The obvious answer is that we are made of atoms, like everything else physical in the Universe. Some of these atoms were made from other atoms inside stars that then exploded.
This is of course correct, but it misses a lot of crucial points. For example, the atoms in a living body are being continuously replaced with new atoms, a process called metabolism. Like the Ship of Theseus, our bodies are continuously being rebuilt, with none of the original atoms remaining in place.
Not all the atoms in the body get replaced over time. There are things that do not take part in normal metabolism, yet are considered parts of our bodies, like tooth enamel, or the lenses of the eyes. DNA material in post-mitotic cells, like most brain cells of an adult, could also remain essentially the same atoms in the same molecules for decades.
But as the brain is the most metabolically active organ in your body, the atoms that remember an event that happened ten years ago are, for the most part, not the same atoms that experienced it at the time.
Single atoms have no memories, and no identities, only large collections of them do. But not just any heap of matter is alive.
The smallest collection of atoms and molecules that can be said to be alive is the cell. We are made of cells that have grown together, and specialized into the different parts of our bodies. In biological sense, the correct “atom” of the human body is the cell.
But this division is not truly atomic either. We cannot take apart a human at cellular level, and reconstruct him again from individual living cells (By the way, there is one class of animal that can spontaneously do exactly this, the sponge). Individual cells in our bodies are not just in different positions, they are also specialized to very different tasks. Even if all cells in the body shared the same DNA, cellular specialization occurs by turning different genes on and off, based on inter-cellular communication.
Individual cells in our bodies die all the time, and are replaced with new ones. In fact, most of what you see when you look at any bigger animal is the dead tissue covering the surface. Yet the organism lives.
Life is not made or designed, it exists through growth, and refuses to be confined to any single level of definition. As conscious beings we take all the unconscious processes in our bodies for granted. As apex predators, we live on top of a pyramid of life which we mostly take for granted. As social and specialized animals, we live as dependent members of the global economy, which we mostly take for granted.
In the end it makes little difference what parts our bodies consist of at any moment. As growing things our bodies are not finished. Nothing alive ever is.