Ever since 70 years ago, when James Watson and Francis Crick described the structure of the double helix in all its elegant simplicity, science has been seduced into supposing that the inscrutable complexities of biology might ultimately be made knowable and explicable at the most fundamental level of the genetic instructions strung out along those two intertwining strands of DNA. From the mid 1970s onward, the massive onslaught of novel techniques for characterizing and manipulating genes promised to do just that. “The search for the Holy Grail of who we are has now reached its culminating phase,” claimed Leroy Hood of the California Institute of Technology. Those techniques, John Savill of Nottingham University Hospital anticipated, “would, like a mechanical army, systematically destroy ignorance, . . . with unprecedented opportunities for science and medicine.”

The subsequent explosion in basic biological knowledge has generated mountains of data. And yet for all the excitement and enthusiasm, the tens of thousands of scientific papers and acres of newspaper coverage, those “unprecedented opportunities” have turned out to be surprisingly elusive.

In As Gods, Matthew Cobb, professor of zoology at Manchester University, provides an insider’s interpretation of the fortunes of genetics over the past few decades — his title a reference to the supposedly godlike powers (for good or ill) that those novel techniques might confer on scientists. “Built on dreams,” the science of genetics “has provided insights into the whole tree of life,” he writes, but it “has equally provoked nightmares.” Still, “its promises are rarely realised, the worst fears never come to pass.”

He opens with a discussion of the basic tools for decoding and manipulating genetic instructions: first, the discovery of one set of viral enzymes that could cut up those lengthy strands of DNA into manageable bits, and then another, reverse transcriptase, that could identify the specific genes within them. Most important of all was the supreme, previously inconceivable achievement of sequencing, of spelling out the arrangement of the 3 million chemical nucleotides of the genomes of many diverse forms of life, from the very simplest to ourselves.

The possibilities seemed (or were presented as being) immense: genetically “engineering” bacteria to produce useful drugs and hormones, boosting agricultural productivity with pest-resistant crops, and gene therapy, the replacement of faulty genes responsible for thousands of inherited disorders. Some of those possibilities have indeed been realized, though scarcely on the scale originally anticipated, primarily because finding a responsible gene is just the first step; the underlying mechanisms of gene action have proved to be vastly complex. But the recurring theme that emerges from Cobb’s account — and, in a sense, the more interesting part of the story — is that, from the beginning, the prospect of what those techniques might be expected to achieve has been consistently oversold.

Here his telling of the saga of genetically engineered “human” insulin is particularly instructive. In 1976, molecular biologist Herbert Boyer and venture capitalist Bob Swanson founded the first biotechnology company, Genentech, with a modest investment of $500 each. They expected that the gene for insulin would soon be identified (it was, a year later) and, when inserted into the DNA of a bacterium, would instruct its ever-multiplying descendants to produce large quantities of the hormone.

Ingenious, certainly, but to no real purpose, as there was more than enough insulin available from animal sources (pig and cow) whose structure, being virtually identical to that of the human version, served its therapeutic purpose well enough. Boyer and Swanson’s imaginative genius lay in seeing that people could be persuaded that this way of making drugs represented “the future” and that their insulin, by being human (i.e., made by a human gene), was necessarily superior to anything from a different species of mammal.

Brilliantly marketed, “Humulin,” as it was named, soon replaced the animal version and became (and remains) biotechnology’s most successful product. When Genentech was floated on the stock market in 1981, Boyer and Swanson’s modest initial investment yielded them more than $80 million each.

The general verdict was that Humulin was, at best, only a marginal medical advance, but still, as Cobb observes, “the combination of the new science and the financial rewards it promised was a heady mix for all concerned.” The Genentech flotation initiated a wave of biotech start-up companies funded by billions of dollars of venture capital. Only a handful proved to be commercially viable. Biotechnology as a revolutionary way of making drugs did indeed make a few people very rich. It also became, in time, the biggest money-losing venture in history.

To be sure, not all gene research has been disappointing. Genetically modified (GM) crops have led to a substantial (and most welcome) decline in the use of chemical pesticides, the classical example being Bt cotton, engineered to contain a toxin made by a bacterium (Bacillus thuringiensis) lethal to leaf-munching caterpillars. Still, as Cobb describes in detail, GM crops have never fulfilled their potential — stymied by widespread apprehensions about the consequences of meddling with nature and by suspicion about the motives of global agribusiness companies, most notably Monsanto. The case in their favor has floundered in the face of such antipathy, amplified to great effect by the stunts of eco-activists dressed in hazmat suits trashing fields of GM crops. “No country has enthusiastically embraced” the introduction of GM, Cobb notes.

Medical gene therapy has proved to be the most disappointing of the new opportunities, but it was never realistic to suppose that it might be possible to use a normal-copy gene of the lung, for example, to correct the faulty gene responsible for cystic fibrosis. There is simply no way of ensuring that the vector, a virus engineered to carry a normal copy, would infect a sufficient number of the millions of cells lining the respiratory tract to have a clinical effect, or of ensuring that it would then be inserted in the right place in the genome without interfering with the function of other genes, or that it would be “expressed” in the right way at the right time. Despite the insuperable obstacles, hundreds of experimental treatments were initiated before the National Institutes of Health pulled the plug on its research program (funded at the rate of $200 million a year), having concluded that “significant problems remain in every aspect of gene therapy.”

Still, there is no predicting what the future may bring. In the closing chapters, Cobb turns his attention to CRISPR (clustered regularly interspaced short palindromic repeats), a relatively new tool of genetic manipulation that is much more powerful than previous innovations. A family of DNA sequences in single-cell organisms, CRISPR bypasses the need to introduce, for example, the human insulin gene into another organism to achieve some desired effect. CRISPR permits scientists to target and edit the genome directly, deleting errors in the code and inserting sequences or genes in the right place for them to function properly. It has, notes Cobb, revived the fortunes of genetic research, being used to create mutant zebra fish and flies, manipulate genes in animals, and repair disease-causing genetic errors in laboratory cultures of human cells. There are as yet no drugs derived from CRISPR, though it has been used therapeutically in the treatment of sickle-cell anemia and some rare forms of cancer.

The necessary corollary of such a relatively simple form of genetic manipulation is that its misuse can be seriously consequential — those “nightmares” so often warned against might, after all, be realized. Cobb describes two as being of particular concern. The first is germ-line therapy, or editing the genome at the earliest embryonic stage so that, throughout life, the corrected gene will be replicated every time the cells of the body divide. This would seem to be a much more viable proposition than those previous attempts to replace a faulty gene in the millions of cells lining the respiratory tract, for example, as with the gene responsible for cystic fibrosis.

Germ-line therapy has never been sanctioned, however, as it is impossible to predict the repercussions that might flow from so drastic a form of genetic manipulation at the beginning of life. Come the advent of CRISPR, it was only a matter of time before the ban would be broached. In 2018, Chinese researcher He Jiankui reported the creation of the first babies (twins) from an edited germ line. He had not, it turned out, sought prior approval for the experiment, for which there was no conceivable clinical justification. Subsequent analysis of the babies’ genomes would reveal that the edited germ line contained a series of potentially harmful mutations. He was dismissed from his institution and given a three-year prison sentence. His fate would, one might suppose, discourage others. Still, the genie was out of the bottle, Cobb observes, anticipating further such “godlike” experiments to come.

No such prohibition applies to editing the genomes of viruses and bacteria, we learn disturbingly. Enter the second major nightmare. Gain-of-function experiments to render organisms more virulent or transmissible remain licit in China, as in most of the world. There have been as yet no confirmed reports of adverse consequences, though it would be reasonable to speculate that the Covid pandemic arose from just such experiments at the Wuhan Institute of Virology.

Early ambition and subsequent anticlimax, two strands interwoven like the double helix itself, run through Cobb’s commendably ambitious, Olympian survey of the “Genetic Age.” This has undoubtedly been one of the most thrilling, paradigm-shifting episodes in the history of science. But the second strand, the realities of the modest benefits achieved so far, is in its way just as significant — a cautionary as well as reassuring reminder of the limits to what science can know or do.