Science is currently flying high on the political agenda. New developments in genetic science are feared by some and hailed as a revolution by others. Yet the benefits or challenges of the technology are not yet fully known.

Though much of the debate raging about human gene technology has taken place even before the basic research has been done, both pro and anti agree that the potential is limitless.

Some see new biotechnology as dissolving humanity and heralding a new nightmare world of genetic haves and have nots. Others that the huge benefits offered could indeed alter our nature - but for the better - improving on humanity.

In his new book, Our Posthuman Future, Francis Fukuyama explores what he sees as the profound political, social and spiritual implications of biotechnology.

The professor and adviser to Washington's RAND corporation is widely regarded as one of the West's leading public intellectuals - best known for his 1992 book The End of History - and sits on US president George W Bush's bioethics council.

He believes that "sterile debate"over less interesting short term issues" needs to give way to a searching debate on the implications of biotechnology to society.

"We ought to start talking about it now before we find ourselves in the mist, having gone through lot of imperceptible thresholds over the next 25 or 30 years and finding ourselves in a world we don't any longer recognise."

Gregory Stock agrees. The former Clinton adviser and director of medicine, technology, and society at UCLA's School of Medicine argues "these technologies are bound to do more than revolutionise medicine and healthcare".

"They're going to eventually change the way we have children. They're going to give us more control over our emotions. They're going to probably alter our life spans."

Both thinkers view the technology as holding out immense possibilities for transforming humanity and both are poles apart. Stock thinks that with genetic "enhancement" humanity can redesign itself and move forward into a better world. While for Fukuyama that vision holds out the prospect of new all-consuming social conflicts.

And in an inversion of the usual American divide between left and right, Republican and Democrat, Fukuyama backs strong regulation in the name of the common good while Stock opposes restrictions to allow experimentation and free up individual choice

Fukuyama's book takes a leap outside his usual area of political economy to discover within our biology the source of human nature.The constellation of our genetic inheritance gives us our essential humanity, setting our limits but also giving us our soul, he claims."It is clear that, while human beings are cultural animals that are shaped heavily by their environment and socialisation, genes put limits on the toxicity of that behaviour and creates certain uniquely human ways of dealing with the world."

While "our scope for believing in our freedom to make what we want is necessarily restricted"there is a human essence, which I call Factor X, that is the basis of human dignity and which entitles us to human rights", he argues.

In a genetic reworking of the old idea of natural rights, Fukuyama believes our biological nature gives us the basis for civilised relations between people."It is my view that human rights ultimately depend on some understanding of human nature and if you alter that human essence - that is the basis of our rights and belief that we are accorded a certain dignity not due to the other parts of the natural world - then you undermine the basis for rights themselves."

Tampering with nature, he says, threatens the current political order and corrodes the foundations of modern democracy.

"We say you should not discriminate against gay people or against people of another ethnicity.

"What you're saying is that those things like skin colour or sexual orientation are secondary attributes, not as important as some essential underlying attribute that is what gives you human dignity. And that is Factor X."

Opening the door to a world composed of the genetically enhanced and unenhanced, Fukuyama cautions, would take us back to eugenics and scientific racism.

"One of the great achievements of the 20th century was to end those kind of theories based on good scientific, empirical knowledge about the relative homogeneity of the human species and to reopen that issue deliberately, I think, invites undermining our entire system."

Also taking a historical view, Stock sees humanity as science's final frontier and views current skirmishes in the gene war as "little eddies in the flow that is moving forward".

"The next frontier is not space, it is our own selves and the critical choices are not really about cloning or GM foods."

And the science is not a matter of if but when: "I don't think these new possibilities are a ticket to utopia but the benefits in my view greatly outweigh the risks and humanity is definitely going to explore these grounds."

For Stock "it is a question of whether we're going to continue to embrace the possibilities of the future or whether we're going to turn away from them in fear and hand their development to other braver souls in other regions of the world."

The UCLA scientist believes that the new technology will triumph because it engages with the fundamental, unstoppable human desire to change and improve our lives.

"We're human and lots of us would actually like to be a little bit healthier, to live a little bit longer, maybe a little bit more talented or a little smarter and lots and lots of parents would like to give those benefits to their children if they could and with very little risk involved."

But it is just this kind of vision of individual choice that Fukuyama fears most.

His dystopic vision does not envisage a coercive Nazi-style state telling people how to "design" babies but the unintended consequences of individual choice, what he calls "negative externalities".

"There are many decisions that individuals make, that are rational for that individual but are harmful to society as a whole," he says.

Fukuyama cites the use of reproductive techniques in south Asia to select the sex of children as "rational from an individual Asian parent's perspective in Korea, China or India but disastrous on a social level".

And the rise of genetic inequalities, could, Fukuyama is convinced, let in eugenics via the backdoor of a genetic welfare state.

"Something I can see as a real possibility"involves the state getting back into the eugenics game because how are you going to guarantee a minimal genetic endowment for everybody unless there is a complete state takeover of the reproductive process?"

Fukuyama's fears are scorned by Stock, who argues that huge changes in life expectancy have already been absorbed by society without collapse or crisis.

While he concedes "we're going to have to balance the interests of children, the interests of society and the interests of society", the UCLA scientist suggests a simple formula.

"As a first cut at it, I think parents are pretty cautious and pretty good about watching out for the interests of their family and thinking about what is best for their children."

"I would give them the latitude to make these decisions and when problems arise, then we need to intervene. But they have to be clear concrete ones."

A packed and polarised audience at a recent Institute of Ideas' book launch for both writers - at one of London's largest venues - gives an indication that in the UK, as in the US, the human genetics debate touches deep fears and speaks to progressive hopes.

But it seems that Britain may have struck the right balance in the short term.

Sounding a rare note of agreement, both professor Fukuyama and Dr Stock praise the UK's regulatory model - especially the role of The Human Fertilisation and Embryology Authority, in the area of embryo cloning and stem cell research.

Even so, Stock cautions that a climate of fear could smother the new science with regulation before it even gets off the ground.

"I think that we must be very, very cautious about legislation that is based upon very abstract, general fears about the future, especially if it means restricting basic research, medical research, and I believe that we actually have adequate structures in place today to protect against immediate dangers that are facing us and that we have ample time to develop new controls if and when problems arise."