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What happens when you cross a human and a mouse? Sounds like the beginning of a bad joke but, in fact, it's a serious high tech experiment recently carried out by a research team headed by a distinguished molecular biologist, Irving Weissman, at Stanford University's Institute of Cancer/Stem Cell Biology and Medicine. Scientists injected human brain cells into mouse fetuses, creating a strain of mice that were approximately 1 percent human. Dr. Weissman is actively considering a follow-up experiment that would produce mice whose brains are 100 percent human.

What if the mice escaped the laboratory and began to proliferate in the outside environment? What might be the ecological consequences of mice who think like human beings, let loose in nature? Dr. Weissman says he would keep a tight rein on the mice and if they showed any signs of humanness he would kill them. Hardly reassuring.

In a world where the bizarre has become all too commonplace, few things any longer shock the human psyche. But, experiments like the one that produced a partially humanized mouse at Stanford University stretches the limits of human tinkering with nature to the realm of the pathological.

The new research field at the cutting edge of the biotech revolution is called chimeric experimentation. Researchers around the world are combining human and animal cells and creating chimeric creatures that are part human and part animal, reminiscent of the ancient Greek chimeric myths of human-animal hybrids.

The first such chimeric experiment occurred many years ago when scientists in Edinburgh, Scotland fused together a sheep and goat embryo-two completely unrelated animal species that are incapable of mating and producing a hybrid offspring in nature. The resulting creature, called a Geep, was born with the head of a goat and the body of a sheep.

Now, scientists have their sights trained on breaking the final taboo in the natural world-crossing human and animals to create new human-animal hybrids of every kind and description. Already, aside from the humanized mouse, scientists have created pigs with human blood running through their veins and sheep with livers and hearts that are mostly human.

The experiments are designed to advance medical research. Indeed, a growing number of genetic engineers argue that human-animal hybrids will usher in a golden era of medicine. Researchers say that the more humanized they can make research animals, the better able they will be to model the progression of human diseases, test new drugs, and harvest tissues and organs for transplantation into human bodies. What they fail to mention is that there are other equally promising and less invasive alternatives to these kinds of bizarre experiments including sophisticated computer modeling to study diseases and test the effectiveness and toxicity of drugs as well as in vitro tissue culture, nanotechnology, and artificial prostheses to substitute for human tissue and organs. When it comes to chimeric experimentation, then, the question is, at what price?

Some researchers are speculating about human-chimpanzee chimeras-creating a humanzee. A humanzee would be the ideal laboratory research animal because chimpanzees are so closely related to human beings. Chimps share 98 percent of the human genome and a fully mature chimp has the equivalent mental abilities and consciousness of a four-year-old human child. Fusing a human and chimpanzee embryo-a feat researchers say is quite feasible-could produce a creature so human that questions regarding its moral and legal status would throw 4,000 years of human ethics into utter chaos.

Would such a creature enjoy human rights and protections under the law? For example, it's possible that such a creature could cross the species barrier and mate with a human. Would society allow inter-species conjugation? Would a humanzee have to pass some kind of "humanness" test to win its freedom? Would it be forced into doing menial labor or be used to perform dangerous activities?

The horrific possibilities are mind-boggling. For example, what if human stem cells-the primordial cells that turn into the body's 200 or so cell types-were to be injected into an animal embryo and spread throughout the animal's body into every organ? Some human cells could migrate to the testes and ovaries where they could grow into human sperm and eggs. If two of the chimeric mice were to mate, they could potentially conceive a human embryo. If the human embryo were to be removed and implanted in a human womb, the resulting human baby's biological parents would have been chimeric mice.

Please understand that none of this is science fiction. The American National Academy of Sciences, the country's most august scientific body, issued guidelines for chimeric research on April 25, anticipating a flurry of new experiments in the burgeoning field of human-animal chimeric experimentation. What would be the ramifications of creating hundreds, even thousands, of new life forms that are part human and part other creature? Creatures that could mate, reproduce, and repopulate the Earth?

Bio-ethicists are already clearing the moral path for human-animal chimeric experiments, arguing that once society gets past the revulsion factor, the prospect of new, partially human creatures has much to offer the human race. And, of course, this is exactly the kind of reasoning that has been put forth time and again to justify what is fast becoming a macabre journey into a monstrous Brave New World in which all of nature can be ruthlessly manipulated and reengineered to suit the momentary needs and even whims and caprices of just one species, the Homo sapiens. But now, with human-animal chimeric experiments, we risk even undermining our own species biological integrity in the name of human progress.

With chimeric technology, scientists now have the power to rewrite the evolutionary saga-to sprinkle parts of the Homo sapiens species into the rest of the animal kingdom as well as fuse parts of other species into our own genome and even to create new human subspecies and superspecies. Are we on the cusp of a biological renaissance, as some believe, or sowing the seeds of our own destruction? Perhaps it is time to ask what we mean by progress.
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