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. Something happened to it; corruption and decadence leading to collapse and regression? Natural disaster? Precursor Killers? 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. 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!

Live-Action TV

 * Battlestar Galactica: The Twelve Colonies believe themselves to be descended from humans staying on an ancestral homeworld called Kobol.

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.

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.