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The frenzy was triggered by England’s Human Fertilization and Embryology Authority, which is pondering the legality of “chimeras,” meaning organisms that carry both human and animal genes. Such creatures may seem like science fiction, but in less spectacular form they’re already common, from cows injected with human stem cells in order to produce a human protein in their milk, which is extracted and used to cure hemophilia, to mice with human neural cells in their brains in order to test treatments for Parkinson’s and Alzheimer’s diseases.

Those examples may seem relatively benign – after all, a cow producing human protein is still basically a cow – but it is fear of a slippery slope toward confusion between human and animal that really causes conniption fits.

That’s the terrain, for example, of Michael Crichton’s new Jurassic Park-style thriller, titled Next, about a researcher at the National Institutes of Health who mixes human and chimpanzee DNA, and then tries to pass off the resulting child as fully human. (Perhaps inevitably, wags label it a “humanzee.”) The riddles that would surround such a creature – what rights it might enjoy, whether it could be exploited for manual labor or have its organs forcibly harvested, and for the religiously inclined, whether it would possess a soul – give most ethicists and theologians a migraine.

Then there’s the “yuck factor,” the basic repugnance many people feel about species-bending mutants whipped up in labs. Such doubts notwithstanding, experts say the technology is largely in place to make it happen – and human history, they ruefully observe, is not exactly replete with examples of technologies that, once developed, were never used out of a sense of restraint.

Welcome to the brave new world of the biotech revolution.

Debate over chimeras swirled in the United States in 2005, when U.S. Sen. Sam Brownback, now a Republican presidential candidate, introduced his “Human Chimera Prohibition Act,” which would have banned the creation of animals with genes from the human brain, or animals with any human genes if they could reproduce, both labeled by Brownback as affronts to human dignity. (He also contended that chimeras could exacerbate the transmission of diseases across species boundaries.)

The bill did not come up for a vote, but analysts expect Brownback to reintroduce the measure, which the U.S. Conference of Catholic Bishops supports, and probably to campaign on it.

U.S. President George W. Bush called for a ban on “human/animal hybrids” in his 2006 State of the Union address.

Redemptorist Fsther Brian Johnstone, a moral theologian at The Catholic University of America in Washington, said that the church does not have any official teaching directly on chimeras. But documents on transplants have carved out a clear principle: Transferring genetic material across species lines is OK, as long as the identity of the individual, and its offspring, is maintained. Anything that blurs the distinction between human beings and the rest of creation goes too far.

Religious leaders concerned about human dignity are not the only forces raising questions about chimeras. Animal rights groups generally approach the issue from the other end, objecting to the exploitation of animals for human use, while environmentalists worry about the genetic manipulation of nature.

Whatever one makes of them, chimeras exemplify the rapidly developing, and occasionally creepy, ethical challenges that arise on the frontiers of today’s genetic science. The Vatican’s Congregation for the Doctrine of the Faith announced Jan. 28 that it’s working on a new document on bioethics, a successor to 1987’s Donum Vitae, to address this sort of new moral conundrum.

Conflict from the start

In Greek mythology, the original chimera was a ferocious mixture of lion, goat, dragon and snake, whose fury caused storms and shipwrecks. (Students of the classics will recall that the chimera was slain by Bellerophon while riding another genetic amalgam – Pegasus, the winged horse. Conflict, it would seem, has surrounded the chimera from the start.)

Today, the term is used to indicate any individual carrying two distinct genetic patterns.

As opposed to a hybrid, where the genetic materials of two species fuse, in a chimera the genes remain separate. Technically, a human being with a transplanted pig liver could therefore be considered a “chimera,” but such procedures have not generated serious ethical qualms – mostly because nobody really believes that carrying around a pig’s liver diminishes personal identity, turning the recipient into a pig-human mutant.

The basic idea for a chimera is not new. Futurists have predicted such creatures for centuries. Two decades ago, Harvard researchers patented a mouse which carries a human cancer gene, known as the ...