Why environmentalists should be concerned: humans have dangerously destabilized the Earth's ecological system. If we now begin altering our evolved interdependence with nature, we will only accelerate the destabilization. (Environmentalists).

It's not as if environmentalists really need something new to worry about. The planet's temperature is set to rise four or five degrees--every glacial system is already in rapid retreat, and icebergs measured in units of U.S. states (the size of Rhode Island!) are calving off the Antarctic. Species disappear daily; acid rain; and you know the whole damn litany. We could be forgiven for wanting to take a pass on human genetic engineering.

And yet I think it may turn into the single greatest battle environmentalists have ever fought, the one for which the Grand Canyon and the African elephants and Amazon deforestation and Love Canal were preparing us. The real test.

Some of the reasons for thinking so are pragmatic. Changing the human germline is an almost preposterous override of the precautionary principle, the idea that if you don't know something's safe you shouldn't do it. We have rushed with blinding speed through the first phases of the biotechnological revolution--what was experimental a decade ago now grows in half the corn and soybean fields on this continent. Now we seem bent on going just as fast with our plans to tweak the human genetic code that until now we have hailed as nature's finest achievement--already teams are competing to produce the first human clone, a precursor of genetic enhancement. The ideas come thick and fast, from visionaries who foresee improving the intelligence of our offspring, or increasing their muscle mass, or bettering their character. In the words of James Watson, the first director of the Human Genome Project and co-discoverer of the double helix, "If we could make better human beings by knowing how to add genes, why shouldn't w e? What's wrong with it?"

For environmentalists with a sense of history, such words recall earlier promises of grand utopias: power "too cheap to meter" from nuclear plants. What we know about how human genetics works is dwarfed by what we don't know--and experimenting on our own genetic heritage seems unwise to say the least. If history is any guide, the experiment will come with dubious side effects, likely to be visited upon the weakest and poorest parts of society. If internal combustion, a century later, yields global warming, then what does this crash course in scientific breeding promise? At the very least, the demand that we exploit this technology immediately seems suspect (except to the venture capitalists who have made the investments). Which is not to say the scientific progress need grind to a halt. There's plenty of work to be done this side of tampering with the germline--almost everyone concedes, for instance, that using gene therapy to help existing human beings with existing problems makes perfect sense.

But where engineers and many environmentalists part company is precisely on this question of trying to "improve" the species. And they disagree, I think, in large measure for emotional reasons as well as pragmatic ones. The human instinct that looks at a free-flowing river and sees something that could be dammed to make power (or money) collides with the human instinct that values, deeply and sometimes at a level almost beyond words, the very free-flowingness of that water. The engineering impulse to tinker, bend, twist, patent, sell comes up against the environmental impulse to appreciate, preserve, protect, cherish. And that impulse, on both sides, extends to the human genome as surely as it does to the Colorado River, the Arctic National Wildlife Refuge, the grassland savannas of Africa.

At first glance, a human being seems an unlikely candidate for wilderness designation. We are shaped by a thousand different forces--in a consumer society those forces grow ever cleverer, often overriding even the desperate attempts of our parents to shape who we are. And yet, so far, there is something irreducibly wild about each of us, the result of that particular assortment of DNA that we ended up with. Not random--but not defined, either. We are, as yet, unprogrammed. Or, at least, the programming is weak enough (our friends, our schools, our origins) that we can, albeit at some cost, override it. Or not. That's what life is often about, that choice.

And if the improvers have their way, then life will be about something else: about the cells of our bodies expressing the particular combination of proteins that someone believes will produce a particular result. And no change, not even the climatic havoc we are now wreaking on the planet, will be as large as that. If, as Thoreau insisted, we are rich in accordance with how much we can afford to leave alone, then this will be the ultimate test of whether we're rich enough. For conservationists, the final frontier lies, literally, right beneath our fingertips.

RELATED ARTICLE: HUMAN ENGINEERING TIMELINE

1953 James Watson and Francis Crick determine the "double helix" structure of DNA. This discovery is a major breakthrough in the study of genetics and reinforces the idea that an organism's DNA is the primary and dominant determinant of its inherited traits.

1973 Stanley Cohen and Herbert Boyer create a transgenic organism using recombinant DNA technology, which allows the manipulation and transfer of pieces of DNA from one species to another.

1976 The first genetic engineering/biotech company, Genentech, is founded by Boyer and Robert Swanson. It is the beginning of the commercial use of genetic engineering technology, an industry which by 2002 is generating revenues of $25 billion a year in the U.S. alone. Within two years scientists at Genentech splice the human gene for insulin production into E. coli bacteria, which then synthesize human insulin.

1978 Louise Brown, the first "test-tube baby" (in vitro baby) is born in England, demonstrating the feasibility of growing embryos outside of the womb. In vitro fertilization is done by putting sperm and an egg together in a lab dish, where chemicals facilitate fertilization, and then implanting the embryo into a woman's uterus.

1980 U.S. Supreme Court rules that genetically engineered microorganisms can be patented (Diamond v. Chakrabarty), setting a precedent for patents on lifeforms.

1983 Kary Mullis devises the Polymerase Chain Reaction (PCR) technique, which rapidly replicates DNA sequences. This process of gene amplification makes gene mapping and forensics easier and cheaper.

1990 Human Genome Project is begun by an international consortium of scientists, with most of the funding coming from the U.S. National Institutes of Health and the Wellcome Trust, a medical philanthropic organization based in London.

1996 A sheep named Dolly, the first mammal to be cloned from adult cells, is born at Scotland's Roslin Institute. Previously, cloning had only been carried out with embryo cells.

1998 Dr. James A. Thomson (University of Wisconsin) and colleagues are the first to isolate human embryonic stem cells, which have the potential to develop into almost any type of tissue. This innovation opens up the possibility of harvesting stem cells for use in treating human diseases.

2000 In June, scientists at both Celera Genomics (a private company formed in 1998) and the publicly funded Human Genome Project announce that they have completed a draft of the human genome. The announcement evokes hopes about medical advancements based on understanding of the genome, as well as controversy about the issue of public access to the information.

2001 In February, scientists at Celera Genomics and at the Human Genome Project report that the number of human genes is probably about 30,000, only about twice as many as the number of genes in a fruit fly and far less than the long-standing textbook estimate of 100,000.

2002 As reports circulate that some scientists may have already begun to implant cloned embryos in women, the U.N. begins work on a global ban on cloning.

compiled by Vanessa Larson

STEPS IN MENTAL DEVELOPMENT

From an article describing a "clearinghouse for mental defectives" established in 1913 by the New York Department of Public Charities. This sort of analysis often accompanied early efforts to improve the human gene pool, and still remains a part of the thinking behind some 21st-century genetic research.

Bill McKibben is a former staff writer for The New Yorker. His books include The End of Nature; The Age of Missing Information; and Hope, Human and Wild.