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. 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.

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 be able to make the Moon (and, in the manga, the other planets of the Solar System) habitable, until there was a war that destroyed everything. Modern humans know nothing about the old civilization.
 * Laputa: Castle in the Sky: The Laputians 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.

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.

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.

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.)

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.

Real Life
And then I was My Own Grampa.