Are any of our cells our original ones? Yes, many of the cells you have today are your original cells. Most of your heart
cells, your nerve cells, your bones and cartilage, and your brain cells among others are original to you- but the atoms in them that make them up have changed.
You can read about that in this article.
It is said that 98% of the atoms within you change every year. Why is that?
We are made mostly of water- about 60% plus by the time we are adults. We drink and pee a lot over the course of a year. Multiply that by a lifetime.
We also breath air in and out and exchange oxygen, carbon dioxide and nitrogen and water- your breath is moist- along with other gases.
Some cells in our bodies die and are replaced frequently such as many in our intestinal track and our skin cells.
Others in the areas mentioned above- neural cells, brain cells, heart cells live on, but the atoms inside them are changed and replaced over time. They are the same cells but with different atoms making them up.
So if our cells change are you, still you? I suppose it depends on how you define YOU.
You are the aggregate of your memories, thoughts, and your identity. You are still you. But you are not the same atoms you once were. Neither is anyone else.
Those atoms that made us up and make us up now are billions of years old. They were created in stars and the explosion of those stars over the life of our universe.
They were here long before you, and will essentially exist for perhaps another 40 trillion years.
Many of the atoms that make you up today were part of somebody else before. How?
We breath air that circulates, we drink water that recycles, we eat food that passes through us, decays, and for millennia became fertilizer for plants that grew and died and themselves became organic fertilizer for more plants some if which were eaten, or became coal, or oil, — you get the picture.