The Uniqueness of Human Embryos

Before gastrulation, a human embryo can split into two embryos (identical twins) or two embryos can merge to become one (tetragametic chimerism). If a human embryo is a human person with a soul from the moment of conception, is a new human person and soul created when that embryo splits in two? Which of the two embryos is the same as the one that existed before - or is neither one the same and did the previous ensouled embryo die? As for a tetragametic chimera formed by the merging of two embryos (usually non-identical twins), does one of the two ensouled embryos die in this process? Is the resultant chimera embryo the same as one of the two that merged to create it? If so, which one? Or is it a different person and did the two ensouled embryos that merged to create it die in the process?

Or, can our understanding of human personhood allow one human person to split into two different human persons without either resultant human person being the same as the preceding one person but also without the death of the preceding one person? Can our understanding of human personhood allow two human persons to merge to become one human person different from either preceding person but without the death of either preceding person?

My vocabulary book appears to be a little bit skinnier than yours, but the concepts don’t seem too hard to figure out to me.

When the sperm and egg unite a new person is created. At a later stage of development if there is a split and an identical twin is formed, that twin becomes a person at the moment he becomes distinct from the original twin. In a related matter, if scientists pursue immoral cloning techniques and create another identical twin (aka clone) decades later, that new person will also have a soul without affecting the soul of the person he came from.

I’m not aware of the biology involved in the chimera you mentioned but if it involved distinct twins that at some point grow together physically and only one child is actually born, he is still only ONE of those original twins. The other died, even if some of his cells live on in his twin. Think about it: your SOUL isn’t altered if you get a kidney transplant, right? Seems like a similar outcome.

How do you know which one of the identical twins is the same person as the original single embryo - and how do you know that one of them has to be the same person?

The same with the chimera - here you have two complete human bodies (albeit embryos) merging to become one. It is not the same as one donated organ being added to an otherwise complete human body. How do you know which of the two original embryos continues on as the merged embryo and which one dies? How do you know that one must carry on and the other must die?

I think that in order to avoid allowing abortion, we are starting to define aspects of the beginnings of human life that we do not fully understand. Even if every embryo is a distinct ensouled human person, I am not sure that the conclusions you draw have to be correct. St. Thomas Aquinas obviously did not fully understand these matters, and I do not think that even with modern science we fully understand them now.

We do not know exactly when ensoulment occurs. This has nothing to do with abortion. The Church has always forbidden the abortion of innocent children. Your examples of things which may happen in the first hours or days after conception do not in any way justify violence against preborn human life.

You ask several times “How do you know… ?” What is it you need to know?

We know that life is good, life is a blessing, and we should not kill. Your examples of twinning or merging of embryos tell us also that life has mysteries and wonders that we do not fully understand, though we may understand better in time. Praise God, who created all the natural processes associated with life.

There is no official teaching; any of your explanations ^above^ would be permitted.

Or, can our understanding of human personhood allow one human person to split into two different human persons without either resultant human person being the same as the preceding one person but also without the death of the preceding one person? Can our understanding of human personhood allow two human persons to merge to become one human person different from either preceding person but without the death of either preceding person?

These explanations are not acceptable. A person’s soul is unique, and cannot split. If an embryo were to die when it splits so to new lives may be conceived, we could only trust in God’s justice for the fate of the deceased.

We don’t know that you are the same person who will respond to this post as the person who created the prior one. Your mother, father, spouse or child does not KNOW that you are the same person who went to sleep in your bed last night.

If this epistemic problem is sufficient to allow abortions, it should be sufficient warrant to allow your family to arrest you for impersonating the person they thought you were but have insufficient evidence to show you still are.

That is a problem with the nature of being human, but we can’t use it to justify killing some humans but harboring others. No?

  1. I don’t know which one was the original because our ability to monitor and track such things doesn’t allow it. But why would I need to know which one came first? There’s no distinction or hierarchy of value there.

  2. No, a chimera isn’t really “two distinct COMPLETE human bodies merging.” That would be conjoined twins, which really do still have distinct souls. But if one twin dies and only the other completes the development process to birth, only that twin is still alive, even if he still has some cells of the other within him. I amy not fully comprehend how completely this process occurs, but I highly doubt that such people have blended brain cells, for example. What little I know suggests that there is one live person with some remnant cells from the deceased twin.

Aquinas and Augustine lived in eras that lacked the proper scientific framework to determine where a quantifiable difference in substance occurred. They labored under the false model of man planting his seed in the woman’s womb and then wondering when that seed became something else. We no longer labor under that faulty scientific framework, so it is not so difficult to see the stage at which the substance of the biology changes. The physical and spiritual are innately linked after all. We should expect to see a physical change corresponding to ensoulment. Conception really is such a substantial change.

  1. Because a human soul (psyche) cannot be divided, neither can two of them merge.

  2. Should their future bodies merge to form Siamese twins, there would be two souls, provided there were two heads/minds. Two souls do not share a single body, but may share conjoined embodiments.

3). When stuff happens at the embryonic level, we do not know, but there is no need to imagine that one of them “died.” After all, there has been no death biologically. If there
was one live embryo and then two, no death has taken place. Why would any soul have to die? Does the difference in embryonic ages really matter?

If two embryos merge, God is sovereign over that. Might one never have been ensouled, and that possibly why they merged? There is no reason to assume that a soul died.

Only one soul per body. Extra genes do not constitute a body.

IMNAAHO, of course.


We don’t know when ensoulment occurs?

If we don’t know with certainty that a fertilized human egg has a soul, what makes it any different from any other human cell that is part of a larger body?

I am not talking jumping from saying there is some ambiguity concerning the uniqueness of human personhood in the early stages after conception to saying that abortion is morally permissible - but I am claiming that fear of allowing abortion can cause people to come up with explanations about human personhood that are really beyond our current understanding.

Even if you observed the joining of two embryos with a microscope you would not know which “person” is the one that continues to live, if the resultant human person can even be said to be the same person as one of the two pre-exising embryos. If someone here knows I’d like to know if scientists understand if there is any way to predict, in a chimera like the one I am describing, which cells from which embryo wind up dividing to make which organs in the resultant human body or whether it is a matter of chance. Also, does the resultant human body have a ratio of cells with the genes of one embryo compared to cells with the genes of the other that tends to be the same across all such chimeras or is that also up to chance?

The only reason people are claiming that you can’t have splitting or merging of souls is because they believe in the first place that a fertilized egg has its own soul from the moment of conception. The same with human personhood - you have to say that one person “dies” when two embryos merge because you believe that a fertilized egg is a human person from the moment of conception.

No, the human soul cannot divide because it is unitary.

Only an indivisible being can be immortal. We die because soul and body can be separated, for instance; and our bodies die if the head comes off the body, etc.

(In eternal life, our souls and bodies will be made inseparable.)

So for the human soul to be immortal, as Scripture holds, it must be unitary and indivisible. And because a soul that could form by merging two others must then be able to divide again, it follows that souls do not merge, either.

It really isn’t about conception, except tangentially.


But does a fertilized human egg have a soul? Being a human life, being a human person, and having a soul are three different things. They can overlap, but do they have to?

And it seems to me a really big deal that one embryo can become two and that two human embryos can merge to become one. If being a human life or a human person means having a unitary and indivisible soul this requires an arbitrary assignment of which embryo is the continuation of the original in the case of the split, and which embryo “survives” in the case of a merger. These explanations don’t seem satisfactory and I think that means there is something lacking in our understanding of ensoulment and personhood with regards to the earliest stages of pregnancy.

You are trying to diminish the sacredness of human life. Resist such temptation.

You’re making this much more complex than it is. It appears certain that science will soon be able to clone human beings (though we OUGHT not do it). Think about that. Cloning a human being is no different in essence than creating a time delayed identical twin. Now, is it really your assertion that upon completion of the cloning procedure, the DNA donor will have a different soul than he had before the procedure (i.e. it divided him somehow?)? No way. The clone will have it’s own unique soul, just like identical twins do.

Since you’re the one asserting the ball is in your court to demonstrate the existence of these “chimera” people that truly are a substantial blend of different DNA sources rather than being primarily one person with remnant portions of a miscarried sibling within them.

If we don’t know with certainty that a fertilized human egg has a soul, what makes it any different from any other human cell that is part of a larger body?

A human zygote is a separate life; a human somatic cell shares life with the larger body.

The soul is the life, therefore a human zygote can hold a soul, whereas a human somatic cell only participates in the soul of the larger body.


Read my post on the meet and greet thread. I’m way beyond whatever hopes you might have for me at this point.

The types of chimeras that I am talking about do not form through the miscarriage of one of two twins. They occur very soon after fertilization when both twins are balls of cells of roughly the same size.

This news article calls the two twins that fused “eggs” but what they mean is embryos:

It’s actually a pretty famous case so I am sure there are better articles about it but I am too lazy to look them up.

You can also look at the wikipedia about this type of chimerism if you wish:

Medical terminology agrees with you that “pregnancy” doesn’t start until implantation. But this is essentially euphemism adopted to allow all sorts of unpleasantness with the veneer of respectability.

In reality, if two eggs are fertilized, then there are two children. In the case you cited, it appears that one of those children did not develop to adulthood, but instead fused together with her and actually formed her ovaries at least. That’s still no evidence of any sort of “blended soul” that you assert. She merely has leftover cells from her sibling that failed to implant.

I never said that human life or human personhood does not start until implantation. I also never said that it I believe that it is definite or even possible that two souls can combine. I am merely saying that we are trying to construct theology about things we really don’t understand that well even with modern technology. Of course, we have to have some moral standard. I just think we should recognize that we still have a lot to learn about the earliest stages of human life.

It seems that from an embryological standpoint, a new and distinct individual of the human species has its beginning at conception. That’s how we all begin. The only question is: after a new human being begins, up till what point in its development is it acceptable to kill it?

DISCLAIMER: The views and opinions expressed in these forums do not necessarily reflect those of Catholic Answers. For official apologetics resources please visit