If humans are nothing more than the interaction of DNA and environmental stimuli
There is an awful lot going on in your question, and some of it I do not feel qualified to respond to. In particular, I think a complete answer to your question would require a good deal of work from contemporary theories of the mind, as well as how these theories inform questions of personal identity. So what I am about to say is only a very partial response (and may be inadequate even at that!).
To be frank, I think the whole idea of life after death is--at least in the various ways I generally hear it characterized--probably nonsense. To see this, let's begin with your experience of yourself--what it is like being you. Think about this for a minute to bring in into focus (as best as you can), and then try out a few of the popular afterlife options:
(1) Now imagine being both you and also, say, a chicken. Peck, peck...cheep, cheep...nice beak! Nah--you have no idea what it would be like to be a chicken, and whatever that would be like, it most certainly can't be anything like what it is like to be you. Maybe your atoms or molecules could become atoms or molecules in a chicken, but that hardly makes the chicken a new embodiment of you. Now I cannot "channel" chicken minds, of course--indeed, I can't even channel yours, despite our shared humanity (this is the famous "problem of other minds"). But I think you will find it is more than just a mystery to think about you being or becoming a chicken. So I am inclined to think that whatever your causal relations with the chicken who lives (on) after your death may be, there is no sense in thinking that the chicken will in any cogent sense be you. So much for transmigration of souls.
(2) OK, now imagine you having a different human body. Suppose now you are an adult male Chinese. OK, now imagine being an infant female from the Aka tribe of Africa. Wah! Got it? ...I doubt it. As I said, there is not only a problem of other minds here, there is also the problem that our personal experiences seem to be gendered (which can sometimes create very difficult challenges for transgendered people, whose personal experience--you might say personal identity--is one of the opposite gender than their own body). But then, what would it be like for you to be a member of a completely different ethnic group, with a completely different personal, familial, and cultural history, and so on? In what sense would that person be you? To go back to my example, you were an infant once (in some historical sense, at least). What was that like? You may have childhood memories that seem to be part of your current identity, but I doubt that you have any idea what it would be like to be you and at the same time be an infant. But if there is "life after death" in some meaningful way, there would need to be continued personal identity before and after death. That's my question: does it really make sense to think there could be an identity relation between you and the Aka baby girl? Say what??? So much for reincarnation.
(3) OK, well try this one now: imagine being both you and also a disembodied soul. Look Ma! No hands!!! Yeah, and no arms and no legs and no shoulders or hips; no belly or chest, no head, no brain...nothing but...well, whatever! What would that be like? Well, speaking just for myself here, I can't wrap my mind around the idea at all. Everywhere I have gone in life, my body has been with me all the while. Now, some people claim to have experiences "astral projection" in which they have experiences as of floating outside of their bodies. Could be I am stupid about this simply because I haven't had such an experience myself, but I wonder what they were seeing with since they didn't have eyes at the time? What would that be like? Damned if I know... To be frank, from an experiential point of view (in order words, in terms of my sense of self), to be me is to be embodied as I am. Even if some disembodied thing (whatever that might be!) conceived itself as in some way identified with me, I'd be inclined (at the moment, anyway!) to deny the conception and identification: whatever it is to be a disembodied thing, that can't be aligned with what it is to be me. So much for the separation of soul or spirit from the (dead) body.
Here's the litmus test I am proposing: what it is like to be you is either consistent with whatever the afterlife experience is supposed to be, or else whatever follows your death will not be you in an afterlife. Given the various kinds of nonsense I have heard about "the afterlife," I find absolutely no reason to think that there will be such a thing that I could count as me surviving my own death. As for you...well, I don't know what that is like, so...