A most remarkable event occurred in the weeks preceding the June 2000 announcement of the completion of the first draft of the human genome DNA code: One of the leaders of the genome project publicly called for strict limits on what the scientific community should be permitted to do with the human genetic blueprint now in hand.

At a conference at MIT, Dr. Eric Lander, leader of the team that decoded the largest portion of the genome, called the conference to attention with this surprisingly stark suggestion:

Already, there are well-meaning discussions about improving the human DNA. I find this somewhat hubristic myself. [The human genome] has been 3.5 billion years in the making. We've been able to read it for the last, oh, I don't know, year or so. And we suddenly think we could write the story better? It's very amusing.

There is the prospect that by changing things we might put off aging, prevent cancer, improve memory. I find it a very difficult question. For my own part, I would put an absolute ban in place on human germline gene therapy. Not because I think for sure we should never cross that threshold. But because I think that is such a fateful threshold to cross that I'd like society to have to rebut that presumption someday, to have to repeal a ban when it thought it was time to ever try something like that.

The "germline gene therapy" being referred to involves altering the genes not just of an individual, but a procedure that embeds the genetic changes in a person's reproductive cells – their sperm or eggs – so that the genetic alteration is heritable by all future generations.

Lander's comments are remarkable on many levels. One is his frank acknowledgment of the enormity of the consequences of "crossing the germline." Most noteworthy, though, is his willingness to recommend "an absolute ban" on a technology with the stipulation that any decision to overturn this ban be made by society rather than by scientific or policy experts. In doing so, this widely respected scientist violated the ruling dogma held by much of the life sciences enterprise: First, that no strictures, and certainly no statutory ones, should be placed on the emerging technologies; and second, that decisions should be left to each individual.

Many scientists agree with Lander's sentiment regarding germline genetic interventions, but most base their position on the technical obstacles that prevent implementing it safely in the foreseeable future, and they would object to a socially determined legal ban.

As his own comments suggest, Lander's appeal did not emerge from a vacuum. There are geneticists and others who adamantly advocate genetically re-engineering the human genetic germline – double-helix co-discoverer James Watson most prominent among them. From his vantage point, anything that could contribute to human health and betterment should be considered. "If we could make better human beings by knowing how to add genes, why shouldn't we do it?" he challenged at a 1998 UCLA conference. I had the opportunity to check in with Watson at a conference this February sponsored by Time Magazine, called "The Future of Life." By "adding genes" was he referring to genes from other species or novel genes created in the laboratory, I asked. "Anything!" he said.

Making "Better" Easier

Watson's rhetoric of "making better human beings" is eerily reminiscent of the twentieth-century American eugenics movement, which forcibly sterilized more than 60,000 citizens and helped inspire the Nazi "racial hygiene" laws. (One of the US eugenicists' key institutions was called the Human Betterment Foundation.) Nobel laureate Watson does little to dissolve such comparisons. Shortly after the Time conference, a British documentary on Watson was aired that had "Honest Jim" declaring that genetic engineering should be used to rid the world of "stupid" children – stupidity, in his view, being a condition that affects 10 percent of kids – and "ugly" girls.

While in previous writings Watson has disowned the notion of creating a superspecies, others are not so restrained. Princeton biologist Lee Silver has famously written of a genetically enriched class of beings emerging from this technology. "One way to identify types of human enhancements that lie in the realm of possibility – no matter how outlandish they may seem today – is through their existence in other living creatures," Silver writes. "If something has evolved elsewhere, then it is possible for us to determine its genetic basis and transfer it into the human genome."

Whatever technical objections might be raised by Silver's proposal, he does not lack support in the social sciences. "Instead of saying, 'gills? wings? I don't think so,' it's much more interesting to say 'gills? wings? why not?'" Leonard Glantz, associate professor at the Boston University School of Public Health, told Technology Review in November 2001:

The dream of all Americans is that the next generation will be better physically, mentally, economically, and socially. Certainly germ-line genetic modification is a way to guarantee that future generations will be better.... I'd have to hear the arguments against modification – and I haven't heard a good one yet. Humans can fly right now, we just have to pay for a ticket. We can exist underwater, we just need submarines.... Once someone develops people-can-fly technology, wherever it is, people will line up for it!

However fantastical Glantz's examples, they serve the purpose of moving the boundaries of the debate and legitimizing genetically modified humans. If germline genetic engineering is to be realized, it will focus on the manipulation of embryos, thereby assuring that the genetic changes are reflected in every cell of the newborn, including reproductive cells. One of the most likely techniques for achieving this is known as gene targeting.

Used widely in model animal organisms, gene targeting permits precise genetic alterations to be made in an embryo, and therefore the germline. The power of gene targeting is such that Dr. Francis Collins, director of the National Human Genome Research Institute, worries that if technical improvements are achieved that would permit its use in humans, "there will be some who will be arguing that it is time for us to take charge of our own evolution...and see if we can improve ourselves into some higher state."

Recently, scientists at the University of Wisconsin announced that they had carried out successful gene targeting with human embryonic stem cells. As one of the UW scientists put it, this technique "allows us to manipulate every part of the human genome that we want."If for no other reason than the remaining technical obstacles, we are not yet at the fateful Rubicon described by Lander, Watson, Silver and others. But the riverbanks are clearly within sight. Bill McKibben's new book, "Enough: Staying Human in an Engineered Age," takes us on a tour of what might lie on the other side and urges us to be content without crossing. In doing so, he confronts the great conundrum in science and technology: Is it possible, is it permissible, is it perhaps sometimes necessary, to say "Enough"?

"Too Much" is Simple, "Enough" is Difficult

Readers familiar with McKibben tend to know his book "The End of Nature" and subsequent fine articles laying out the science of and ecological threats from global warming. "Enough" is an extension of that earlier work: The focus now is not the threat to our relationship with nature but to our own human nature. Germline genetic engineering is one of the technologies that McKibben views as representing such a threat, and is the leading thread running through this personal essay. Robots and nanotechnology are others. Following to some extent the analysis of Bill Joy, the Sun Microsystems genius turned prophet of technodoom, McKibben worries about the human apocalypse that could arise from the merger of all three technologies opening the door to the posthuman. "Enough" is a synthetic work, with little in the way of original interviews or research. McKibben's technique is to pull the key ideas and arguments from his voluminous readings and artfully weave them together so that we can gain a vision of the genetic-nanobotic schemes in the works.

Utilizing the ability to manipulate and order atoms precisely, the nanotechnologists envision a future in which all our tasks and desires – including eternal health – will be carried out by invisible armies of molecular robots. The future that germline genetic engineering could enable is one of a ruling genetic caste and ultimate alienation of ourselves from ourselves. The threat from nanobotics is not just the emergence of posthumans but the wholesale replacement of the human, genetically altered and otherwise.

Under the best scenario, the potential benefits of enhanced health and reduced labor from unrestrained technological advance will lure us into a "soft dehumanization." Enticed by technical solutions to poverty, illness and even mortality, McKibben is concerned about the consequences should these elixirs turn out not to be illusory lures but actual realities. Since the meaning of our lives comes from confronting our human limits, and especially our mortality, in McKibben's view the stakes are nothing less than the survival of any human meaning whatsoever.

McKibben's methodology in laying out the dangers is to draw from the research and writing of the scientists and technologists, and integrate their data, visions and arguments so that the personal and social results come into view. Layer upon layer of the science and the techno-utopian project are unrelentingly piled up. At this stage it may be impossible to know whether the envisioned genetic and nanobotic futures are cloud-castles or a genuine New Jerusalem, but McKibben is convincing in relating the scope of what many of the scientists and technologists, ensconced in the world's most prestigious universities, laboratories and think tanks, are actively considering.

The title of the book, "Enough," intentionally provokes the accusation of "Luddite." McKibben addresses this directly, and convincingly demonstrates the fatuousness of such attacks. He would not, for example, withdraw antibiotics or smash the computers. Instead, like Lander's outlook on germline therapy, he believes that we have reached a threshold, a turning point in which the decisions about some of the new technologies represent a radical break from the human project launched with the Enlightenment. Germline genetic engineering is not the same as in vitro fertilization, he argues as one demonstration of this assertion, since it opens the way for permanently altering not just the individual but all descendants and, with time, even perhaps the species.

McKibben's argument is that we must recognize the difference between the technological choices that confront us now from all that have come previously, and not fall prey to objections that we are standing in the way of progress, dooming ourselves to a new dark age, blocking evolution. He rightfully asserts that "to cure the ill or feed the hungry ... lie within our present powers or within the steady, foreseeable, noncontroversial progress of science and medicine. They don't require a posthuman future."

Choosing a human future means drawing upon what for McKibben is the most definitive feature of our species: the ability to recognize and live within limits, to face and accept our own finitude. While he admits that this tradition is not most dominant in our culture, he suggests that it is a powerful one, and cites John Muir and Martin Luther King Jr. as among its major standard-bearers.

In this way, the argument of "Enough" is simple, and that is its beauty. McKibben does not take us through hermeneutic somersaults or demand an analytic purity to justify his position. Instead, he draws upon lived experience – his own and that of his family and friends. He relies, in other words, on the social and embodied conditions that we all share. Agree or disagree with his examples, with this or that particular threat from the new technosciences, the philosophical and practical problem remains: When, if ever, can we and should we say, "No more?" Indeed, what kind of ethics are adequate if drawing a line is not a reasonable and ethical choice? But, as McKibben amply demonstrates, and as anyone who has ever interacted with the scientific community has experienced, the cry for limits is met with derision or worse by the technoscientists and their representatives.

McKibben, unfortunately, gives them ammunition aplenty with which to launch their attacks. His understanding of genetics is not strong, and he actually gets it wrong in several phrasings and examples. There's no such thing as " a single base pair of genes," nor is it " the different pairings of DNA that cue the production of different proteins and hence different people." Nothing will so warm the cockles of his opponents' hearts than that their rival doesn't have the science down pat. His examples of nanobotic possibilities are also fraught with some of the more questionable claims. The prospects of medical nanobots "cruising our bloodstreams, attacking pathogens within our bodies and building new cells, even organs," challenge even Jules Verne. I recently ran into award-winning science-fiction writer Kim Stanley Robinson and raised the nanobot issue with him. "I've looked at some of that stuff," he told me, " and a lot of it's science fiction. And I know science fiction." In searching for past or existing examples of the ethic he wishes to promote, McKibben unfortunately invites further accusations of Luddism. Fifteenth-century Chinese rejection of large sailing ships, sixteenth-century Japanese rejection of guns in favor of samurai swords and the contemporary Amish rejection of phones in their homes are instances he offers.

Anyone sympathetic to his viewpoint cringes in advance at the sarcasm with which this will be met. But the missteps do not undermine McKibben's essential argument, which even evolutionary biologist E.O. Wilson endorses as "the burning philosophical question of the new century ... how to control the technoscientific juggernaut before it dehumanizes our species."

McKibben's protracted essay is aimed at changing our individual hearts and minds. Yet he does not identify any socioeconomic structure that bears particular responsibility, that needs to be addressed and reformed or brought under control. But at least when it comes to genetics, the corporatization of our research universities and the patenting of life forms certainly have eroded much of the ground for public contemplation of the issues and further impelled the drive toward commercialization. Microsoft billionaire Paul Allen and Oracle billionaire Larry Ellison now gather the molecular biologists on their yachts and estates to plot the conquering of brain development and function. While the benefits to human well-being are certainly a driving force in this research, no doubt their ambition includes grasping the holy grail of the contemporary scientific quest, consciousness itself. The patent application that would follow this success would be most interesting.

When, if ever, enough hearts and minds are changed to say "enough," real interests and structures of power will have to be confronted. McKibben is not naïve about such matters – but his approach does not permit addressing them here. (On the personal level, he's honest enough to admit the enticements of the techno-utopian future to himself. He only hopes that he would choose against them.)

The ultimate challenge to his argument, though, may not come from the benefits offered to each of us as individuals but from attempts to address the suffering of others brought on by the human dark side – whether it's the degradation of our biosphere or the slaughter of innocent peoples. I attended a McKibben speech in September, and I asked him whether he ever allowed himself to consider the possibility of re-engineering the species to address this part of the equation. "It's entirely possible to make the case that human beings are a huge problem and that we'd be better off with something else in their place," he told me. "Of course, everyone who's ever dealt seriously with environmental issues or issues of war and peace, or any other of the great human failings, can think that. For me, we remain a sweet, interesting, intriguing species, full of enormous potential that we have yet to fully realize. I think that we've got all kinds of room to find out good ways of being human within our biological limitations."

Whether that sentiment is enough to rally a counterforce to the threats depicted in "Enough" is open to debate – a debate Bill McKibben urgently invites his readers to enter.

Ralph Brave, a science writer based in Davis, California, is currently on a fellowship at the Center for Genetics and Society. He has written on the human genome for the past six years, including articles in Salon, the Washington Post and the Baltimore Sun, and is working on a book on the subject.