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 such XX/XY intersex 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.