Current favorite picture of things: I am an organism, essentially (so: Animalism). But must I think I began to exist as a zygote? Maybe not. First there was a zygote, which developed and grew into an embryo, and then a fetus (etc.) and at some point the fetus became me. There's a lot to say about why that picture is attractive, and why it might also be unattractive ... but I want to focus on just one issue here: does it makes sense to suppose that a zygote becomes a mature organism, without being identical to it? Can A become B, in the absence of an underlying entity C, such that A=C and B=C? Or to put it another way, does it make sense to think of A becoming B, when it's also the case that A goes out of existence?
One indication that this might make sense involves monozygotic twinning. Suppose a zygote Z becomes two embryos, E1 and E2. Z can't be identical to either of the successor embryos (since they're not identical to each other), so Z goes out of existence. Nevertheless, it's fair to say that Z becomes E1 and Z becomes E2. This is becoming without identity. There's no underlying entity that persists, as Z becomes E1 (and also E2). Jeff McMahan points out that this sort of going out of existence via division isn't problematic in The Ethics of Killing:
There does not, however, seem to be anything problematic about the claim that, if an organism begins to exist at conception and twinning occurs, the organism simply ceases to exist. Ceasing to exist through division is not the same kind of event as death and does not leave dead remains behind. Thus, for example, when an amoeba divides, it ceases to exist though it does not die. While living entities may cease to exist by dying, some may also cease to exist in another way, by dividing. (p. 27)Could it be the case that even without twinning, a zygote/fetus goes out of existence when it becomes a baby? If "ceasing to exist through division" is a possibility, why not also "ceasing to exist through transformation"? This, again, would be the sort of ceasing to exist that doesn't leave remains behind. When there's radical transformation, you might say, A becomes B, but A also goes out of existence. There's no underlying entity, C, that endures "underneath" the transformation.
In fact, isn't this exactly how we think about death (or at least could think about it)? There is a living organism--A. It dies, and there are so many radical transformations involved that the corpse, B, is something new. A both goes out of existence and becomes B. Surprisingly (to me, anyway), McMahan thinks if you take A as essentially an organism, you can't make out the idea that it goes out of existence when death occurs. Since that's absurd, you shouldn't (on his view) take A to be essentially an organism.
But why can't you think of a living organism as ceasing to exist, when death produces a corpse? He writes,
If, however, an organism ceases to exist when it dies, what exactly is the corpse and where does it come from? Merely labeling it the 'remains' of the organism is unilluminating (p. 20).
He comes up with four possibilities: (1) The corpse is "an entirely new entity, one that springs into existence in the area of space that the organism previously occupied immediately upon the organism's death." (p. 20) His assessment: No, of course not, that's silly. (2) The corpse was there all along, a second entity in addition to the living organism, A. His assessment: Also silly! (3) The living organism, A, is just a phase in the existence of a longer-lasting entity, and the corpse is another phase. So the organism goes out of existence in the same sense that a child goes out of existence when she becomes an adult. His assessment: This is inconsistent with thinking that A is essentially an organism, so it's ruled out too. (4) Upon death, the organism, A, basically disintegrates, leaving behind no further entity, B. In a nutshell: there are no corpses. Again: Silly.
But isn't there another option? Let's call it (1A), since it's close to (1).
(1A) The corpse is a new entity, but not "entirely new". Most of the molecules in the corpse come from the intial living body. Much of the organization of the corpse is owing to things that happened to the prior living body. The living body becomes the corpse, but also goes out of existence, since the transformation is too great to sustain identity.
If division of Z into two embryos is a way for Z to go out of existence, and we shouldn't think of twins E1 and E2 as "entirely new" in some mysterious or absurd way, then why not also think of ordinary death as a way for a living organism to go out of existence, though also becoming a corpse? So: becoming without identity, in both instances. And then, why not use the notion of becoming without identity to understand us and our origins? Zygotes become you and me, but we're not identical to them.
When all is said and done, I fear there may be more than one coherent way to think about these things. All I say (so far) is that this might be one of them.