The Scottish Council on Human Bioethics has released a report entitled Embryonic, Fetal and Post-natal Animal-Human Mixtures: An Ethical Discussion where they discuss what scientists are doing with mixing human and animal cells and the ethical issues arising from this work.

Genetic Human-Mouse Chimeric Fetuses

Recently Scientists at Stanford University injected human neuronal stem cells into mouse fetuses, creating mice whose brains were about 1% human. By dissecting the mice at various stages, the researchers were able to see how the added brain cells moved about as they multiplied and made connections with mouse cells [91]. The same scientists now want to add human brain stem cells that have the defects that cause Parkinson's disease, Lou Gehrig's disease and other brain ailments and study how those cells make connections. Indeed, scientists suspect that these diseases, though they manifest themselves in adulthood, begin when something goes wrong in early development.

Because of this, the Stanford team is also thinking about making chimeric mice whose brains are 100% human. However, they suggest that if the brains look as if it is taking on a distinctly human architecture - a development that could suggest a specific amount of 'humanness' - they could be killed. On the other hand, if they look as if they are organising themselves in a mouse brain architecture, they could be used for research [92,93].

In January 2005, an informal ethics committee at Stanford University endorsed the proposal to create mice with brains made nearly completely of human brain cells. The chairperson of this committee indicated, in this respect, that the board was satisfied that the size and shape of the mouse brain would prevent the human cells from creating any traits of humanity. But just in case, the committee recommended closely monitoring the mice's behaviour and immediately killing any that display human-like behaviour [94].

They go on to briefly describe experiments that have been done with chimeric fetuses of human cells with sheep, monkey, and pig cells. In each case human stem cells were injected into locations in animal fetuses. They also discuss the potential to introduce human stem cells at a much earlier stage in development where the injected cells can be expected to become a larger percentage of the resulting animal's total cell count.

In 2003, Scientist at the South Korean firm Maria Biotech, were reported to have injected human embryonic stem cells labelled with a fluorescent protein into 11 mouse blastocysts which later developed. The embryos were then carried by foster mice, whereby five offspring were born with fluorescence in tissues including the heart, bones, kidney, and liver. However, the scientists terminated the project after having to address "severe protests" from the public [101].

They take a hard line against any tinkering that results in a creature that has human neurons sharing a brain with animal neurons.

• The incorporation of human stem cells into post-blastocyst stages of non-human embryos should only take place if it can be demonstrated that they cannot contribute to the germline or brain cells of the animal.
• The incorporation of non-human stem cells into post-blastocyst stages of human embryos should only take place if it can be demonstrated that they cannot contribute to the germline or brain cells of the human being.
• The incorporation of human pluripotent or totipotent stem cells into a non-human blastocyst or its preliminary embryonic stages should be prohibited.
• The incorporation of non-human pluripotent or totipotent stem cells into a human blastocyst or its preliminary embryonic stages should be prohibited.

This is an interesting position. They draw the line against mixing human neurons with non-human neurons. Perhaps they see human neurons as somehow sacred.

Okay, what is the appeal of this position? First off, it avoids the really difficult problem of defining what is a rights-possessing entity. Make sure nothing that is a mix of human and non-human mind comes into existence. Then we never have to face that question. Well, that's the hope anyway. But the hope is wrong. We will end up having to face that question anyway when someone tinkers with another species and just changes its DNA to make it smarter without using human DNA to create the smarter result. We will face the question when people start creating human offspring that have modifications of genes that govern cognitive ability. We will face that question when artificial intelligences are created.

The need to create a scientific definition of humans will be forced upon us by technological advances. That definition (or, rather, definitions since consensus will not be possible) will threaten religious definitions, ideological definitions, and other definitions based upon fantasies of what we wish to believe is nature.

But before we start modifying human nature or creating other intelligent lifeforms we already increasingly face another threat to how we view ourselves: Genetic and neurobiological advances will gradually undermine many beliefs about the nature of humans. Worse, the challenge of what should be considered human will (I predict) be challenged when looking just at genetic variations which exist in humans.

For example, more genetic variations that contribute to violence and criminality will be found. Probably some genetic variations will be found that contribute to psychopathy. Should we consider amoral totally unempathetic minds as humans? We'd be unwise to grant rights to an artificial intelligence with those qualities. Should our standard for rights possession be lower for humans than for AIs or animals uplifted by genetic engineering that raises their intelligence?

Delaying the day we have to face the question of whether chimeras possess human rights or human souls might make sense even if creation of chimeras is eventually allowed some day. The longer we delay the better will be our scientific understanding of human nature and of the cognitive qualities needed to maintain a rights-based society.

A second reason for keeping humans unmixed with other species is that doing so preserves the ability of many humans to feel that humans are special and apart from the rest of life. Many people see humans as special due to having souls which other species do not have. But if a human-chimp chimera could be created the question arises: Would it have a soul? Suppose that a creature looked perfectly human but somehow had 10% of its brain cells from another species. Would it have a soul? It'd have more human brain cells than, say, an Alzheimer's patient. So would it have a soul?

A third reason to oppose creation of human-animal chimeras is to avoid suffering in the resulting creatures. But suppose scientists include a small enough percentage of human cells that the resulting animal thinks and acts like an animal of its type. Or suppose the human cells were genetically modified to be more compatible with, say, neurons in a mouse's brain and that the mouse brain was kept as small as a normal mouse's brain. Potentially that'd avoid the problem of creatures which are not shaped in a way that causes them suffering. Also, the mouse would not possess any higher level of awareness than a normal mouse has.

It seems to me that the biggest benefit of totally banning the creation of human-animal chimeras is that it avoids our feeling confused about how we should treat the results. But we are going to have to wrestle with all the ethical questions that chimeric creatures present us with whether or not we create chimeric creatures. Also, even if the creation of such creatures is banned inevitably people will create them illegally. So we'll still have to decide at some point what criteria to use when weighing what rights to grant them or whether to destroy them upon discovery.

Before taking the line that we should just ban anything that seems yucky or weird consider the potential benefits from letting scientists create chimeras. For example, scientists can study human diseases by putting human cells with human genetic disease into animals. Also, use of animals to grow organs for transplant might work if the organs were grown from the fetal stage using human stem cells injected into the fetus.