The disappointment of the double helix

From the time of Plato onwards, the grandeur of the universe and the richness of the living world spoke of a hidden reality beyond appearances and the reach of the human mind to comprehend fully.

This is scarcely the modern view, where for science the unknown is merely the waiting-to-be-known. And it certainly might seem this way given that now, for the first time in the history of our species, we can hold "in our mind's eye" the entire history of the universe-from the moment of the Big Bang to yesterday.

Still, the paradox of this most impressive of recent intellectual achievements is that it forcefully brings to our attention what we can never know-what preceded the birth of the universe, the origin of matter and the laws that govern it. So too for the phenomena of life.

For the best part of 60 years, ever since the discovery in 1953 by James Watson and Francis Crick of the double helix, the elegant simplicity of that structure has seduced us. We see the genetic instructions strung out along its two intertwining strands, and we suppose that the biological complexities of life might be made knowable.

From the mid-1970s onwards, the massive onslaught of modern genetics has promised to do just that, culminating in the very recent past with the ability to spell out the full sequence of genes (the genome) of man and mouse, chimp, worm, fly, and many others. But paradoxically, as we now know, the composition of those genomes has turned out to be virtually the reverse of that anticipated-with a near equivalence of a modest 20,000 genes across the entire range of the complexities of life all the way from the millimeter-long worm C. elegans to ourselves.

More astonishing still, we now know that those genes are interchangeable between species where, for example, the same gene that orchestrates the formation of the fly's compound eye does so too for our very different camera-type eye and so on. There is, in short, nothing in the genomes of fly and man to account for why a fly has wings, six legs, and a dot-sized brain and we should have two arms, two legs, and a mind capable of comprehending the origins of the universe.

The genetic instructions must be there, of course, for otherwise flies, humans, and the tens of millions of species with which we share this planet would not reproduce themselves with such fidelity from generation to generation. But we have moved in the light of these extra­ordinary findings from supposing that they are at least knowable to recognizing we have no conception of what they might be.

The explanation must lie in the simple elegance of the double helix that for the past 60 years has held out the enticing promise that we might understand the program that makes an organism. But the elegance of its structure cannot be because it is simple but because it has to be simple-if it is to replicate the genetic instructions every time the cell divides.

And that obligation to be simple requires the double helix to condense within the one-dimensional sequence of chemical genes along its intertwined strands, those billionfold complexities that determine the unique three-dimensional form and attributes that so readily distinguish one form of life from another. The semblance of simplicity then becomes a measure of the double helix's inscrutable profundity.

The challenge for scientists' claims to knowledge of the living world is obvious enough. So while, for example, snowdrops are as they are­-because they are made that way by their snowdrop genes-now we are forced to recognize that science cannot tell us how those genes fashion those delicate drooping flower heads with their evocative white and green coloring. Once again those snowdrops, indeed the whole glorious canopy of nature, are infused with the deep sense of the mystery: "How can these things be?" -James Le Fanu is the author of Why Us: How Science Rediscovered the Mystery of Ourselves (Vintage, 2010)