Sort of a complicated question, I'll try to ask. How was I born me and not another?

[FatAndFaithless]

You couldn't be born as a different species because then you wouldn't be "you" anymore.  It's not like your mind would just be inhabiting a cat's body or something.  The only possible set of circumstances that could result in "you" are the set of circumstances of which you are currently a product.

Oh my god, did you really just tell me that my mind couldn't have been inside of a cat? UGH!! I know that. I'm just saying... holy crap how do I even rephrase this differently anymore? Is there any scenario where a person could have not become a person? Like, all the particles that make up our body from birth, they were present there from the big bang, they probably composed some sort of molecules floating around space before the earth was created. The particles in my body probably existed in some form or another from way before I was even born. All of these molecules go into creating one thing or another, and yes a lot of species on earth are related and I could have become a cat, or a monkey, or whatever. But those molecules could also just be sitting there inside of a rock, I could have never come to be. I was just thinking, why am I sitting here right now and not just some primordial substance sitting around somewhere else, not formed into a human being?

...Because "you" are defined by what you are now.

And "Is there any scenario where a person could have not become a person?" makes...no sense.  If a person wasn't a person, it wouldn't have been a person in the first place, so no.  

I mean, some of your old skin cells that you used to have are probably in a tree or rock or bug somewhere, but those skin cells aren't "you."