In experiments like those, Greely told the academy last month, "there is a nontrivial risk of conferring some significant aspects of humanity" on the animal.
Greely and his colleagues did not conclude that such experiments should never be done. Indeed, he and many other philosophers have been wrestling with the question of why so many people believe it is wrong to breach the species barrier.
Does the repugnance reflect an understanding of an important natural law? Or is it just another cultural bias, like the once widespread rejection of interracial marriage?
Many turn to the Bible's repeated invocation that animals should multiply "after their kind" as evidence that such experiments are wrong. Others, however, have concluded that the core problem is not necessarily the creation of chimeras but rather the way they are likely to be treated.
Imagine, said Robert Streiffer, a professor of philosophy and bioethics at the University of Wisconsin, a human-chimpanzee chimera endowed with speech and an enhanced potential to learn -- what some have called a "humanzee."
"There's a knee-jerk reaction that enhancing the moral status of an animal is bad," Streiffer said. "But if you did it, and you gave it the protections it deserves, how could the animal complain?"
Unfortunately, said Harvard political philosopher Michael J. Sandel, speaking last fall at a meeting of the President's Council on Bioethics, such protections are unlikely.
"Chances are we would make them perform menial jobs or dangerous jobs," Sandel said. "That would be an objection."
A Research Breakthrough
The potential power of chimeras as research tools became clear about a decade ago in a series of dramatic experiments by Evan Balaban, now at McGill University in Montreal. Balaban took small sections of brain from developing quails and transplanted them into the developing brains of chickens.
The resulting chickens exhibited vocal trills and head bobs unique to quails, proving that the transplanted parts of the brain contained the neural circuitry for quail calls. It also offered astonishing proof that complex behaviors could be transferred across species.
No one has proposed similar experiments between, say, humans and apes. But the discovery of human embryonic stem cells in 1998 allowed researchers to envision related experiments that might reveal a lot about how embryos grow.
The cells, found in 5-day-old human embryos, multiply prolifically and -- unlike adult cells -- have the potential to turn into any of the body's 200 or so cell types.
Scientists hope to cultivate them in laboratory dishes and grow replacement tissues for patients. But with those applications years away, the cells are gaining in popularity for basic research.
The most radical experiment, still not conducted, would be to inject human stem cells into an animal embryo and then transfer that chimeric embryo into an animal's womb. Scientists suspect the proliferating human cells would spread throughout the animal embryo as it matured into a fetus and integrate themselves into every organ.