Humans Are Their Own Precursors

No, humanity, you are the precursors.

There are all kinds of stories where there are Precursors.

In this case, humanity is its own.

Once, there was an advanced ancient human civilization, far back enough in the past as to count as prehistory to modern or near-future humanity, or at least long ago enough to have been forgotten about. Something happened to it; corruption and decadence leading to collapse and regression? Natural disaster? Precursor Killers? War either civil or with an external party that itself failed in time? The possibilities are endless and may be explained as the plot advances, or they may not, or And Man Grew Proud and all that is left is Future Imperfect. Whichever the case, humanity survived but had to rebuild itself back from the hunter-gatherer stage. As a result, they might find and use the occasional piece of Schizo-Tech. There may be an Advanced Ancient Acropolis or Lost Technology to unearth, all of which remained hidden until now. Time Abyss survivors that somehow escaped the downfall without losing everything are also possible. Atlantis is a common subtrope.

The purpose is to create a sense of mystery and wonder for the ancient precursors, and establish a contrast between the humanity of the present and of the past. If some technology is left over from the ancient humans, this can be used to justify Schizo-Tech. It can also be used to create a sense of scale or tension for an apocalyptic event or threat; if the event is capable of wiping out that much of humanity, it must be serious, and if the threat could overcome the more advanced previous era, how can modern mankind win? A sense of loneliness for the new civilization can be created, knowing they are on their own after their advanced ancestors collapsed. If you want to use this trope in your writing, make sure to develop plenty of differences between the old and new humans. This ensures readers feel there is a sense of purpose in how the trope is used. Try to create an immersive mood, centred on the old precursors.

Compare Earth All Along, where the unknown planet is revealed to actually be Earth, and Humanity's Wake, which is about our modern civilization being this to whatever succeeds us. TV Tropes calls this "Advanced Ancient Humans".

Some examples may entail spoilery reveals; reader beware!

Anime and Manga

 * The "Silver Millennium" from Sailor Moon was sufficiently advanced to make the Moon (and, in the manga, other planets of the Solar System) habitable, until there was a war that destroyed everything. Modern humans knew nothing about the old civilization.
 * In Laputa: Castle in the Sky, residents of the titular Laputa had flying cities, robots with Energy Weapons, and Wave Motion Gun Kill Sats, but died out apparent centuries before the story's setting, which appears to be the 1920s. Almost every bit of technology matches the setting, except for a massive Castle in the Sky with weapons of mass destruction casually cruising through the air. The process of the human precursors slowly rising and disappearing is shown in the opening credit sequence.
 * The human nations of El-Hazard: The Magnificent World are Arabian Nights-themed and of perhaps late medieval tech level. They are founded on the ruins of an extremely advanced civilization that apparently destroyed itself in an apocalyptic war, leaving behind few examples of its technology, mostly weaponry. This includes one moon-sized device permanently floating in the sky.
 * In both the Nausicaä of the Valley of the Wind manga and anime, the story is set a thousand years after the Seven Days of Fire, a catastrophic event in which many humans and animals perished. The general technology level in the world regressed, but the magnificent machines constructed by the precursor humans still remain, resulting in a Schizo-Tech setting. In the manga, far more elements of the human precursors are explored, including plans of how they want to restore the world.

Comic Books

 * The Eternals of Marvel Comics are proto-humans genetically experimented on by the Celestials a million years ago. They have been living among us ever since, wielding superhuman powers and superior technology, with some taking up the God Guise.

Film

 * Atlantis: The Lost Empire: As the title says, there was an Atlantis in the past that was advanced beyond anything at that point. They had, among other things, lasers, floating vehicles and monuments, and massive machines. A Heroic Sacrifice prevented cataclysm from destroying the entire civilization, but sealed it off from the outside world; by the time visitors find it again around World War I's time, it has regressed terribly, and almost all that tech has been lost.

Literature

 * Empire From the Ashes: Twenty Minutes Into the Future astronaut discovers that the Moon is actually an AI-controlled disguised warship from a now-defunct human polity from 50,000 years ago called the Fourth Imperium. Escapees from the ship after a failed mutiny were the progenitors of humanity as Earth thinks it knows it, and some of the still-alive mutineers have been living amongst us, biding their time.
 * H. Beam Piper's Genesis: Humans had become technologically advanced on Mars, but were faced with extinction as the planet dried up. They managed to start a colony on Earth, but their technology and history were lost to Finagle's Law.

Live-Action TV

 * In the remake of Battlestar Galactica, the Twelve Colonies believe themselves to be descended from humans staying on an ancestral homeworld called Kobol. (The original BSG also has the Twelve Colonies originating from Kobol, but doesn't have the spoilered plot twist.)
 * The Ancients of Stargate, at least those that haven't become or ceased to be Energy Beings, look identical to modern humans and many modern humans retain enough Ancient genetics to use their gene locked technology. Atlantic is indeed their creation, though it's a spaceship city.

Radio

 * The Hitchhiker's Guide to the Galaxy: Modern humans are revealed to be descended from the most useless members of the Golgafrinchams, an ancient extra-terrestrial race.

Tabletop Games

 * Mage: The Awakening: There was allegedly an Atlantis where magic was widespread until those who are now known as Exarchs ascended to the Supernal Realms and destroyed the Celestial Ladder after them, destroying the old society and causing damage to reality that shut off most of humanity from magic. Allegedly, because Atlantis might have been Ret-Goned, or it might not.
 * Claimed by the Geonee (a Human subrace) in Traveller, based on the evidence that they discovered in their own star system. They are, of course,.

Video Games

 * Civilization: An unknown people or peoples left Ancient Ruins behind. Any technology recovered from one will always be more advanced that what the player has, implying this.
 * Halo: The Forerunner Saga and later games reveal that there was an earlier human civilization that encountered and fought both the Flood and Forerunners in the past, but were regressed forcibly after being defeated by the Forerunners.
 * Honkai Impact 3rd: There was a Previous Era of humanity 50,000 years ago that also had to fight the Honkai. They had Lost Technology the likes of which the Current Era either cannot replicate or can only do so imperfectly, such as the Divine Keys and MANTIS Super Soldiers, but failed to defeat Honkai in their time. Whether they had any more Previous Eras of their own is unknown.

Visual Novels

 * Fate stay night: Gilgamesh is disgusted with modern mankind because he believes that we are weak compared to how things were back in his day. Because of the Older Is Better nature of magic in the Nasuverse, he might actually be right.

Web Animation

 * RWBY: In a time lost to memory, the ancient humans had magic, which gave them the power to fight off the Grimm and make wonders.  Other than a single immortal individual, though, they were completely wiped out by their gods after that individual persuaded them to attack the gods.  After the gods abandoned the planet, humanity eventually reappeared on Remnant along with the Faunus (although exactly how has yet to be revealed as of V8), but without the presence of the gods they had no magic.  They did have Dust, though -- which some viewers believe is the fossilized magic of Humanity V1.

Real Life
And then I was My Own Grampa.