Some scientists are now creating biological chimeras in order to carry out various types of research. A chimera is a mixture of two or more individuals in a single body and is named after the mythical Greek creature with a lion's head, a goat's body and a serpent's tail.
The creation of animal-human chimeras raises profound ethical questions and guidelines are not yet in place to signpost what is and what is not acceptable practice in this area.
Most people when first confronted with the idea of an animal-human hybrid are instinctively revolted by the notion and are reminded of the book The Island of Dr Moreau written in 1896 by HG Wells. It tells the story of the outlaw doctor who developed creatures that were part animal and part human.
In a limited sense, human chimeras are not unnatural. For example, most mothers carry some cells from each of their children in their blood and most twins carry some cells from their sibling twins. Recipients of organs donated by other humans are chimeras and if you have had a heart valve replaced by a valve from a pig or a cow you are an animal-human chimera. Also, for many years now, scientists have been putting human genes into bacteria, and also into farm animals, in order to make useful human proteins such as the hormone insulin. We are used to all these things and they have not caused an ethical furore. However, the situation becomes more problematic when we consider growing entire human organs in animals, and sensitivities reach a peak when the cells being grown in animals are human brain cells.
Research on animal-human chimeras is largely an outgrowth of human stem-cell research. Stem cells are undifferentiated cells that have the capacity to develop into any of the approximately 200 different types of tissues that make up the adult human body. It has been possible, by adding human stem cells to animal foetuses, to grow certain largely human organs in these animals. For example scientists at the University of Nevada have created sheep whose livers are up to 80 per cent human. The idea is to make these livers available to people who need liver transplants.
The power of chimeras as a research tool to study cell development in general and brain development in particular was demonstrated in dramatic experiments about 10 years ago. Small bits of developing brain from quails were transplanted into the brains of developing chickens. Amazingly, the resultant chickens displayed the characteristic vocalisations and head bobbing of quails. Complexity of behaviour had been exported across species.
A much more radical approach that has not been attempted to date would be to inject human stem cells into an animal embryo and then allow that chimeric embryo to develop in the womb. It is likely that the human cells would be incorporated into every tissue of the mature chimera. Such humanised animals would be very useful in research. For example they would be much better subjects on which to test drugs than the ordinary mice that are currently used.
But a bizarre complication could arise if human stem cells were incorporated into an animal embryo, eg a mouse embryo. These stem cells could find their way into the testes and ovaries of the developing chimeras, producing human sperm and egg cells. If two such chimeras mated, a human embryo might result trapped in the womb of a mouse. This embryo could be removed and developed in a woman's womb. The eventual baby would find itself in the unique position of having two mice as its parents and of giving a whole new meaning to the question "Are you a man or a mouse?"
Incorporating human brain cells into animal brains is particularly controversial. Irving Weismann's team at Stanford University injected human nerve stem cells into mouse foetuses and created mice whose brains are about 1 per cent human. By dissecting the brain at various stages they learned much about how the human brain cells developed, knowledge that would have been very difficult to obtain quickly in any other way. He now plans to use brain stem cells that have defects that cause diseases such as Parkinson's disease to see how these cells develop.
Weismann has also proposed creating a mouse-human chimera with a 100 per cent human brain. He would watch how these human brains developed. If they took on mouse brain architecture they would be used for research. If they took on human brain architecture, which might indicate they were developing human consciousness, they would be killed.
No doubt many readers will be appalled at some of the ideas and proposals I have described in this article. I understand. I am quite taken aback myself by much of this stuff. The main argument used by those who advocate stepping briskly down the animal-human chimera pathway is that this route will fast-track the acquisition of basic biological knowledge and in particular the discovery of cures for fatal human disease. This could well be true, but an "ends justifies the means" argument is only tenable where the means are ethical and it is not at all clear that the creation of significantly humanised animals would be ethical.
Our instinctive reservations may have deep ethical significance. I wouldn't automatically rule out forever any or all research involving chimeras but, at the very least, this whole area needs slow and sure deliberation before we go anywhere significant with it.