LEON KASS, chairman of President Bush鈥檚 bioethics council, calls it 鈥渢he wisdom of repugnance鈥�. Others know it simply as the 鈥測uck factor鈥� 鈥� that visceral feeling that there is something wrong, even if you cannot say what. That gut feeling, Kass argues, is telling you that a moral boundary has been crossed and it is time to turn back.
The yuck factor has made more frequent appearances in recent years as biotechnology has advanced. Most recently it has emerged in discussions of human-animal chimeras 鈥� living entities that have both human and animal cells. One researcher, for example, wants to create a mouse with a brain composed entirely of human neurons. That might sound like science fiction, and for now it is. But scientists have already created similar human-animal amalgamations, and there are many more on the drawing board.
Researchers鈥� attraction to human-animal chimeras goes hand in hand with the evolving field of stem cell therapies. They want to uncover the potential of stem cells for both restoring health and destroying it 鈥� which tissues stem cells can differentiate into, and whether they will develop into tumours. This can only be tested by injecting human stem cells into animals, which inevitably means creating chimeras. Chimeras could also give us revolutionary new models of human diseases such as cancer and Alzheimer鈥檚.
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Animal rights campaigners are bound to see chimeras as yet another example of unwarranted exploitation. While most people may not share this view, and even though stem cells have extraordinary promise, there is still something deeply unsettling about the prospect of creating animals that are partly human. Explaining why, however, is tricky.
It cannot be just their 鈥渦nnaturalness鈥� 鈥� that they never existed before humans decided to create them. After all, we have been tinkering with the living world since prehistory, from hybrids to recombinant DNA technology. Such creations are often controversial, but they do not send us into a moral quandary.
There is no doubt that involving humans in experiments raises the stakes: organ transplants and IVF certainly triggered deep moral objections when they were introduced. Yet until recently, human-animal chimeras have not done the same. Few are repulsed by the idea of having a pig valve transplanted into their heart, or an injection of pig neurons into their brain to treat Parkinson鈥檚 disease, even though both turn the recipients into chimeras.
But imagine if that injection of pig cells made its recipient less human 鈥� if it made their thoughts and feelings a little bit pig-like. That is an unsettling thought, and it reveals what is really troubling about human-animal chimeras: the prospect of creating something that is somehow sub-human, a person who is not quite human or one trapped inside an animal鈥檚 body. Where would such a creature belong in a world used to clear distinctions 鈥� social, moral and legal 鈥� between humans and other animals?
鈥淲hat is really troubling is the prospect of creating something that is somehow sub-human鈥�
On present evidence, the chances of a mouse becoming conscious or feeling human emotions are very slim. When human neural stem cells were injected into a mouse brain they followed the rules for mouse neural stem cells. It is likely that 鈥渉umanness鈥� resides not in individual brain cells but in the organisation of those cells. One curious consequence of these experiments is that they expose how little we know about what it means to be human: they may even provide some answers.
The US National Academy of Sciences issued sensible guidelines earlier this year to minimise the potential for 鈥渉umanising鈥� animals and vice versa. That guidance calls for scientists to take extra precautions when altering an animal鈥檚 brain or germ line (lest future generations grow ever more human). And it warns of the need to be alert to the emergence of any human characteristics in animals 鈥� whether in behaviour or appearance.
To reassure the public and ensure that animal experiments remain acceptable, work on chimeras must be carefully monitored and limited to tests that are essential to tackling human disease. 杏吧原创s should also be specific about their purpose: if trials are basic research, relevant only in the long-run, they should say so. There is no room here for the 鈥渨hat would happen if I did this?鈥� type of experiment. The risk of creating a monster, provoking public anger and destroying a field with huge potential benefit is just too great.
