Twenty years ago, bioengineers fiddled with plastic and wires and transducers while building gizmos. Today, a bioengineer is more likely to be tinkering with cells and chromosomes and genes while deciphering the stuff of life.
 Gregory Stock’s book “Redesigning Humans: Our Inevitable Genetic Future” preaches the promise of bioengineering-longer life and health spans, smarter children, better genes. The question Stock addresses is not “Should we genetically redesign humans?” or even “Are there limits to such molecular tinkering?” but rather “How do we best get to our inevitable future?”
 Stock is well positioned to lead such a discussion. Holding a doctorate in biophysics from Johns Hopkins University and a MBA from Harvard University, Stock directs the Program on Medicine, Technology, and Society at UCLA’s School of Public Health. His passion is the exploration of critical technologies that are primed to have large impacts on humanity’s future and the character of medical science. “Redesigning Humans: Our Inevitable Genetic Future” won the Kistler Book Prize for science books and was nominated for a Wired Rave Award.
 From page 1, Stock shows himself to be an enthusiastic optimist, pragmatic about the redesigning of humans from the genes up. He writes, “We know that Homo sapiens is not the final word in primate evolution, but few have yet grasped that we are on the cusp of profound biological change, poised to transcend our current form and character on a journey to destinations of new imagination.” This is no Darwinian non-progressive evolutionary process, but evolution as directed, conscious human advancement.
 Peppered throughout the book, bold claims about the potential to manipulate our evolutionary trajectory challenge our deepest values and push us towards a serious public debate on the future of genetic medicine and bioinformatics. Although Stock would happily lead us towards a broad-based acceptance of human evolutionary control, the result may be more modest-an increase in the reader’s understanding of the science poised to bring in the genetic future and a sense of incredible awe and healthy concern about that future, inevitable or not.
 Stock realizes that as we understand how genes work to shape who we are and to influence our health, we will want to make decisions about our own genetic make-up and, more importantly, that of our children. “Redesigning Humans” focuses on the technology for and the implications of choosing certain genetically influenced characteristics for our offspring in order to protect them from disease, increase their intelligence, and give them other desired attributes. Technologically, how might we go about this? Genetic engineering? Inserting auxiliary chromosomes? Reproductive cloning?
 Importantly, Stock sees the interest in human reproductive cloning as the red herring that it is-a needless distraction from the vital issues of embryo selection, genetic manipulation, and genetic enhancement. Although the idea of a delayed genetic twin is hauntingly strange, the birth of a “cloned child”-if and when it occurs-will have little lasting impact on most of us, on our values, or on our children.
 Human redesign and evolutionary control will come about through germline genetic engineering-that is, specifically changing the genetic complement of the fertilized egg so that the alterations are passed to subsequent generations. Extensive modifications of human hereditary material of the kind Stock envisions will require safe and dependable methods for germline intervention. Performing genetic surgery one person at a time is simply unworkable. Germline technology will allow humanity to rig the genetic lottery-ostensibly guaranteeing genetically enhanced, disease-resistant future generations.
 In Stock’s view, such “germline interventions” will involve the addition of artificial chromosomes-for example, adding numbers 47 and 48 to the zygote’s original 46. Since artificial human chromosomes provide a stable platform for adding desired genetic material to cells, they will provide the precision and reliability that germline engineering requires.
 These artificial human chromosomes will be chemically identical to those naturally occurring in our cells-twisted ladders of DNA. Possessing packets of genes and their control sequences, artificial chromosomes could become a universal genetic delivery system. Since there is no manipulation of the 3 billion base pairs on a person’s 46 naturally occurring chromosomes, artificial chromosomes minimize the possibility of derailing existing genetic interactions. In fact, modules of genes could be delivered via such auxiliary chromosomes and turned on and off with the flick of a chemical switch.
 According to Stock, the vast potential of human auxiliary chromosomes to combat disease and to upgrade genes will become so desired that “laboratory conception” will become obligatory in order to avoid the potential harms of “natural conception.” If he is right, we will have arrived at the gates of the genetically engineered world of the movie GATTACA.
 Stock engages the reader in a thought experiment. Remembering that humans have 46 chromosomes, imagine that a future father has chromosome 47, version 2.0, containing a dozen therapeutic gene packets, injected into the fertilized egg that nine months later is his daughter. However, when the daughter is grown, she finds this version to be utterly prehistoric and wants her son to get the upgraded version 5.9, which better regulates gene expression and provides protection against many more types of cancer with fewer side effects than version 2.0. The daughter cannot imagine giving her son an outdated version of chromosome 47 nor reverting to the old days of 46 naturally occurring chromosomes. Wouldn’t all parents choose version 5.9 as the daughter has done?
 Stock denies that scenarios such as this are impossible or sheer fantasy. Our impulse is to do whatever we can to benefit our children. Hence, we will intervene and, “[t]he biggest challenge we will face from germline technology is not from its failure . . . . Success is what will tax our wisdom, because that would force us to come to grips with the medical, social, political, and philosophical implications of self-directed human evolution. The injection of artificial chromosomes loaded with genetic supplements . . . may prove to be humanity’s best hope, and its worst fear.”
 Indeed, Stock’s longed-for, inevitable genetic future ought to tax our wisdom, but not in the way he imagines. His overarching theme is that selecting and altering embryos through germline modifications-the most promising method for human redesign-will “write a new page in the history of life, allowing us to seize control of our evolutionary future.” Bold technological advances in the understanding of human genetics and reproduction as well as animal transgenics will allow humans to make fundamental choices about the genetic make-up of their children and their children’s children. Such selective reproduction is much more efficient at spreading successful genes that traditional procreation. The future human will, according to Stock, be “much more than simply human.”
 But is our proper goal to be “much more than simply human” or to be “simply human” in the best way possible? As any viewer of GATTACA ‘s predictable world of genetic “valids” and “in-valids” knows, there’s much more to being simply human than “successful genes.” And, even if we could steer human evolution, toward what end would-or should-we aim?
 Stock does not ignore consequences; indeed his thinking is all about the future. He thoughtfully raises ethical concerns arguing that the moral crux is not the potential for abuse of genetic technology but that we do not know the long-term effects of its use. However, rather than appealing to the precautionary principle-that in instances of profound uncertainty and sizable risk, it is best to err on the side of caution-Stock insists that slowing down and devising an appropriate course of action is “a mirage.” We simply cannot find the brakes.
 Seemingly, nothing can thwart our drive to manipulate human heredity. The title’s mantra of “inevitability” permeates the book. For Stock, “[r]emaking ourselves is the ultimate expression of our humanity.” There is really nothing to be done, as genetic and reproductive technologies will be “impossible to control.” And to those who hesitate, who not only acknowledge but worry about consequences, both predictable and unforeseen, he simply says, “If biological manipulation is indeed a slippery slope, then we are already sliding down that slope now and may as well enjoy the ride.”
 I am no fan of traditional slippery slope arguments as they often leave us paralyzed by the fear of slipping into the ethical abyss. I am less a fan of Stock’s reverse slippery slope argument that leaves us no choice about taking the plunge. Nonetheless, confronting the slippery slope reveals something important about our humanity-that we are apt to be carried away with ourselves. Since we cannot be trusted to stop, traditional arguments would have us avoid the slope altogether. Since we cannot be trusted to go, Stock would give us a shove.
 Most of us travel through life traversing slippery slopes. The first time we fib, we set foot on a slippery slope that can lead to fraud-yet few of us inevitably go from white lies to jail time. Our daily ethical practice involves walking down and stopping on slippery slopes. The ethical task is to erect carefully considered barriers below which we refuse to fall.
 Ethics is about setting limits, and Stock’s argument from inevitability neither does us justice nor does justice to us. “Is” does not equal “ought” and “can” does not imply “should.” Because any and all germline interventions impact future generations, it is the responsibility of the present generation to choose wisely. That we will face such choice is “inevitable”; a particular choice, or a particular future, is not.
 Despite the sense of “inevitability” or perhaps because of it, “Redesigning Humans” helps set the stage for personal and public deliberation on the genetic future. Stock has penned a real page-turner, a book written with a clarity that can be appreciated by scientists and genetic novices alike. Although one can-and should-question his lavish predictions regarding genetic diversity and evolution, Stock’s use of real-life examples and thought experiments grounds the cutting-edge science. Whether you agree with his thought-provoking vision of humanity’s future or not, you will develop an appreciation for genetic science and its potential applications to alleviate human suffering and to control human evolution.
 Although more modest than Stock’s intent, in my view, “Redesigning Humans” makes two important contributions to our thinking about the genetic future. The first is his leading the reader through the “small step” approach to germline modification by demonstrating that there are merely a series of small steps from in vitro fertilization to embryo screening-both of which are done today-to germline engineering. He writes, “Direct germline intervention is the logical conclusion of our ongoing progress in reproductive biology and the ultimate expression of it, and its realm will likely be human enhancement.” In fact, all science works by increments. What appears to be a giant leap is really a series of small steps-each of which can raise ethical and theological questions.
 The second notable contribution is that biotechnology in general and genetics in particular are currently governed by an ethics of free choice and free markets. Stock accepts market controls but few other limits. “A major safeguard that capitalism offers against questionable reproductive technologies is that unless they appeal to a significant number of people, the procedures have no profit potential and fade away or are not even developed. . . . [T]he reproductive and genetic technologies . . . we see first will likely be those with large potential markets.” What determines right from wrong seems to be the desire of parents to pay for particular traits and not for others. Emphasizing the role of financial interests in science, Stock cedes the ethical ground to the marketplace. Although I find Stock’s position in this regard troubling-how do markets assure safety and guard against discrimination, for example-it is crucial to realize the role that financial incentives and profit play in 21st century medicine.