Cell biology is weird

You know, when I think about this cell bio stuff I'm studying, it makes my head spin. I found myself daydreaming about the complexities of DNA and how complicated it is that every living thing on earth uses the same chemical, DNA, to build the proteins that make up its structure from teh lowest bacteria to the hoity toity complex mammals. Perhaps I'm in need of a nap, but there seems to be some really cosmic metaphysical stuff lurking in this course material. We are so complicated. How did all of these random things all end up together is a seething mass of all that which is me? How do the memories of my life last in the chemicals in my brain from 30 years ago when the cells aren't the same, and even the atoms they're made of aren't the same as when that happened?

I get the theory about how we all evolved from a single cellular organism over billions of years through mutation, predation and luck but someone please explain to me where the hell that single celled organism came from. How did that just happen?