It is an important and seemingly simple question, with an equally simple answer: We just don't know. 

Not being able to decide definitively should not stop us asking the question, though. Our answer, however tentative, has too many ethical and policy implications.

First, the context: the UK Parliament will shortly debate the Human Fertilisation and Embryology Bill, which has a number of controversial elements related to embryo research, screening, cloning and the structure of the family. As it currently stands, the Bill will permit the creation of animal-human hybrid embryos to be used for embryonic stem cell research. 

The creation of 'cybrids' would be allowed, by adding a human nucleus to an animal egg emptied of its chromosomes, providing they were destroyed after 14 days. Scientists claim that cybrids are effectively 99.9% human because the nucleus is human and only a few animal maternal mitochondrial genes will remain in the animal egg cytoplasm. Others claim that the numbers of animal mitochondria in the eggs and early embryos, and the proportion of animal material including DNA in each cell, would in fact be considerably higher than 0.1%. In addition, unknown effects of these mitochondria on the later development of the 'human' embryo could further blur the species boundary. 

The creation of a 'true' hybrid embryo would also be permitted by the Bill, either through fertilising a human egg using animal sperm or vice versa. Chromosomally, these entities would be 50% human and 50% animal with genetic mixing in every cell of the entity. The creation of hamster-human hybrids using hamster eggs is already legal under the HFE Act 1990, to test human sperm for treatment and research, but the entity must be destroyed at the two cell stage, when it becomes a hybrid. The Bill proposes to permit hybrids to remain alive up to 14 days and to extend the purposes for which they could be created.

In addition, the Bill would legalise new forms of chimerical embryos, created via the insertion of some animal cells into human embryos and would also, finally, legalise human transgenic embryos, created by inserting sequences of animal DNA into human embryos. The resulting embryos would again have to be destroyed after 14 days. Although transgenic animals have already been made by inserted a human gene into non-human DNA, and have provided valuable medical information, there appears to be no current medical or scientific interest in creating the type of transgenic embryo described in the Bill.

Why the push for cybrids? The answer, primarily, is the shortage of human eggs, which are necessary for cloning to create human embryos, which are then dismantled for their stem cells. Scientists are now careful about making claims for producing immediate therapies and cures. Rather, the focus is on the need to create research models to improve the efficiency of cloning techniques, so that fewer human eggs are eventually needed for human embryo cloning.  

Opponents claim that huge volumes of research on egg function and basic cellular and molecular biology must be ignored to justify animal-human hybrid embryo research. In the words of one scientist, James Sherley: 'Not a single new experiment is necessary to know with certainty that human-animal cloning will not provide faithful models for human-human cloning. This is also a particularly troubling state of affairs, given the increasing scepticism regarding the actual therapeutic potential of human-human cloning.'

Moreover, new reprogramming techniques have led some scientists, including Ian Wilmut of the Dolly the Sheep fame, to abandon embryonic, though not adult, stem cell research altogether.

So how important are species boundaries? There are practical questions, such as the questionable usefulness of the research and genuine concerns about viruses crossing species barriers. But are there ethical reasons why should we retain clear boundaries between the species?

The creation of all types of hybrid embryos strikes at the very heart of what it means to be human. The mixing of human and animal will create a new spectrum of entities in which it will be very difficult to know whether one is dealing with animal or human. Where exactly does the animal end and the human begin?  How do we decide this? Does moral status also change across the spectrum?  And what legal status should be afforded to mixed species entities?

Should a definition of 'human' be based on the molecular content of cells? Accordingly, cells with more human molecules would be more human.  Or should it be, as with cybrids, if there is a human genome? If cybrids are not human, what are they?  Are they simply 'a clump of cells which would never go on to develop beyond 14 days'? (Prof John Burn, Newcastle University), implying they cannot be regarded as human by any measure?  Does the number of human or animal genes make a difference? Or can an embryo legitimately be defined as being 99% or 50% human?  If so, should one be treated with more respect than the other? Or can both be destroyed for research purposes?

Xenotransplants do not raise this dilemma. Provided that no brain or reproductive tissue is used, it is generally accepted that a human with a transplanted animal organ does not jeopardise their essential human identity. However, the US President's Council on Human Bioethics stated in 2004 that society should not be put into a position to judge the humanity or moral worth of ambiguous hybrid entities. Yet the HFE bill is forcing us to do exactly that.

Ultimately, the answers we give will depend to a certain extent on prior philosophical commitments. How we conceptualise human life and nature will determine our answer. The Christian response will lean strongly on Scripture and Tradition, which makes it plain that there is a significant, absolute, God-given boundary between humans and other animals, which we should respect. The creation of people in the 'image of God' gives humans a special worth and dignity that calls for protection, particularly for the most vulnerable of humans, even if we are not 100% sure of their human status.

Philippa Taylor is a bioethics consultant, a member of the Family Law Review for the Centre for Social Justice and is studying for an MA in bioethics.