Via bioethics.net comes news of a National Geographic News article called “Animal-Human Hybrids Spark Controversy”. The article is, in my judgment, really confused: confused in a way that makes it much more sensationalistic than it should be, and obscures the really interesting questions that human/non-human chimeras raise.
For instance, it says that “at Stanford University in California an experiment might be done later this year to create mice with human brains.” Now, I don’t know what, exactly, they are planning to do at Stanford, though I can guess. But it’s really unlikely that they are going to try to make mice with human brains. Why? Well, for starters, think of the size problems. You have a little mouse body a few inches long, weighing maybe an ounce, and then attached to it a mouse head big enough to house an entire human brain, weighing a couple of pounds. Leaving aside such questions as, how would it walk? How huge would its neck muscles have to be? and so forth, just ask yourself how, exactly, a human brain is supposed to fit inside a mouse cranium, even assuming that crania have some capacity to expand early in development.
You might at this point be thinking: silly hilzoy! Obviously, what the article means is that they will create a mouse with a mouse-sized human brain, just as, if someone said they were going to build a Matchbox car with a working automobile engine, they would mean a Matchbox-sized engine, not a regular one. But how would this work? A mouse-sized brain made of human neurons would not be (what we normally think of as) a human brain, any more than something small enough to fit into a Matchbox car chassis, but made of (a small number of) normal-sized engine parts, would count as a normal working engine. The obvious solution would be to make the engine, or the brain, out of tiny replica parts. But we don’t have tiny little replicas of human neurons. Nor is there any reason to think that it’s even possible to create a tiny version of the human neuron that works the way a human neuron works, so that if we arranged those tiny neurons the way normal neurons are arranged in the human brain, they would work (a) at all, or (b) the way a human brain does. So the idea of a mouse with a human brain, in anything like the normal sense of that phrase, is just a non-starter.
Likewise, the article raises this (im)possibility (quoting a bioethicist who should know better):
“an experiment that would raise concerns, he said, is genetically engineering mice to produce human sperm and eggs, then doing in vitro fertilization to produce a child whose parents are a pair of mice.”
Just try to imagine how the logistics of this might work. The two mice mate, and conceive a child. It begins to develop. Then what? Presumably, one of two things happen: the fetus dies, or the female mouse bursts. What could not possibly happen is that a female mouse could actually, literally, carry a human infant to term and then give birth to it. (Through a mouse pelvis? After spending the better part of the entire mouse lifespan pregnant with a child that would, at birth, weigh on the order of a hundred times as much as she does? Please.)
As I said, though, all this just serves to obscure some interesting questions, to which I will now turn. (Warning: it’s going to be one of my wonky posts. But it will be interesting to me to see whether anyone makes it through, and if so whether they think I’m right.)
Read more
