Is ours just the most recent creation?
The idea of a pre-Adamic creation is usually based on one word in Genesis 1, the word replenish. Replenish means to re-fill and implies that the earth had once been filled by a previous creation.
Genesis 1:28 (KJV) And God blessed them, and God said unto them, Be fruitful, and multiply, and replenish the earth, and subdue it: and have dominion over the fish of the sea, and over the fowl of the air, and over every living thing that moveth upon the earth.
Actually, He doesn't tell Adam to replenish (i.e. re-fill) the earth. The Hebrew word translated "replenish" means simply "fill" and is so translated by all modern Bibles, even the New King James Version.
Genesis1:28 (NKJV) Then God blessed them, and God said to them, "Be fruitful and multiply; fill the earth and subdue it; have dominion over the fish of the sea, over the birds of the air, and over every living thing that moves on the earth."
I hope you have found this helpful.