Skip to main content

Human-animal hybrids? Church teachings on cutting-edge genetic experimentation

LOS ANGELES, Calif. (The Tidings) - Dr. Adams enters the room of her patient with good news. A heart has become available for a transplant for Robert, whose life-expectancy is limited to only months without such surgery. Dr. Adams explains that the heart that is available is not from a human donor but from a pig. The pig was conceived and raised at a “medical farm” for pigs that have received human stem cells at an embryonic stage, thus allowing their hearts to be compatible for human transplantation.

This short scenario actually offers us an opportunity to exam two distinct but overlapping moral concerns; xenotransplantation (using non-human organs for human transplantation) and chimeras (creating persons or animals with two genetically distinct cell types). In fact, these are not issues that are part of science fiction but are actually at society’s front door.

On Nov. 19, 2004 the Washington Post published an astonishing article that told us, among other things, “In Minnesota, pigs are being born with human blood in their veins. In Nevada, there are sheep whose livers and hearts are largely human. In California, mice peer from their cages with human brain cells firing inside their skulls.”

Less than a year later (June 27, 2005), Scientific American followed up with a more in-depth story on the same issue, human-animal chimeras, individuals who are composed of two genetically distinct types of cells, e.g. human and mouse):

“In Greek mythology, the chimera was a monster that combined the parts of a goat, a lion and a serpent. With such a namesake, laboratory-bred chimeras may sound like a bad idea born of pure scientific hubris. Yet they may be unavoidable if stem cells are ever to be realized as therapies. Researchers will need to study how stem cells behave and react to chemical cues inside the body. Unless they are to do those risky first experiments in humans, they will need the freedom to test in animals and thereby make chimeras.”

The same article further notes that Weissman of Stanford University pioneered chimeric experiments in 1988 by creating mice with fully human immune systems to study AIDS. Later he also transplanted human stem cells into the brains of newborn mice as models for research. Also, Zanjani of the University of Nevada at Reno has created adult animals with human cells integrated throughout their body.

The Church weighs in

What does the Church have to teach us about xenotransplantation and chimeric research?

Is someone breaking the natural law by receiving an animal transplant? Are we playing God when we create a chimera? Even if a mouse is not granted “human” status because it has a brain made of human cells, does it acquire a greater innate dignity than a regular mouse? What percentage of human cells would be necessary for an animal to “become” human? Fifty-one percent?

These are important questions for all of us who are Catholics and struggle to bring our pro-life vision into a society that all too easily reduces persons to commodities and moral methodology to a revived form of utilitarianism.

Regarding the first issue, xenotransplantation (receiving organs from animals), the Church’s response is clear and makes sense. There is nothing inherently immoral with transplanting animal organs into humans, assuming all aspects of good medical care and bioethics have been followed.

The Pontifical Academy for Life, in Prospects for Xenotransplantation (2001), addressed this issue specifically:

From our point of view, supported by the biblical perspective that asserts that every person is created “in the image and likeness of God” (cf. Gen 1: 26-27), we reaffirm that humans have a unique and higher dignity. However, humans must also answer to the Creator for the manner in which they treat animals. As a consequence, the sacrifice of animals can be justified only if required to achieve an important benefit for society, as is the case with xenotransplantation of organs or tissues, even when this involves experiments on animals and/or genetically modifying them (Prospects, n. 9).

The point should also be made that Catholic theology does not have preclusions, on a religious or ritual basis, in using any animal as a source of organs or tissues for transplantation to man. The question of the acceptability of an animal organ --- once it has been established that personal identity is not affected by xenotransplantation, and once all the general ethical requirements of transplantation have been met --- becomes one on the cultural and psychological level (Prospects, n. 10).

Genetic enhancement

Regarding the Church’s teaching regarding developing animals with “human” genes or cells that may enhance the effectiveness of xenotransplantation, the Pontifical Academy for Life addressed the issue in the same document (n. 15):

As we have already observed, the possibility of working out such genetic modifications, using genes of human origin as well, is morally acceptable when done in respect for the animal and for biodiversity, and with a view to bringing significant benefits to humanity … recognizing that transgenesis does not compromise the overall genetic identity of the mutated animal or its species, and reaffirming humanity’s responsibility towards the created order and towards the pursuit of improving health by means of certain types of genetic manipulation…

Thus, xenotransplantation does not violate the natural law or our human nature. Rather it can be a legitimate means of preserving life. Self-preservation is clearly a mandate of the natural law.

Nor does xenotransplantation violate the natural rights of animals. Genesis clearly recognizes the human right to legitimately use animals for the good of humankind, and the preservation of life and health clearly fall in this category. If it is moral to consume animals to maintain life, it certainly is legitimate to use animal organs for transplantation to maintain life also.

Animal, human and ‘para-human’

Still, other issues remain. Is there a moral limit to the proportion of human cells that can be placed in an animal embryo? When does an animal with human cells become a “para-human” or even a “human”? Or can it ever?

In 2003, researchers at the Shanghai Second Medical University in China reported that they had successfully fused human skin cells and rabbit eggs to create the first human chimeric embryos. The embryos were actually allowed to live for several days before they were killed in order to procure the stem cells.

In 2005, scientists at the University of California created a chimera of a human protein known to suppress allergic reactions and a cat protein. When tested in mice the new protein stifled the cat allergy because the human part was more dominant.

Is there a significant moral difference between attempts to create human-animal embryos and those to create human-animal genes and proteins? Is it moral to create chimeric proteins in order to treat allergy sufferers?

The answer is clear that there is a moral difference. Creating chimeric genes and chimeric proteins would certainly be morally possible, assuming all other ethical factors were considered. However, as Marilyn Coors has stated on behalf of the U.S. Bishops in “Genetic Enhancement: Custom Kids and Chimeras” (2005), there must be limits with human-animal chimeras:

“The possibility that a human or quasi-human brain might be imprisoned in an animal’s body is reprehensible.”

Chimeras and the Catechism

Regarding human chimeras, we make a distinction between “natural chimeras and artificial chimeras.” There is indeed a natural chimerism that occurs in the human population, a human-human chimerism. These are persons who are found through DNA tests to have cells in their bodies with different DNA.

On the one hand, most human chimeras are “blood chimeras,” in other words, non-identical twins who shared a blood supply in the uterus and thus continue to manufacture throughout their lives, blood cells from stem cells received from their twin while they were in the womb. Those who are not, are thought to have blood cells from a twin that died early in the womb. Even non-twins can carry foreign blood cells that may have come from their mother or a blood transfusion.

On the other hand, natural human chimeras who possess organs, not just blood cells, that have different DNA, are rare but do exist. But what about creating human chimeras? Adding animal cells to humans or human cells to animals?

While there is nothing in the Catechism that directly addresses the issue of the scientific creation of human chimeras, the moral issues that speak to the status of the human embryo, the understanding of the human person as created in the Image of God, the moral limits to the genetic manipulation of humans, and the condemnation of embryonic stem cell research are all clearly applicable to the issue of creating human-animal chimeras:

— It is immoral to create human embryos outside the womb.

— Any action that reduces a human being (at any stage of development from conception through natural death) to being an object of research or a commodity is immoral.

— Any action that attacks or disrespects the dignity of the human being at any stage of life is immoral.

— Even when the human being is not reduced to an object or commodity and there are legitimate therapeutic reasons to create a chimeric gene or protein, the possibility of harm to persons must be taken into consideration. The complexity of human genetics makes this a very difficult process.

Marilyn Coors makes this clear in her conclusion to “Genetic Enhancement”:

“It is theoretically possible that genetic enhancement could be truly beneficial for individuals and society, and, at the same time, respect the origins of life and the integrity of the human person as a unity of body and soul. The present state of the scientific process does not meet those requirements, however. For these reasons genetic enhancement of human embryos is immoral under Catholic teaching.”

(Vincentian Father Richard Benson is academic dean and professor of moral theology at St. John’s Seminary, Camarillo.)


This story was made available to Catholic Online by permission of The Tidings (, official newspaper of the Archdiocese of Los Angeles.



E-mail:       Zip Code: (ex. 90001)
Today's Headlines

Sign up for a roundup of the day's top stories. 5 days / week. See Sample

Rate This Article

Very Helpful Somewhat Helpful Not Helpful at All

Yes, I am Interested No, I am not Interested

Rate Article


Leave a Comment

Comments submitted must be civil, remain on-topic and not violate any laws including copyright. We reserve the right to delete any comments which are abusive, inappropriate or not constructive to the discussion.

Though we invite robust discussion, we reserve the right to not publish any comment which denigrates the human person, undermines marriage and the family, or advocates for positions which openly oppose the teaching of the Catholic Church.

This is a supervised forum and the Editors of Catholic Online retain the right to direct it.

We also reserve the right to block any commenter for repeated violations. Your email address is required to post, but it will not be published on the site.

We ask that you NOT post your comment more than once. Catholic Online is growing and our ability to review all comments sometimes results in a delay in their publication.

Send me important information from Catholic Online and it's partners. See Sample

Post Comment

Newsletter Sign Up

Daily Readings

Reading 1, Hebrews 11:1-2, 8-19
Only faith can guarantee the blessings that we hope for, or ... Read More

Psalm, Luke 1:69-70, 71-72, 73-75
and he has established for us a saving power in the House of ... Read More

Gospel, Mark 4:35-41
With the coming of evening that same day, he said to them, 'Let ... Read More

Saint of the Day

January 31 Saint of the Day

St. John Bosco
January 31: What do dreams have to with prayer? Aren't they just random ... Read More