Scientists working on ageing are hesitant to say it openly, but most believe we may extend the human life span significantly within the next generation or two. Vital life spans of even a 100 to 150 years would shatter the world as we know it, and yet it is only one of the possibilities sweeping towards us. Soon we may be altering the genes of our children to engineer key aspects of their character and physiology. We stand at the threshold of an extraordinary, yet troubling scientific dawn that will alter our lives, challenge our basic ideas about what it means to be human, and perhaps even reshape our very selves.

Twenty-one years ago, on July 5, 1978, Louise May Brown, the first "test-tube baby," was born. The event was greeted with consternation and hand-wringing about science gone amok, human-animal hybrids, and the rebirth of eugenics, but today in-vitro fertilization is the unremarkable choice for tens of thousands of infertile couples whose only complaint is that the procedure is too difficult, uncertain and expensive. What was once so deeply disturbing now seems just another part of the modern landscape. In a few decades, will the same be said of cloning? of dramatically extended human life spans? of children with genetically enhanced intelligence, endurance, and other traits?

It may be too disturbing for many of us to contemplate, but the technological power we have hitherto used to remake the external world is now potent enough to transform us as well. And we are unlikely to be any more hesitant about "improving" our biology than we were about changing our environment. James Watson, co-discoverer of the structure of DNA, put it bluntly at a symposium I convened last year: "No one really has the guts to say it, but if we could make better human beings by knowing how to add genes, why shouldn't we?" DNA chips, artificial human chromosomes, and other recent breakthroughs in molecular genetics are paving the way to human genetic engineering and the beginnings of human biological design.

The human species is moving out of its childhood. It is time to acknowledge our growing powers and begin to take responsibility for them. We have no choice in this, for we have begun to play god in so many intimate realms of life that we could not turn back if we tried. Some, of course, believe we should stop our audacious incursions into the very fabric of human biology - at least until we can summon up more wisdom. But the way to find wisdom about our newfound capabilities is not by trying to deny them (and thereby relegating their exploration to outlaw nations and scientific renegades), but by using them judiciously, by carefully feeling our way forward, and yes, by making mistakes and learning from them.

Until now, the appropriate timescale for measuring change in the biological world has been thousands if not millions of years, but today we can barely imagine what we will become in a few hundred. What we can say with near certainty is that we will not long remain the same. The forces pushing humanity towards self-modification are too powerful and seductive to resist.

Some people curse these new technologies, fearing they will pervert our values and destroy all we have struggled so long to achieve, tearing at our institutions, our philosophies, and our lives. Others see the same technologies as ripe with possibilities, a wonderful chance to transcend human limitations.

Whether these advances bring a tragic end to life as we know it or usher in a glorious new era is uncertain. Ironically, both may occur. Millennia from now, when future humans look back on our era, I believe they will see it as a difficult, turbulent, and yet extraordinary moment that laid the very foundation for their lives. The unraveling of our biological blueprint and the initiation of a conscious reworking of our biology is an unprecedented development in life's long history. It is bound to redefine and reshape us. Humanity is in the midst of a transition more profound, more difficult, and more far reaching than any before. And we are its architects, its audience, and its objects.

(The copyright of this article belongs to Gregory Stock. It cannot be copied or reproduced without his express permission. This article represents the personal view of Gregory Stock and not that of Horizon.)

You can find out more about Gregory Stock at http://research.mednet.ucla.edu/pmts/Stock.htm).

Greg recently constructed an extensive public website - Engineering the Human Germline: Best Hope or Worst Fear? - to deepen international discussion about the possible application of germline engineering technologies to humans.

Transcript of online chat with Prof. Gregory Stock.

Horizon: Designer Babies programme page.