Regardless of how many there are (twins triplets, etc), science tells us that an offspring of human beings is a human being. They have the DNA of a human being and their development is part of the life cycle of a human being.
You do not share DNA with an animal or a tree because obviously, you're a human being because your parents are human beings.
So first, the bolded part is wrong again. Science
doesn't say shit about this. In fact, it
cannot. This is a philosophy question, i.e. 'what is a human?' Science cannot tell you how to define a new human life any more than science can define the word "pink": people philosophically/socially decide about where pink should be (e.g. a lightish red), then science can go about determining what wavelengths of light/what RGB numbers fall into that category, but if the word changes, as they do over time, then the wavelengths of light that are classified as pink change as well. Note that the science didn't change, but the definition.
Fortunately, for something so fundamental, a more permanent definition is asked for, and we can base it on reasoning/philosophy, rather than social norms as we would for colors.
See, all science tells us is that a zygote, like egg and sperm cells, have different DNA than the parents, and are (usually, but not necessarily) inside the parent. But People can even have multiple different types of DNA in the same body, called Chimerism. And groups of existing human cells aren't special either: after a person dies, the cells still survive for a fair bit of time, and much longer if preserved for an organ transplant or the like. That does not make a human either.
Honestly, here's how to classify it: if pre-thought fetus is inside the mother, it's part of the mother's body. If it's outside, it's human tissue, just like a newly dead body, a cut off hand, etc. If it has a brain, it's a person.
Now you try to add "and there development is part of the lifecycle" but that doesn't work either. If I take that hand, extract skin cells from it, turn those into stem cells (I've heard there's a process to do this), then turn that into a line of cells that could turn into a human if implanted, is that now a human? How is that not just my body? If it isn't a human, how is it different from IVF creating a zygote?
Seriously, where did the new human life begin? According to you, it would have to be post implantation, as only then does it resume the lifecycle.
And going back to the chimerism/some cells splitting off but not duplicating enough to survive: both (arguably) the absorbed twin and (definitely) the split cells didn't really go through a life cycle. Do they thus never count as being alive as a separate person? Not by your definition. But if the split cells were saved by a doctor, then implanted later and grew, they might become a full kid.
-Argues that killing twins is somehow proof that killing one kid isn't bad-
Seriously?
-Argues that natural deaths somehow proof that murder isn't murder-
Seriously?
I'll just point out, I think that we should kill humans, from time to time. However, I will freely agree that abortion is, in fact, at the very least, killing a potential human.
I also think that aborting the ones who'll never be anything of worth, at the very least, should be pushed. I've had repeated contact with retarded people, and I do not want, even slightly, to ever support them.
If the mother wants to keep that kid, let them pay for it.
None of this changes the killing in question.
No, I'm arguing that there is no clear line one can draw other than at human thought. The zygote example is about "life begins at conception". The chimerism example is combating "but the mother and kid have different DNA", noting that people can have multiple types of DNA in their body.
