And its tail is a snake

I wondered this morning what the possibility of human chimeras does for bioethical concerns about identity. I figured there must be literature on this, but my internet search only turned up the odd fact that Mary Ann Chimera is president of Democrats for Life of Ohio. So here is some half-baked musing.

Sometimes when there are two or more fertilized eggs, they don’t separate enough for there to be twins or even conjoined twins. The cells develop into just one ordinary looking body, making it so that different parts of the body have different genomes. The resulting organism is called a chimera.¹ This might have observable results, as when somebody’s eyes are different colors, but often it will have no noticeable effect.²

Now consider the Christian pro-life doctrine that life begins at conception. When a sperm enters an egg, God links a soul to the cluster of developing cells and makes it a person. It seems to follow from this that a chimera which results from two eggs would have two souls.

One could just bite that bullet and insist that chimeras are unnatural mosters, but it gets worse.

The ensouled-at-conception view goes naturally with a kind of dualism on which the soul is where conscious experience and agency happens. So a chimera ought to be able to tell by introspection that they are a chimera. Imagine a chimera with the dividing line down the middle of their body. Each soul should only have experience and control agency over one half. They ought to be like some tragically conjoined twins. But, of course, that’s not what happens.

Chimeras also raise problems for more mainstream bioethics. One of the stock issues in reproductive ethics and population ethics is the non-identity problem. Discussions of it often assume a kind of essentialism according to which a person is determined by their genetic identity. If a different sperm or different egg had met, then a different person would have been born.³ On this way of thinking, it makes sense to ask how my life would have been different if I’d had a childhood disease but not to ask how my life had been different if I had been born with a heritable disease. In the latter case, it would have been a different genome and not me at all.

Now consider a chimera who has genome A throughout most of their body and genome B in (let’s say) their liver and kidneys. Can we imagine the same person without chimerism? One possible intuition is that we cannot, for non-identity reasons. Another possible intuition is that we could if we imagine them having genome A, but not with genome B.⁴ This starts to put pressure on the essentialism that underlies thinking about non-identity.

1. The name is taken from the mythical creature.
2. One often-cited estimate puts the number of humans who are chimerical at about a third, but there really isn’t a whole lot of evidence. It’s hard to detect even with genetic tests, because you’d have to sample lots of cells.
3. M. A. Roberts at the SEP writes cautiously, “a distinction in sperm and egg cells would seem in most cases sufficient to insure the conception of a distinct child.”
4. The idea, suggested by how we think about transplants, is that liver and kidneys aren’t identity constituting.