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The boundary line between human beings and animals has never been altogether clear. Now -- as we read about laboratory mice with human genes and contemplate a future of pig-to-people organ transplants -- the ancient fantasy of a chimera, a being part human and part animal, is fast becoming a scientific reality.

The news from the laboratories is spectacular and more than a little science-fictionish. Among the new creatures that have been created is the "oncomouse," which develops human cancers, and other mice that model other human ailments such as Alzheimer's and diabetes.

Most of us will never see a genetically-engineered mouse, but the odds are very good that in the decades ahead we will see -- or be -- people with hearts, livers or other organs transplanted from animals. Pig valves are now commonly used in heart operations, but so far no attempt to transplant a whole organ from an animal to a human being has succeeded.

That will change fairly soon, as researchers create transgenic animals whose organs are less likely to be rejected by the human immune system. One group has bred three generations tTC transgenic pigs, and clinical trials with human beings are the next step. "Now," reports BioScience magazine, "biologists are rushing to develop a potential new source of donor organs: farm animals. Researchers envision organ farms, where pigs, sheep and perhaps other animals may be raised not just for their meat but also for their major organs."

This means that we are on the verge of a new chapter in humanity's haunted and ambivalent relationship with animals. People from the earliest times have dressed themselves in the hides and horns of animals, and drawn on their strength symbolically in rituals, myths and totems. Animals stalk, gallop, fly and slither through family names, coats of arms, military insignia, commercial brands and athletic teams.

But despite our dependence on all kinds of nonhuman creatures -- or perhaps because of it -- human beings have always feared becoming too deeply connected with animals, slipping into animalism themselves. Animals figure heavily in taboos as well as totems, are often devils as well as gods. In Christian folklore the Evil One is often shown to be half-animal, with tail and horn and cloven hoofs. People in medieval Europe were executed for the abominated (but apparently all too common) sin of sexual intercourse with animals, and often their poor partners in this crime were put to death as well.

This phobic anxiety about maintaining the animal-human boundary was a major source of opposition to Charles Darwin's theory of evolution. Although "The Origin of Species" really didn't explicitly say there was a family connection between apes and human beings, the general impression was that the book did assert such a connection. People who rejected the idea proudly proclaimed themselves to be on the side of the angels rather than the side of the apes.

The same concern was evident in the controversy that arose two centuries ago over the cowpox vaccination. You might think that Edward Jenner's discovery that he could induce a case of a relatively harmless disease and thereby trick the immune system into developing a resistance to smallpox would be hailed by one and all. Smallpox was a major blight that killed people -- mostly children -- by the millions, and left millions more blind, maimed and disfigured.

But vaccination was opposed by Thomas Malthus, the original too-many-people theoretician, who foresaw that it would remove control on population growth. It was opposed by clergymen who believed that it was an unnatural act, contrary to God's will, to give human beings a disease from cows. Chimeric images played a large part in this long public debate. A cartoonist of the time drew a fantastic picture of vaccination recipients growing miniature udders and cow's horns out of their arms.

The opponents united to found anti-vaccination leagues in Europe and the United States, and for decades they did strenuous battle with the public-health reformers who kept pushing for national vaccination programs. Eventually the public-health forces prevailed, and in 1977, the last known case of smallpox -- a man in Somalia -- was reported. By that time, people no longer feared that cowpox would make them turn into cows.

The debate about the new chimeras has already begun. A group of clergymen, led by author Jeremy Rifkin, recently issued a declaration against the patenting of transgenic animals, which they say violates the sanctity of life. Meanwhile, 40,000 people are waiting for heart transplants, hoping that animal-transplant research will ultimately eliminate the problem of lack of donors.

As more people step across the human-animal boundary, many may feel they have violated a taboo. But most will gladly accept the gift of nonhuman life, and respond with something like the heavy-handed humor of Senator Jesse Helms, proud recipient of a pig valve in a heart operation, who recently told a reporter: "Every time I pass a plate of barbecue, I cry. It might be one of my relatives."

Walter Truett Anderson's next book is entitled "Evolution Now: The Augmented Animal and the Whole Wired World." He wrote this commentary for Pacific News Service.

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