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Scientists in China recently reported growing monkey embryos containing human cells for the first time. The team of researchers injected monkey embryos with human stem cells and watched them develop. At least three of the human-animal hybrids—known as chimeras—survived to 19 days after fertilization.

What are chimeras?

Chimeras are animals composed of cells that originate from two or more different species. To create a chimera, scientists introduce cells from one species into the developing embryo or fetus of another. (The name chimera comes from Greek mythology and describes a creature with the head of a lion, the body of a goat, and the tail of a serpent.)

Creating chimeras can involve the use of cells from two animals, and does not always refer to the use of human cells or embryos.

What are stem cells?

The human body contains more than 200 types of cells. Most of the cells are of a particular type and have a specific function. For example, leukocyte cells find and destroy microbes, which helps keep the human body free of infection. Stem cells are different, though, in that they are relatively undifferentiated and unspecialized, meaning they have not yet obtained a special structure and function.

These cells are either multipotent, meaning they can give rise to several other differentiated and specialized cells of the body (e.g., liver cells, kidney cells, brain cells) or pluripotent, which means they can become any or all of the cell types that make up the body.

All specialized cells arise originally from stem cells, and ultimately form a small number of embryonic cells that appear during the first few days of development. The flexibility of such embryonic cells is why they are valued for biomedical research

Haven’t human-animal chimeras already been created?

Yes. In 2003, Chinese scientists fused human cells with rabbit eggs to produce the first human-animal chimeras. A few years later, American researchers at the Mayo Clinic created pigs with human blood in their veins and scientists at the University of Nevada created sheep whose livers and hearts are largely human.

In 2017, scientists from the Salk Institute in California tried to grow the first embryos containing cells from humans and pigs. And earlier this year, Japanese scientists created a mouse-human embryo that contained up to 4 percent human cells—the most human cells yet of any chimera.

What are the ethical concerns about chimera research?

Humans have been engaging in xenotransplanation, that is, transplanting nonhuman tissues or organs into human recipients (e.g., pig skin grafted onto burn patients) since 1838. And for almost 100 years, researchers have inserted human genetic material into animals for the purposes of creating treatments (e.g., animal insulin). Many Christian bioethicists consider such uses morally legitimate, though a few believe it violates the species barrier instituted by our Creator.

The creation of chimeras, however, is more broadly problematic. As the Christian Medical and Dental Association (CMDA) notes, there are several compelling moral reasons to refrain from applying biotechnology to create chimeras or hybrid organisms that are partly human and partly nonhuman. For instance, we should not create intermediate or indeterminate species sharing human and animal genetic material (1 Cor. 15:38–40).

And as David Prentice and Chuck Donovan explain, the intermixing of genetic material becomes more morally problematic when it is done at the embryonic stage:

The problematic aspect is that when added so early in development, the human cells could end up, well, anywhere in the developing animal. In the worst case, the human cells could end up in gonadal tissue and form human gametes (eggs or sperm) within the animal’s body.

Further, the human genetic material could affect the brains of the animals:

The breeding of new forms of life—human-animal hybrids—could then be in view, or even the development of an animal with a largely human or fully human brain. NIH’s answer to objections like these seems to be to preclude such animals from breeding (this would likely not be 100 percent effective—just ask anyone who has run an animal facility).

There are also other ethical problems to be concerned about. As Jeffrey Keenan, president and medical director of the National Embryo Donation Center, says, “This technology also runs the risk of violating the principle of informed consent on human subjects” and “could even enable animals to contract human infections and diseases, and vice versa.”

Should Christians completely reject this type of research?

In deciding whether to reject this research, Christians should become generally informed about the issues involved, interpret this information through a biblical framework, and then follow the leading of their conscience as guided by the Holy Spirt.

For example, a first step in applying a biblical framework would be to distinguish between the different uses of animal parts. In 2005, Dr. Ben Carson, who at the time was director of pediatric neurosurgery at the Johns Hopkins Medical Institutions and a member of the President’s Council on Bioethics, said in a hearing on human-animal chimeras: “I think it’s very important as a council that we make sure that we distinguish between using human or animal parts across species, such as insulin, heart valves, things of that nature, and mixing the genetic material that has proliferative capacity. I mean, there’s a huge difference between those two things. We need to make sure that the public understands that we are distinguishing between those two things.”

A next step is to determine what biblical limits we should place on the use of such animal parts. As theologian Jordan Ballor explains,

The created purpose of animals was one that was different from plants. Animals, in sharing the status of beings with the “breath of life,” possess a level of importance that is not reducible to merely instrumental or pragmatic value.

The reduction of animals to pragmatic use as a source of food is a result of sin, illustrated in Genesis 9. But even here, at the depths of sin’s corruption of relationship, there remain limits and boundaries.

We should view the possibility of interspecies mixing and the creation of human-animal chimeras as just this sort of limit, because it undermines and violates the created order, which distinguishes between plants, animals with the breath of life, and humans created in the image of God.

Many Christians will likely be able to follow the CMDA’s lead in endorsing some forms of chimeric and hybrid research and technology designed for the benefit of humankind. They should know whether these forms “are safe and do not degrade the unique status of humankind.” And they should oppose chimeric and hybrid research or technology that “fundamentally alters human nature as designed by God.”

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