In Greek mythology, they were monsters - with the head of a lion,
the body of a goat, and the tail of a serpent. But today's chimeras
(pronounced ky-MIR-uhs) are being crafted for a far different
purpose. Scientists hope that by mixing genes of different animals
they can better understand human biology and eventually test new
drugs more safely and accurately, harvest organs for transplant into
humans, and find new cures for human diseases.
There's just one twist: the genes being mixed are animal and
human. Already in Israel, researchers have put human embryonic stem
cells into chick embryos. In Switzerland, they've created mice with
human immune systems; in Minnesota, pigs with humanlike blood; in
Nevada, sheep with near-human livers.
Many people find the whole idea morally repugnant, even if they
have a hard time articulating just why this "yuck factor" bothers
them. Bioethicists and others are also struggling to reason their
way through the issue, weighing possible benefits and risks. The
President's Council on Bioethics is studying human-animal chimeras.
Next month, the National Academies of Sciences will release
guidelines on whether they're ethical to make. Congressional aides
are investigating them, too, with thoughts of drafting legislation.
Inevitably, ethicists are led to debate several questions. Just
what makes humans unique? When would a chimera become too human?
And, if it did show human physical or behavioral traits, why would
that be wrong?
After all, pig heart valves have been used to replace human ones
for years. But experiments involving brains, which seem to come much
closer to affecting the identity of a man or beast, bring deeper
discomfort and more troubling discussions.
Irving Weissman, a researcher at Stanford University, for
example, has proposed creating mice with brains containing 100
percent human neurons. By studying such mice, researchers might find
cures for diseases such as Parkinson's and Alzheimer's, he reasons.
"Scientists are under a lot of pressure to be creative," adds
Jason Robert, a bioethicist at Arizona State University in Tempe,
"to come up with solutions that will help lessen the lag time
between basic research and [medical cures]. But it's not clear that
building chimeras is going to solve the problem."
Another complicating factor: The research involves the use of
human stem cells, already a hot-button issue. "We've focused so much
on the moral status of the [human] embryo that we've forgotten that
there are other [ethical] issues coming to the fore," says Cynthia
Cohen, a senior research fellow at the Kennedy Institute of Ethics
at Georgetown University in Washington and an adviser to the
Canadian government on stem-cell research.
"What we're really worried about is creating some sort of
creature that would be functioning like a human being and yet having
very strong animal-like features," Dr. Cohen says.
What, then, makes humans distinctive? Cohen argues that there are
"a cluster of characteristics" either that are unique to humans or
that humans express to a greater degree than animals, including the
ability to distinguish right from wrong, make decisions and act on
them, do complex thinking, and develop empathy. To keep these
qualities distinct in humans preserves human dignity, she says.
But the proposed Stanford experiment would have had little
likelihood of producing mice with any human behavioral
characteristics, says Henry Greely, a professor of law at Stanford
and the chairman of an ad hoc university committee that two years
ago informally advised Weissman on the ethical implications of his
venture. "Certainly if the mouse stood on its hind legs and said,
'Hi, I'm Mickey!' we'd be worried," says Professor Greely, who is
also director of the Stanford Center for Law and the Biosciences. …