Blending humans with animals is not the sort of ethical issue that most people deal with from day-to-day. But it is becoming increasingly important in stem cell science. Esmail Zanjani, of the University of Nevada at Reno, has already created adult animals with human cells integrated into their bodies. Irving Weissman of Stanford University has created mice with fully human immune systems to study AIDS.

There are currently no international standards for experiments which mingle human and animal tissue. The resulting beings, called chimeras, are useful research tools which enable scientists to study how cells function. But there are fears that scientists could blur the line between animal and human. Dr Weissman has proposed, for instance, creating a mouse whose brain is composed of human neural cells. This is unlikely to become Stuart Little — but what if he used a chimpanzee?

This ethically fraught area could be the next battleground for stem cell science. In March Senator Sam Brownback introduced a Human Chimera Prohibition bill. Also in March, the President’s Council on Bioethics teased out some of the moral implications of such research in a meeting in which it became clear that both the questions and the answers about this controversial research are quite muddled. Then the National Academy of Sciences recommended that the creation of most types of chimeras be permitted. Now New Scientist magazine has argued that “revulsion is not enough to stop [this] new research”.

A debate over chimeras could easily become a proxy debate over evolution. David P. Barash, a psychology professor at the University of Washington, enthusiastically endorsed the creation of viable chimeras recently as a way of discrediting creationism. “In these dark days of know-nothing anti-evolutionism… a powerful dose of biological reality would be a very healthy event. After all, perhaps the most important take-home message from evolutionary science is the one that radical fundamentalists find most unacceptable: continuity [between the species]. And this is precisely the message that chimeras, hybrids or mixed-species clones would drive home.”

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