When does (human) life begin?

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That’s an argument often used from the pro-life side. They claim that life begins at conception; when a sperm cells fuses with an egg cell to form a zygote. The zygote is said to be a human person. However, setting aside the abortion debate, real life biology poses some issues. Specifically the instance of monozygous twins.

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If one maintains that human life begins at the zygote, then one has to maintain that the identical twins that result from twinning are actually one human.

Clearly, most people would not accept this. Especially twins. Alternatively, if we would maintain that a zygote is a human individual, and the twins are still separate individuals, we would have to believe one of the following:

(1) Only one of the twins is the same human individual as the original zygote, but the other twin is a new individual who began during or after the twinning process. But we can’t tell which one is the original.

Or (2) Neither twin is the original. All twins began during twinning, while the human individuality of the original zygote is lost.

In either case, this would still mean that human life does not begin at conception… at least for some individuals. And since twinning can occur at various points during development (see image above) we would have to admit that (during this period) we are not able to objectively conclude that the zygote’s / embryo’s “destiny” is to be one discrete individual.

Things get even hairier when we consider genetic chimerism. This is almost the opposite of twinning. Here, the embryo’s of two non-identical twins fuse into one and develop as one body. Sometimes the fusion can create conjoined twins where seperate body parts are still recognizable. However, sometimes the fusion is seamless, such that the person can be a chimera and live their life never knowing this fact. And sometimes, this fusion can involve an embryo with an XY karyotype type with one with an XX karyotype. One example of an XX/XY cellular mosaic intersex case is Foekje Dillema, who unfortunately faced undeserving discrimination due to her condition.

In such a case, maintaining the belief in life at conception would mean that a genetic chimeric person is actually multiple persons, or that one of the original zygote person dies during or after fusion.

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As someone who supports the pro-choice side, I find arguments over when “life” begins pointless. As Sam Harris says, the skin cells on our nose are alive so, if “life” is the issue, every time we scratch our noses it’s another Holocaust.

The more pertinent issue, IMHO, is: At what point does a living human conceptus become a person, with all the rights that entails?

Your post provides some good arguments why the definition of “life” is a non-sequitur in this debate.

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There’s an alternative that still saves the definition: perhaps one individual can become two or two can become one. If it happens for cells, why not for people? (Disclaimer: it’s a silly and arbitrary definition anyway.)

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Not really necessary to the discussion, but there are plenty of other corner cases to consider: blood donation and organ transplants, transplants from cadaver donors, cell migration from fetus to mother, cloning.

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From an evolutionary point of view, human life started (depending on your definition of “human”) about 1 million years ago, or perhaps as recently as 800,000 years ago. And it hasn’t stopped yet.

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Give it time

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But that’s poses some philosophical questions. When one individual splits into two, what happens exactly to the original individual. Are there two new people (the original is lost) or is one of the two the original and the other the novel clone?

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Those may be philosophical questions, but they are not biological questions. It is interesting, and disconcerting, that philosophers find those questions to make sense.

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Again, there are more alternatives. Maybe they’re botth the original individual.

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Sounds like Science Fiction to me …

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How about this?: because of semi-conservative replication, each one is half the previous individual, but as cells continue replicating, the original individual becomes vastly outnumbered by copies. Still, there remains in each clone a single cell that’s half the original.

Sorry, forgot about independent assortment as well as diploidy. In each clone there are 92 cells that are 1/4 the original.

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At UW there was a philosophy professor who thought that was a worthwhile question: Charles Marks, not to be confused with Engels’ friend. I took his intro course back in 1979 and the main thing I learned was that sometimes philosophers are interested in very silly questions. I suppose your paths are unlikely to have crossed.

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I don’t agree that the possibility of twinning means that the original embryo was not a human. I could be cloned today, but I am a human. If one embryo merges into another, I would say that one embryo died of natural causes. The possibility that one may die of natural causes does not mean that one is not a human.

Broadly speaking, “when is this a person?” can either be answered with a discrete moment or characteristic (conception, heartbeat, etc) or some kind of spectrum. A spectrum is problematic because it may imply that some people (eg, those with higher intelligence) are more human than others. And most of the yes/no criteria one could choose are pretty arbitrary (Eg, why is a heartbeat morally significant? Am I not human if I’m in cardiac arrest?). The moment that there is a distinct human body has the advantage of being an objective fact.

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At 40?

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This is a poor argument for at least two reasons.

  1. It is a non sequitur. It is simply not true that a non-discrete “spectrum” of when something arises (“being human” or “being conscious” or “being old” or anything else) implies that the things being assessed are themselves on a gradient. Once you’re old, you’re old. Ask me how I know.
  2. The certainty that you seek doesn’t exist, because whatever you want to measure (“conception,” heartbeat, doesn’t matter) is also something that happens on a gradient. There is no “moment of conception.” We can certainly identify before and after, but we can’t pinpoint the moment. Think about it then: how is “conception” different from another developmental process like, say, the emergence of brain activity of some kind?

This doesn’t mean that I think you are factually wrong to advance “conception” (by this I assume you mean fertilization) as a reasonable definition of the beginning of a human individual, and I think you are wise to criticize measures of “personhood” that could be used in problematic ways. (You don’t need to talk about human embryology to find disturbing examples of this.) It does mean that you haven’t provided the “advantage of being an objective fact” with your choice.

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Sure if we take intelligence to be a defining characteristic of being human. But, we can just decide not to. All definitions are, of course, made up.

And then you have to define what you mean by a “human body”, and “distinct”.

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Once you’re old, you’re old.

That’s a funny example. At 20, I thought 40 was old. 90-year-olds think 60 is young. “Just remember when you could still tie your shoes!” :winking_face_with_tongue:

As far as fertilization, sure, it’s a process that takes maybe 24 hours. If anyone asks whether there’s a human with rights at hour 12, I’m not sure. My point was that afterward, the existence of a human embryo is an observable fact.

Think about it then: how is “conception” different from another developmental process like, say, the emergence of brain activity of some kind?

How it’s different is that we don’t have decide on which kind of brain activity and how much of it and how to measure it. The simplest answer to “which biological humans have human rights?” is “all of them.” Other answers seem dangerously arbitrary to me.

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Except that this is an obvious over-simplification that is already known to not work in practice. Children have a limited capacity to care for themselves, and so have more curtailed rights. As do some adults whose limited capacity has been legally determined. Brain-dead “biological humans” have very few rights.

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And then you have to define what you mean by a “human body”, and “distinct”.

Set aside the human question for a moment. Isn’t a mouse zygote a biologically distinct organism from its mother?

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You used the phrase “discrete moment” so I’m pleasantly surprised to see that you understand that “conception” is neither. But there’s still a pretty big problem, which is that you are emphasizing the claim of “observable fact” but failing to convince me that this particular developmental event is suitable for defining something as a “biological human” with “human rights.” Any discrete and definable developmental transition would suffice for that. Even if it were true that “conception” were an easy-to-use criterion (it’s not), you would need to argue that it’s a good way to define whatever it is you want to define.

Again, I’m not saying you are factually incorrect; I’m just pointing to something that should be obvious: that you are simply advancing an apparently arbitrary definition of “person” (or whatever) then trying to fit some biology on top of it. It can’t work.

That’s an unconvincing distinction for technical reasons (since we had to decide on fertilization too), but much more importantly: we use brain activity as a major criterion for defining the end of life. It’s okay if you don’t like that, but I’m not seeing even a hint of an argument why.

That has nothing whatsoever to do with how to define anything. It’s really weird to see you wedge that in. Do you somehow believe that those who would use different definitions/criteria are people who wouldn’t answer “all of them” to that question? Really poor choice to do that.

I respect your opinion, a lot, but I don’t think you can see that your choices are also arbitrary, and even “dangerously” so to anyone who draws the lines elsewhere.