Outwitter of viruses

About 60,000 years ago, a new species arose somewhere in Africa, promptly walked out of that continent and spread rapidly around the world. Homo sapiens - bristling with intelligence and cunning - overcame almost everything in its path. Resident populations in Europe of earlier Homo species, the Neanderthals, did not last long after the arrival of modern humans. By 10,000 years ago, humans had invented agriculture. Comparatively easy times followed as food became more abundant. Human population sizes grew rapidly and soon humans found themselves living at high densities. Unwittingly, they had unleashed a set of evolutionary forces that are now beginning to bite back. Stephen Palumbi's The Evolution Explosion tells the story of these forces and their economic, environmental and health effects.

High-density human populations have caused the rapid evolution of pests and other disease-causing organisms. High population densities make it easy for diseases to transmit themselves from host to host. This means that the diseases can afford to become nastier because even if they kill their host, they are likely to have jumped to a new one before the host dies. This principle holds not just for human diseases but for the diseases that plague the plants and animals that humans domesticated, and that they grow or maintain at high densities.

Early attempts to control pests and disease sometimes used extreme measures: diseased individuals were quarantined, abandoned or left to a worse fate. But by the early part of the past century humans had learned how to fight back chemically against disease, with antibiotics, pesticides and anti-viral medications. The trouble is that the pests are capable of matching human ingenuity blow for blow with swift and often sublime evolutionary escapes. Within a few years of the sulfa drugs being introduced, bacteria had begun to evolve resistance. The same disappointing roll call has accompanied the introduction of every new antibiotic, with resistance often appearing within a few years and seldom more than a decade (Palumbi tells the story of a Nobel laureate whose prizewinning pesticide DDT had been defeated by the time his prize was eventually awarded).

Now after about 50 years of dousing bacteria with antibiotics, so-called superbugs have appeared, resistant to all known antibiotic drugs. The same pattern has played out in crop pests. Both plant diseases and plant-chewing insects have evolved to evade most known treatments. HIV, the cause of the human disease Aids, can be treated - and even then only palliatively - only with a cocktail of three drugs given simultaneously: the virus is resistant to any one (and quite possibly to any two) alone.

Simple diseases caused by single-celled bacteria or mere viral particles use two fundamental features of evolution to outwit the most intelligent creature on earth: variation and fecundity. Owing to the way that genes specify proteins, a typical single gene might be capable of producing a staggering 20500 different varieties of protein - an effectively infinite number. Most of these proteins will be of no use, but if even a fraction is functional, the organism is capable of great flexibility, and that is in just this one gene. A typical virus may have five to ten genes such as this typical one. Combine this potential for variability with the extreme fecundity of small organisms - the population of viruses in your body during an infection can easily outstrip the entire human population - and one begins to see the essential ingredients to produce Palumbi's explosion: no matter what sort of antibiotic or anti-viral we assail our pests with, they seem capable of wriggling out by trying one of their many possible forms.

What can be done? Happily, Palumbi's counsel is more than one of despair. The triple-drug therapy works for HIV because it is supremely unlikely that the Aids virus will produce in one leap a mutant form that is resistant to all three drugs, and anything short of this means death for the virus. So here is one glimmer of hope: attacks on pests from several directions at once will at least slow the march of resistance. Rotation of crop pesticides or insecticides is another strategy: the resistant forms favoured in one year are out of favour the next with the result that the pest makes little progress towards evolving resistance. It is not too soon to implement these and similar strategies. Palumbi estimates that the explosion of evolution we humans have unleashed costs us dearly: an extra $30 billion is spent annually to control resistant pests on crops, to treat antibiotic-resistant bacterial infections and to combat viruses. Palumbi's is a sober and authoritative voice reporting on a topic of ecological and economic importance.

Mark Pagel is professor of evolutionary biology, University of Reading.