hurt to leverage some or all of the same things that sustained a hierarchy on Earth.
But let’s go a step beyond that. Imagine that there are gods and that, like the Greek, Norse, and many others, they liked to walk among and interact with mortals, leaving progeny behind. It’s no great stretch to imagine that such off-spring would have some superior attributes. In D&D terms, they would have stat bonuses. In story terms, they might be faster, stronger, more clever than others. One doesn’t have to look far to find such traits in myths, of which Hercules is just one of many examples.
In a fantasy world, who would the gods bestow their amatory favors on? No reason to think gods would choose their mates materially different from mortals: they would seek well-formed mortals with some particular prowess or mental acuity. Where else would you find that than among the well-fed, well-lauded and well-educated (in some settings) nobility? If ever there was a time that divine blood was not in the nobility, the gods would naturally gravitate to them and seed such divinity. Over time, there would be a positive reinforcement: more divine blood would produce ‘better and better’ nobles. By better, I mean only the benefits godly blood might bestow and traits gods might favor (feminine beauty and manly handsomeness, however that might be judged in your world).
It might not take long before there was a marked difference between nobles and commoners, more than just what a pampered childhood and good food could provide. In time, whole new species might arise, depending on how long this preference persists and how genetics works in your world. Before the differences became too obvious, there might be a period where a gifted commoner might join the ranks of nobility, soon intermarrying with the divine nobles and if especially attractive, perhaps bedding a god or goddess. But once this process went on for too long, the differences between the two might be too much to bridge as commoners perhaps became (or seemed) more brutish than the nobles. Might make for an interesting origin story for an elf-like race, in fact, providing a good reason for the differences and ample reason for enmity between the favored and un-favored. (There’s a Norse myth explaining the origins of thralls, commoners and nobles that makes use of divine heritage.)
Of course, there could be many factors working against too much of a discrepancy between blooded and unblooded. Maybe the gods, consciously or not, interbred as much with commoners (due to their larger numbers maybe?) than nobles. Maybe different gods bestow different traits, some of which maybe don’t interrelate well, arising in multiple gene pools that diffuse divine blood. In D&D terms this could be something as simple as Tieflings and Aasimars, humans with demonic and angelic heritage: they probably don’t interbreed much with each other but might still intermix with ‘lesser’ humans.
There’s a lot that has been done with divine blood lines but most often the focus is on a specific lineal decent (something extremely unlikely without divine intervention as I noted in my thoughts on the Da Vinci Code‘s premise.) But divine blood might have an even bigger impact on peoples (perhaps a god or group favor a certain tribe or race, like the elves) and nobles, than a King and his progeny.
While the effect of persistent god/mortal interbreeding isn’t often considered in world building, maybe it could provide that extra bit of novelty to your world?