Every cell in our body comes from the original stem cells that comprised the embryo we once were.
Everything starts with a sperm and egg, which fertilize each other by merging into a single cell thereby becoming the first stem cell from which our entire body will be created. At that point the merged cell starts dividing into 2 cells, then 4, 8, 16, 32 cells etc.
In these early stages, every one of these cells can become any cell in the human body.