Last night we learned that, even though Parliament has not yet approved the creation of human-animal hybrid embryos, scientists at Newcastle University have created the first cow-human embryos. Concluding that this was not some sick April Fool's joke (the traditional noon deadline had already passed), those of us with concerns about this development must engage with the questions this presents at a level that makes sense to those we would seek to influence.

Some would object to the procedure on theological grounds. They do not pretend that the DNA from humans is somehow different from the DNA of other animals - for biochemically they are not. Rather, they maintain that humans have been created in the image of God and are therefore qualitatively different from the rest of creation, that each species should be reproduced "according to their kinds". While theological correct, this argument will carry no weight in today's pluralistic, secular world and will only expose those who make it to ridicule - especially when they use misleading language like "frankenscience" and "monsters".

Others would object on ethical grounds. All human beings should be extended the same human rights, irrespective of age, medical condition, sexuality, race, creed, or stage of development. Yet, our rational Western culture has defined its own new reality. Not only are human beings younger than a fairly arbitrary threshold of 24 weeks of development deemed unworthy of protection, but human rights are already being extended to animals and proposals are even being drawn up around the world to extend human rights in the future to robots. There is nothing precious about a ball of cells that looks down the microscope like a bowl of semolina.

It seems there is only one level of debate at which we might be heard: the pragmatic. Proponents of the experimental technique suggest that the potential benefits of this sort of admixed embryonic research are "so far reaching that many of them have not yet been imagined." They maintain that scientists will be able to create a limitless supply of embryonic human stem cells for research - and appear unconcerned that we should be experimenting on viable human embryos at all. They suggest that experiments on these human-animal stem cells will give us insights into human diseases. Lastly, they promise that such research will lead to a cure to diseases such as Alzheimer's and Parkinson's - and that one day we might create tissues and whole organs for transplant into humans.

In truth, as we noted last week, the science is far less promising. There are four main reservations:

1. Interspecies mitochondrial-nuclear DNA dysfunction

Given that there are varying degrees of interspecies incompatibility between mitochondrial and nuclear functions, the medical risks of implanting cells that contain a combination of human nuclear DNA and animal mitochondrial DNA are considerable. These risks will be even greater in view of the fact that mitochondrial dysfunction is a key factor in many of the diseases claimed to be treatable by therapeutic cloning. Equally, the risks would also be particularly high if such stem cells were used to treat heart or liver complaints, owing to the high level of mitochondrial activity in these organs.

2. Difficulty in interpreting results

Given that there are so many profound genetic and epigenetic flaws in cloned embryos (even using eggs of the same species), the use of human-nonhuman cybrid embryos for research into diseases would be liable to become a study of artefacts. In other words, results would be difficult to interpret and conclusions may prove irrelevant for treatment in humans. Remember the six men hospitalised with multiple organ failure while testing an anti-inflammatory drug two years ago?

3. Risk of creating new diseases

If any of the cybrid embryos were implanted into a host or not kept in a bio-secure environment, there would also be a risk of animal diseases being transmitted to humans or the creation of new diseases.

4. Immunological incompatibility

Finally, cells arising from human-nonhuman cybrids would most likely be immunological incompatible with humans and it is therefore doubtful whether they could ever have any clinical application. Consequently, it is extremely unlikely that tissues or organs would ever be used for transplantation in humans.

For further details about these scientific concerns, see the BioCentre report The New Inter-Species Future?

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