Are 'hybrid' human-animal embryos a good idea?
* They allow scientists to produce embryonic stem cells for research purposes
* Animal eggs are in plentiful supply and hybrid embryos overcome the shortage of human eggs
* No one will allow them to develop beyond 14 days and the stem cells will not be used in medicine
* It is immoral to mix animal and human stem cells, and is demeaning to life
* Stem cells can be extracted from adult humans, so there is no need to create embryos that are then destroyed
* Allowing such research is the slippery slope to the day when someone clones a hybrid embryo and implants it into a womb
I'm not going to commit myself one way or the other on the value of the research, but I want to take a quick look at the moral ideas implied by the first "no" point and the third. I tend to think that the third makes no sense unless it is coupled with the first, because of the following. It is supposed to be an objection to the creation of hybrid embryos that one might be implanted in a human womb at some point in the future. But, unless there's something special about a "pure" human, then it's hard to see why this would be a bad thing. Thus, the idea that there's something centrally important about human life must be driving the third point; and this is the idea expressed by the first.
The first actually contains two seperate ideas. Idea #1 is that the mix of animal and human is immoral. Idea #2 is that human life can be demeaned. Neither one makes much sense, really.
Idea #1 presumes a pretty hard distinction between human and animal. That is, we are supposed to be able to point out clear paradigm cases of human and animal life, and draw nice boxes around them. In some cases, this is easy. I'm human. My cat is not. But what this ignores is the fact that current biological humans developed from other creatures. I am, in some sense, in a direct genetic line with Australopithecus. Is Australopithecus human or animal? What about homo habilis? Or Neanderthals? The point I'm arguing is not that because we came from creatures that could, arguably, be called "animals" we ourselves must be animals. That's nothing more than a genetic fallacy. What I'm arguing is a little more subtle than that: in order to divide the world neatly into humans and animals, there must be something unique and special that humans have and animals don't. When looking for this, though, we must accept that, at a certain point in our historical development, this special thing must have appeared. This is because, at some point, everyone will freely admit that our genetic ancestors were animals and not humans. So, we don't just have some sort of silly demarcation problem -- i.e., where do we draw the line? -- but a much more fundamental worry: on what basis, in principle, could we draw a line? Intelligence? Neanderthals were intelligent, and, for that matter, so are chimpanzees and bonobos. Indeed, many of the characteristics we might consider "specially human" are shared between us and other contemporary and historical apes.
One way to solve the problem, of course, it to claim that what we really want to do is seperate apes (not humans) off from animals. But the problem is just going to repeat itself. At some point in their historical development, apes must have acquired this special property. Where? We would very likely be able to see antecedents of it in other mammals; so, should we try to seperate mammals from other animals?
In short, Idea #1 presumes that nature admits of dichotomies rather than continua. That is, Idea #1 presumes that the world comes to us in neat little boxes with clear and defined edges, when it actually comes to us as a big smooshing rush of creatures and things that are not clearly and distinctly different from each other.
To make matters even worse, not only does Idea #1 make an error of fact, it makes an error of morality. That is, it presumes that because there is a natural difference, then there must also be a moral difference. But, there are natural differences between humans of different races and sexes. Do these also make moral differences? If the speciesist argument against mixing animal and human cells is supposed to be a moral argument that must be taken seriously, then the logic underlying racist and sexist arguments must also be taken seriously, naturalistic fallacy be damned. (This is, famously, Peter Singer's view.)
So, Idea #1 is full of holes. What about Idea #2? According to Idea #2, there's something valuable in human life that can be demeaned. By this I presume it is meant that the value of human life can be reduced in some central or fundamental way. Now, it doesn't follow from my critiques of Idea #1 that there's no value to human life; the only point there is that you can't justify saying that there's a unique and special value to human life qua human life. So, human life could still be valuable, and this value could somehow be reducible. But what is this value supposed to consist in? Presumably, a human life is valuable insofar as it instantiates whatever actually is valuable -- or, as a sop to the moral anti-realists in the audience, whatever is taken to be valuable by society, or whatever my emotional reactions point to as being valuable, etc, etc. So, a human life is valuable insofar as it is a life of friendship, justice, courage, charity, fidelity, and so on. This value can be demeaned, I suppose, if one fails to instantiate these values, or, perhaps, if one instantiates them at one point, but then ceases to do so. Perhaps also the value of a life can be demeaned if it instantiates the wrong values or bad values or insincere/inauthentic values. But, notice what has happened here: in trying to explain how life can be valuable, I have slipped into talking about what makes a life valuable. That is, what makes a particular life valuable, not life in general. And that seems a necessary move, for there are far too many possible paths a human life may follow for one to say, with any hope of being even close to right, that there just is value to life qua life.
Given that, then Idea #2, as presented, rests on a mistake. But, it could be easily repaired by stating that the value of any life would be demeaned if animal-human hybrids were allowed. This, however, points right back to the objections made against Idea #1: namely, this only makes sense on the presumption that a life is only valuable if it is a life of a human rather than the life of something with another genetic structure, like a Neanderthal, a chimpanzee, or a cat (or, for that matter, the life of a Martian or of a god). As said, though, this is badly flawed: there are no clear boundaries to pick out what is valuable about a specifically human life; and there is something deeply wrong with the logic that picks on a natural difference in order to justify a different in moral treatment.
So, if there is something wrong with animal-human hybrids, it's going to have to have something to do with the second point, namely their efficacy as research tools. Since this is an exclusively scientific question, and the body considering the issue is a scientific one, it is this question that they should be considering in detail.