Mendel's Demon
Mark Ridley Weidenfeld & Nicolson, 348pp, £20
ISBN 0297646346
The Biography of a Germ
Arno Karlen Indigo Paperbacks, 192pp, £16.99
A couple of years ago, at a dinner in Barcelona, I sat next to an American scientist called Stanley Miller. This was a potentially tricky occasion. Had his name been mentioned the day before, I might have assumed that he was dead; the Miller contributions I recalled were in the 1950s and 1960s. Not a bit of it - Miller is alive and well and still thinking hard about the chemical origins of life.
Evolution is about living things, yet there must have been something before that. But what exactly? The best bet is what scientists call a "primordial soup". In those early experiments, Miller took very simple molecules, such as water and ammonia, and exposed them to physical forces - heat, lightning - that might have been present in a prebiotic earth. The simple chemicals yielded by these early laboratory experiments were not self-replicating. Mark Ridley, an evolutionary biologist, takes up the story at the point where elaborate nucleic-acid chains began to reproduce themselves in nature. Before the most widely known of these (DNA), there may have been an RNA (which is DNA's close relative) world, although Miller himself has doubts. And before that, who knows - peptide nucleic acids, perhaps? The puzzle of the prion, the agent associated with BSE and what is loosely known as the human form of it, is a reminder that there may be more to self-replication than DNA.
Only this year, scientists have discussed stripping down to the bare essentials the genetic manual of a bug called mycoplasma and recreating that life in the laboratory. On the whole, greater complexity of life forms has been associated with increasing gene numbers. Whether life stands still for aeons, as with bacteria, or whether it progresses to reach a kind of perfection (man, arguably), copying is the key. And the copying of genetic information, like the child's game of Chinese whispers (an image of which Ridley is inordinately fond), is prone to error.
Ridley argues that complex life forms could have evolved only because of the emergence of a series of devices for ensuring that the error rate was reduced. The crowning achievement of the series is sex - rather, a particular feature of the process known as meiosis, which can be summed up as a random shuffle after a paradoxical doubling of genes from two different members of a species. The author uses the image of breeding faulty motorcars to explain how this process is of net benefit in terms of faulty genes passed on.
Mendel's Demon is not an easy book for the non-biologist, despite the engaging style and commonplace metaphors. Ridley tells us that one section began as a lecture, and perhaps that is true of other parts, too. Certainly, there is repetition, and the book might be easier if it were shorter. Genes cannot have motives, but the author comes close, at times, to falling into the trap of suggesting that they do. He tells us, for instance, that if it could be predicted which particular genes would be passed on and which not, the doomed ones would have "rebelled" and the system would have collapsed. But does not rebellion imply that genes have intentions and can respond appropriately to timor mortis?
Still, Ridley has much to say that is provoking about how human evolution could yet proceed (if at all) and about how life might have evolved from similar chemical beginnings on other worlds. The odds do seem to have been hugely stacked against the emergence of complex life forms on this planet.
If, as a general rule, more genes equates with greater complexity, the larger parasitic organisms go the other way. The tick, for example, prospers by having no need for certain genes because it pinches some of what it needs from its involuntary host. And in terms of their life cycles, ticks, like butterflies, are more complex than we immodest humans.
Arno Karlen's germ is Borrelia burgdorferi, a microbe that was eventually found to be the cause of a mysterious disease afflicting people in the US east coast town of Lyme. Lyme disease has had a long history under different names, notably in central Europe and Scandinavia, and has also been detected in workers in the New Forest. This bug lives in the guts of ticks; ticks feed off deer, mice and other warm-blooded creatures, and will attack man. Lyme disease appeared (or reappeared) in the Connecticut River region not, for once, because of man's interference, but because nature was allowed to reassert itself by an eco-friendly middle class.
If New England's ecology, the details of B burgdorferi or the sex lives and eating habits of ticks are not for you, Karlen offers food for thought on the inevitability of scientific discovery. I read this book while travelling to a conference at which the first plenary session turned out to be a historical review of the missed opportunities for identifying the viral cause of type-B hepatitis. With hindsight, one wanted to cry out "It should have been obvious" or "How could they have been so stupid?". As Karlen understands, the retrospectoscope is a powerful instrument. However, several people guessed the cause and vector of Lyme disease, and he concludes that the "long lack of follow-up to their insights merits an entry in the annals of medical myopia".
With hepatitis B, the blame can, in part, be laid on the voice of authority, in the form of the German pathologist Rudolf Virchow, who got it wrong. No scapegoat is obvious for Lyme. Those who fund biomedical research would do well to study history.
David Sharp is the deputy editor of the Lancet
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