Amid much public debate and controversy, the UK's Human Fertilisation and Embryology Authority (HFEA) recently authorised the creation of human-animal embryos for research. In the process, the nucleus of a cow ovum (egg) is removed and replaced with human DNA to create an embryo - the same cloning technique used to create "Dolly" the sheep. The resulting embryo will have mitochondrial DNA from the cow ovum and nuclear DNA from the human donor. Such embryos will not be allowed to develop beyond 14 days and must never be transferred into either a woman or an animal. They will be used for research, including embryo stem cell research.

Making a human-animal embryo -- for which the term "manimals" has been coined -- involves crossing the human-animal species barrier and authorising it sets a precedent that doing so is ethically acceptable. To understand the wider import of this development, we need to look at the full range of possible human-animal combinations and ask whether they also could be approved using the same line of reasoning, namely, that the science made possible by cow-ovum/human-nucleus cloned embryos has great benefits.

We are the first generation of humans ever to have to face such ethical issues. We hold life, itself, and the power to change it radically in the palm of our collective human hand.

There is some difficulty in defining the terms used to describe the various human-animal combinations researchers have created or might want to create, in part, because definitions long used in other areas - for instance, in plant biology - get misused and such misuse gets propagated; there is overlap between definitions which creates confusion; and because new possibilities for such combinations are continuously opening up. They include the following:

Chimeras: Canada's Assisted Human Reproduction Act defines a chimera as "an embryo into which a cell of any non-human life form has been introduced; or an embryo that consists of cells of more than one embryo, foetus or human being." Chimeras are organisms that contain genetically distinct populations of cells derived, for example, from more than one embryo -- whether an animal and a human embryo or two embryos of the same species. (Fully human chimeras occur naturally when two early embryos fuse.)

Transgenic organisms are those into whose genomes a piece of foreign DNA has been inserted and would include the transfer of one or more genes from an animal to a human embryo (which is prohibited by the Assisted Human Reproduction Act) or vice versa (which is not prohibited). For example, human complement genes are transferred to pig embryos for xenotransplantation purposes (using animal organs in humans) or, most commonly, mice are engineered with human genes and then are used to study human disease. The resulting animals are chimeras.

Hybrids are created by fertilising a human ovum with an animal sperm or vice versa, that is through sexual reproduction. The Assisted Human Reproduction Act prohibits such combinations for the purposes of reproduction, which means, for instance, that fertilising hamster ova with human sperm to check on sperms' viability and potency is allowed.

Cybrids, which the British HFEA has just approved, are a sub-category of hybrids: These embryos have the cytoplasm of one species and the nuclear DNA of another. They result from cloning (asexual replication) using an animal ovum and DNA from a human somatic cell nucleus to generate a cytoplasmic hybrid embryo. The prohibition on cloning in the Assisted Human Reproduction Act means such embryos must not be created in Canada.

Synthias are the "new kids" -- or, more accurately, possible "future kids" -- on the molecular biology-genetic-reproductive technoscience block. They result when synthetic biology is used to build or "engineer" life, including from the genes up, and could involve human-animal combinations.

A species is defined by the capacity to interbreed and produce fertile offspring; correlatively, different species cannot do that. Ian Wilmut, the scientist who created "Dolly," the first mammal cloned from an adult somatic cell, said that nature had never anticipated the possibility of crossing the species barrier between mammals and, therefore, has no safeguards against it.

That means we must provide such safeguards through ethics. So, what are the ethics of these procedures?

First, we can make a distinction between "bringing a life into being" through a human-animal combination and "altering a life-in-being" by the transfer to it of genes from another species.

We can also make a distinction between transferring human genes to an animal "life-in-being" and animal genes to a human one. As well, we can distinguish altering the intrinsic genetic essence of human beings with animal genes and changing some of their extrinsic characteristics with animal tissues or organs. So, for instance, there's a major difference between adding pig genes to a human embryo and replacing a person's heart valve with one from a pig.

All human-animal genetic combinations raise enormously serious ethical issues, but they are not all the same ethically. I believe "bringing a life into being" through a human-animal combination is inherently wrong and must not be undertaken.

But within strict limits, some cross-species gene transfer may be ethically permissible.

Most people have an ethical "yuck" reaction to human-animal combinations, for instance, human-animal hybrids. Our moral intuitions tell us that using the human capacity to reproduce to reproduce with an animal is wrong. We must make sure that intuition is not repressed.

Might the long-standing crime of bestiality - having sex with an animal - be an expression of this intuition? Princeton University philosopher, Peter Singer, argues this crime is just one example of a strong disapproval of non-reproductive sex and that bestiality is morally acceptable as long as the animal does not suffer and there is pleasure for both the human and animal. (Singer believes that all animals are equal and distinguishing between human and other animals is "speciesism," a form of wrongful discrimination.)

But might the opposite be true? Might a fear that a mixed human-animal living being could result be at the heart of the prohibition on bestiality and our moral intuitions against it? If so, we must watch that in using science to realise that outcome, we don't suppress those moral intuitions.

Placing a medical or scientific cloak on our actions often results in such suppression. That cloak can disconnect us from the intrinsic nature of what we are doing, and the moral intuitions and emotions we would otherwise experience that warn us ethically. It allows us to focus just on the good we hope to achieve, at the expense of the ethical issues involved.

I propose that making, for instance, a human-animal hybrid or cybrid is inherently wrong, which means it cannot be justified ethically by good intentions or outcomes and must not be undertaken. Might, however, some human-animal combinations not be inherently wrong and, if so, which ones and why?

Let's look at transgenesis.

First, the direction of the transfer seems to matter ethically. Many people see transferring animal genes to human embryos as unethical and, as mentioned, doing so for the purposes of reproduction is prohibited by the Assisted Human Reproduction Act. Transferring human genes to animals is not prohibited, but when is it ethical?

I propose at least three factors are relevant: The nature and function of the genetic material transferred from a human to an animal; the amount of that material in comparison with the animal's genome; and why the transfer is undertaken.

For instance, in "pharming," human genes are transferred to farm animals such as goats or cows, in order to recover drugs from the animal's milk to treat blood clots, anaemia, haemophilia, emphysema or cystic fibrosis.

In my view, that is ethically acceptable. The human genetic material transferred is strictly limited in kind and amount and is not of a "sensitive" nature, for instance, associated with human consciousness; it has a very specific, fully identified function; and it is being transferred to produce a life-saving therapy or one to treat a serious disease.

Leaving aside the host of other ethical issues raised by xenotransplantation, likewise, I believe that transferring human complement genes to pig embryos to make the pigs' organs suitable for transplantation to humans, when, otherwise, crossing the species barrier would elicit a hyper-acute rejection reaction in the human recipient and almost immediate rejection of the organ and possibly death, is ethical.

On the other hand, creating mice by injecting 100,000 human embryonic stem cells per mouse into the brains of 14-day-old rodent embryos raises serious ethical concerns, even among some scientists with liberal views on research ethics, because they could envision "nightmare scenarios" in which a human mind (or some level of human consciousness) might be trapped in an animal head.

We are the first generation of humans ever to have to face such ethical issues. We hold life, itself, and the power to change it radically in the palm of our collective human hand.

This is an amazing, unprecedented power. With great power comes great responsibility. We need deep ethical wisdom and unwavering moral courage in deciding what we may do with it and what we must not.