In the lineage of Sword & Sorcery, the most immediate ancestor is not quite adventure fiction, nor horror – it is, rather, a genre that was huge when S&S was born and raged for another quarter century before it began to fade, and today is little more than a nostalgic blip on the Fantasy radar. I am talking, of course, about Sword & Planet.
Obviously named of a piece with Cloak & Dagger, Sword & Sandal, and Sword & Sorcery, Sword & Planet was an outgrowth of the kind of stuff that is sometimes called Planetary Romance, and which also led to the rise of genuine Science Fiction. In the late 1800s the market for adventure fiction was growing even as the blank spaces on the map were shrinking. Knowledge of the other planets was expanding around the same time, and it all was kicked into high gear by astronomer Giovanni Schiaparelli in 1878.
Schiaparelli got some good looks as Mars through his telescope and described some features he saw as “canali” - which in Italian just means “channels”, but in translation this became “canals” in English, and people lost their collective shit over the idea. Nobody could see well enough to tell whether Mars had life or was even capable of supporting it, but the idea seized hold of the popular mind like a rabid badger. People started writing stories about Mars, life on Mars, and what that life might be like, a guy named Percival Lowell wrote a series of entirely speculative and very imaginative books about what life on Mars might be like, and the whole topic was very much in the public consciousness of the early 20th Century.
Enter Edgar Rice Burroughs, who wrote the genre-defining A Princess of Mars in 1912 (though it did not appear in book form until 1917, before that it was serialized in magazines). It was extremely popular, and was followed by not only ten sequels, but kicked off a whole wave of imitators who dominated up through the 1960s, and who continue even to this day.
The genre conventions of Sword & Planet were these: a male hero from Earth, usually the United States, who is a misfit in his own culture due to his code of honor and restless nature. He is transported somehow (S&P was not big on scientific explanations for anything) to another planet. This may be a planet in our own solar system, or may be a world unknown to the modern age – the location is not really important. On this world he finds humans who live in a barbaric society that he finds suits his temperament much better than our own safe and stultifying Earth.
There will be a woman – usually a princess or a queen – and the hero falls in love with her, but in order to win her he will have to impress her with his prowess, rescue her from multiple threats, monsters, and kidnappers, and often in the process save the entire world/kingdom with his sword-swinging derring-do.
Now it is true that there is a lot more Errol Flynn than Solomon Kane in most Sword & Planet adventures, but that does not mean the genre as a whole did not have an effect on the evolution of Sword & Sorcery. Both genres entail lone heroes who are more virile and badass than the people they encounter. Both take the reader to alien and fantastical worlds, and both entail a lot of solving problems with violence.
Imagery, in particular, is very similar between the genres. Ever since Burroughs decided to make Martians go around functionally naked, warriors in straps and loincloths and princesses in golden bikinis have been a staple of Sword & Planet art. Throw in the fact that the S&P hero will invariably be brandishing a sword in the face of some hideous monster and you are already almost there. The two major differences are that a S&P hero will sometimes also be waving some kind of gun around, and a Sword & Sorcry hero will be engaging in much more bloody levels of violence.
The literary differences are really ones of style and execution. A Sword & Sorcery hero will usually be a primitive from some almost stone-aged tribe, while a Sword & Planet hero will be a person from our modern age, able to explain things to us in a modern vernacular style. There will be no express magic in a Sword & Planet tale (though there may be super-science that mimics the effects of magic). The violence in a S&S story will be much bleaker and bloodier than the more PG-13 violence found in the Sword & Planet genre.
The real different aspect is the thread of Horror that runs through Sword & Sorcery. There will not be a whiff of Elder Gods or prehuman monstrosities in a genuine Sword & Planet story. Right now, writing that, I am thinking how cool that could be, but it would not be a proper story in the genre if you added those elements. Sword & Planet is almost always expressly hopeful, set in a world of black and white morality where the good guys always win. It has nothing like the moral grayness of Sword & Sorcery, with its grim landscapes and fatalistic heroes.
Sword & Planet was born in the early years of the 20th Century, but it remains very much a product of Victorian sensibilities and tropes, while Sword & Sorcery is much more modern to our eyes. It porytays a world without moral absolutes, where a man’s fate may be forged by his steel, but there is no assurance that he will win, or even that he is the hero.
The imagery and ideas of Sword & Planet fiction clearly had an influence on S&S, if not at the beginning, then later, as the genres evolved through the early 20th Century and then when both had a revival in the 60s. The loincloths and swords and monsters and exotic locations, the muscular heroes and scantily-clad heroines, the bright colors and lurid situations – all of it kind of grew together in this pulp-fueled mass, and yet they remain distinct genres because they are doing different things. Sword & Planet has modern heroes and lives in the light. Sword & Sorcery is in love with the primordial, the primitive, and will always dwell in the dark.