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Suffer the children

The effects of radiation don't stop with the people exposed to it

EVEN the great-grandchildren of people exposed to radiation could inherit unstable genomes, a new study suggests. This instability may be the cause of the leukeemia cluster around Britain's Sellafield nuclear plant. Studies over the past few years have shown radiation damage to be surprisingly persistent. It increases mutation rates not just in cells directly zapped by radiation, but also in descendant cells many divisions later. Birth defects are more common in the offspring of mice exposed to radiation, and in the following generation too. Mutation rates are also high in both generations (Nature, vol 405, p 37). Now geneticists have shown that even the fourth generation is affected. A team at the University of Leicester led by Yuri Dubrova took 20 male mice hom three different strains and irradiated them either with high-energy neutrons for just over 2 hours, or a blast of X-rays for 2 or 4 minutes. They were then mated with healthy females from the same strain, and the offspring were allowed to breed with healthy animals to produce great- grandchildren of the original mice. The team measured mutation rates in the mice by looking at two stretches of DNA called short tandem repeats, which are prone to spontaneous and radiation- induced mutations. They found mutation rates for the male and female offspring and in the following two generations wer as high as in their irradiated ancestor, regardless of the type of radiation, the dose or the mouse strain. The results also suggest that the

The results also show that the radiation damage destabilises the entire genome of later generations, not just the sections damaged in the irradiated animal. "There's something global happening that's affecting the stability of the whole genome," says Ruth Barber, a member of the Leicester team.

She thinks this stems from changes that don't alter the genetic code but can still pass to a cell's descendants. But no one knows what changes would cause the genome to become prone to mutations.

Whatever the mechanism, the results highlight the potential dangers facing the children of people exposed to radiation. The team says its results provide "a plausible explanation for the apparent leukaemia cluster near Sellafield".

Childhood leukaemia is around 10 times as common in Seascale in Cumbria, where many Sellafield workers live, as in Britain

overall. However, epidemiological studies have not conclusively linked this to the parents' exposure. And Barber stresses that it's not proven that the unstable DNA causes disease in mice, let alone people.

Studies of people who survived the atomic bomb blasts in Hiroshima and Nagasaki have found no mutations above background levels, says Richard Setlow, a biophysicist at Brookhaven National Laboratory in New York state. But now their children are under the spotlight. "If anything is found, we'll want to go to the next generation," says Setlow. "We need to find out how rapidly these mutations disappear-or get worse." Hazel Muir

More at: Proceedings of the Notiono/ Acodemy of Sciences Ivd g9, p 6877;

Gene warfare

One small tweak and a whole species will be wiped out

FOR the first time' biologists are planning to genetically modify an invasive species with the express purpose of killing it off.

The target is a European species of carp that has taken over many Australian rivers and streams. Researchers are planning to use gene technology to stop the fish producing female offspring, forcing the population to crash. But before that can happen, the scientists will have to show that the strategy, which has already been shown to work in lab fish, won't create more problems than it solves.

Ron Thresher of the CSIRO, Australia's national research organisation, and his team hope to introduce multiple copies of a gene called daughterless into carp which would be periodically released into the wild. Copies of the gene are carried by the males, ensuring that it spreads though the population. "If you turn everything into a male, sooner or later the pqpulation collapses," says Thresher.

Carp, which originally came from Europe, now make up 90 per cent of the fish biomass in the Murray-Darling river system, leaving little room for native species that are under threat from habitat destruction. The daughterless project is funded by the MurrayDarling Basin Commission, a government effort to restore the dying Murray River.

Thresher and his team have already shown that introducing just one copy of a daughterless gene into zebrafish eggs produces a brood that are 80 per cent male. Next, they plan to introduce multiple copies of the gene into mosquito fish, which breed rapidly, making it possible to follow the spread of the gene through many generations in a short time.

In all, the researchers will have to do at least seven years of testing to show the strategy is both safe and effective. Only then could it get the go-ahead for use in the wild, says team member Nic Bax, who models ecosystems at CSIRO.

The tests will assess a host of risks, including whether or not the daughterless gene can move between species. That's unlikely, says team member Peter Grewe, because the gene, actually a modified version of the fish's own gene for an enzyme called aromatase, varies between species. In their search for a carp-control gene, the biologists were also careful to choose one that does not appear to confer any advantage on the GM fish, so reducing the risk of inadvertently creating a "superpest" or a biocontrol strategy that cannot be stopped once it has started.

But they will need to conduct stringent studies to ensure that is indeed the case, says population geneticist William Muir of Pur- due University, Indiana. He supports using genetic technology to benefit the environment, but warns that if the GM carp had even a moderate survival advantage over normal carp they could wreck havoc, especially if they were inadvertently introduced to other parts of the world.

The theory behind GM carp may seem simple, says Anne Kapuscinski, an expert on genetically modified organisms at the University of Minnesota. But even with extensive testing it can be difficult to predict what will happen when GM organisms are subjected to natural selection in the environment. What's more, she says, the altered genes may not be stable. If they stop working, then all you've done is release more of the pest species.

Other experts contacted by New Scier~hst were concerned about the environmental impact of removing a species that has been part of the ecosystem for decades.

But Thresher insists the gene strategy is a better way of controlling pests than methods such as poisoning. "You don't end up wit tonnes of dead carp floating down the river."

Beth Burrows of the Edmonds Institute in Edmonds, Washington, an environmental think tank that researches GMOs, says there must be public debate on the issue before any carp can be released. There is a history of biocontrol efforts backfiring in ways no one could foresee, she says. While fixing the last problem, you run a big risk of introducing another one. Rachel Nowak, Melbourne ducing another one. Rachel Nowak, Melbourne

A fresh start

Life may have begun not in the sea but in some warm little freshwater pond

THE cherished assumption that life emerged in the oceans has been thrown into doubt. New research shows that primitive cellular membranes assemble more easily in freshwater than~ salt water. So although the oldest~wn fossil organisms were ocean dwe~ers, life may actually have developed in freshwater ponds.

Most theories on the origin of cellular life presume that the first step was the formation of a spherical membrane called a vesicle that could enclose self-replicating chemical chains-the ancestors of modern DNA. The idea is that the ingredients for simple membranes were all present on the eaFIy Earth, and at some point formed vesicles spontaneously in water. It seemed ~most likely that this took place in the sea rather than freshwater, largely because of the sheer size of the oceans. With their unique chemistry, deep-sea thermal vents and tidal pools are generally believed to be the most likely sites.

Now research by graduate student Charles Apel of the University of California, Santa Cruz, suggests that this is wrong. Apel and his colleagues were able to create stable vesicles using freshwater solutions of ingredients found on the early Earth, but not salty solutions, they will report in a future issue of Astrobiology. "When sodium chloride or ions of magnesium or calcium were added the membranes fell apart," Apel says. This happened in water that was less salty than the oceans are today.

Geologist L. Paul Knauth of Arizona State University points out that Earth's early oceans were 1.5 to 2 times as salty as they are today, making it even more unlikely that viable cells could have arisen there. Giant salt deposits called evaporites that formed on the continents have actually made the seas less salty over time.

"No one in their right mind would use hot seawater for laboratory studies of early cellular evolution," says biochemist David Deamer of the University of California, Santa Cruz, who is reporting the work with Apel. "Yet for years we have all accepted without question that life began in a marine environment. We were just the first to ask if we were really sure of that."

"This is a wake-up call," says mineralogist

Robert Hazen at the Carnegie Institution of Washington. "We've assumed that life formed in the ocean, but encapsulation in freshwater bodies on land is appearing more likely."

The finding wouldn't have surprised Darwin. Over a century ago he speculated in his personal letters that the origin of life was "in some warm little pond with all sorts of ammonia and phosphoric salts, light, heat, electricity, etc. present". Matt Kaplan

Fire eaters

Did massive black holes guzzle the big bang fireball?

THE giant black holes at the centre of galaxies might have been born in the first split second of the Universe's existence. So say a pair of astrophysicists who think the big bang fireball spawned mini black holes that quickly grew into monsters by feeding on "quintessence", an energy field that might cause the accelerating expansion of the Universe.

Supermassive black holes many millions of times heavier than the Sun are found at the centre of most galaxies, but nobody is sure how they formed. It is widely accepted that these giants started to develop around the same time as galaxies, at least a billion years after.ithe big bang. Each may have been born in one go, as an extremely large cloud of dust and gas in a newborn galaxy collapsed. Or they may have formed gradually as debris and black holes from stellar explosions settled into galactic centres (New Scientist, 30 June 2001, p 20).

But according to Rachel Bean of Imperial College in London, these processes would struggle to make the black holes so heavy. She and her colleague Joao Magueijo have come up with a radical alternative: the black holes grew into monsters in the first split second of the Universe's existence.

Many theories predict the formation of baby black holes during this time. Patches of the big bang fireball would have been so dense that they collapsed to a point, generating black holes weighing anything from a fraction of a gram to a few million tonnes. But it was always assumed that these black holes would be too tiny to survive for long, and would evaporate due to a process called Hawking radiation.

But Bean says they might have grown fat on a diet of quintessence, a mysterious energy field that behaves like a kind of repulsive gravity. Some scientists think quintessence might explain why the expansion of the Universe is accelerating (New Scientist, 2 March, p 7).

According to Bean, a black hole would consume quintessence, absorbing the field's energy and increasing its own mass in the process. "Black holes will eat up anything," says Bean. "They would feed off the quintessence, just like they feed on stars and other matter, and then the quintessence couldn't escape." In a fraction of a second, quintessence would fatten the mini black holes up to several million solar masses.

Much later, the black holes would become shrouded in the raw materials for star formation. "It's interesting to turn the

arguments round and suggest black holes were there from the beginning, and they in turn were the seeds for galaxies," says Bean. "Observations don't rule that out."

David Spergel of Princeton University in New Jersey thinks it deserves further study. "The primordial black hole idea is intriguing," he says, adding that quintessence is a very plausible source of the Universe's acceleration. However, he says, observations of galaxies that host monster black holes hint that a significant part of the holes' masses came from ordinary matter raining onto them from outside. Hazel Muir More at:

Mercy mission Wrap DNA up tight if you want it delivered safely

SCRUNCHING UP DNA into ultra-tiny balls could be the key to making gene therapy safer and more efficient. The technique is now being tested on people with cystic fibrosis.

So far, modified viruses have proved to be the most efficient way of delivering DNA to cells to make up for genetic faults. But viruses can't be given to the same person time after time because the immune system starts attacking them. Viruses can also cause severe reactions.

As a result, researr,hers increasingly favour other means of delivering genes, such as encasing DNA in fatty globules called liposomes tha*an pass through the membranes round cells. But simply getting a gene into a cell isn't enough-for the desired protein to be produced, you need to get the gene into the cell's nucleus.

At around 100 nanometres in size, most liposomes are too large to pass through the

tiny pores in the nuclear membrane except when the membrane breaks down during cell division. Even if cells are rapidly dividing, delivering genes via liposomes isn't very efficient-and it's no good for slowly dividing cells such as those lining the lungs.

But researchers at Case Western Reserve University and Copernicus Therapeutics, both in Cleveland, Ohio, have developed a way to pack DNA into particles 25 nanometres across, small enough to enter the nuclear pores. The nanoparticles consist of a single DNA molecule encased in positively charged peptides and are themselves delivered to cells via liposomes. In cells grown in culture, there was a 6000-fold increase in the expression of a gene packaged this way compared with unpackaged DNA in liposomes.

Trials have now begun in 12 people with cystic fibrosis, who have a faulty gene that means thick mucus accumulates in their lungs. The researchers will first test the technique on nasal cells before trying to deliver genes to the lungs.

"We're very excited about this," says Robert Beall, president of the Cystic Fibrosis Foundation. "Everybody recognises that gene therapy could provide the cure for cystic fibrosis, and it is exciting that this is a non-viral approach."

When Pam Davis of Case Western University School of Medicine tried the technique on mice with cystic fibrosis, she found the replacement gene was expressed in nasal lining and partially restored function-with little or no immune reaction. But that doesn't mean the method will work in people, she warns, because mice have a very different airway structure.

Indeed, there have already been many failed attempts to treat cystic fibrosis with gene therapy. Lungs are especially challenging, says respiratory specialist Duncan Geddes of Imperial College, London, because the lung lining is designed to keep out foreign objects. The build-up of thick sputum in the lungs of cystic fibrosis patients makes the problem even worse.

But the replacement gene only needs to be expressed in a small proportion of cells, Geddes says. "It's extremely interesting and promising." Sylvia Pagan Westphal, Boston

Light Ray

It came from the far reaches of the cosmos and could destroy everything we thought we knew about how the Universe works. Michaet Brooks fears for the future of physics

FOUR years ago disaster struck the Earth, borne on a beam of light. On its 12-billion year journey from a distant quasar, the light had passed through intersteilar clouds of metals such as iron, nickel and chromium. When astronomer John Webb and his team at the University of New South Wales in. Sydney analysed the light, they found these atoms had absorbed some of its photons. But, according to the known laws of physics, they had absorbed the wrong ones. Something was amiss.

This single observation rips apart our most cherished theories of how the Universe works, according to physicists Thomas Banks and Michael Douglas of Rutgers University, NewJersey, and Michael Dine of the University of Santa Cruz. If the astronomers really saw what they thought they saw, then our best explanation for how matter and forces interact-the standard model-is proved woefully inadequate. M-theory, the most successful incarnation of the idea that all matter is made up of vibrating strings, can't cope with the observation either. Banks, Dine and Douglas make this claim in a paper to be published in a forthcoming issue of Physical Review Letters. And they don't make it lightly, since all three have been personally involved in developing M-theory.

The implications of Webb's data are huge. Incorporating the astronomers' finding into M-theory or the standard model would mean changing the theories so much they no longer explain accepted concepts such as inflation, which describes how the Universe expanded rapidly after the big bang. It would mean twisting theories so far that the long-hunted Higgs particle should already have turned up in our particle accelerators. In other words, with our current understanding of the Universe, it can't be done.

How can a flash of light wreak such havoc? Because it suggests that one of the pillars of modern physics-the fine structure constant, or alpha-is not constant at all. Alpha is a "coupling constant": it dictates how photons interact with particles such as electrons, describing, for instance, which photons will be absorbed by the electrons in a cloud of metal atoms. Today, alpha is around X~. But in that original observation of light from the quasar, Webb found that alpha was slightly smaller by about a millionth of its present value. Twelve billion years ago, iron and magnesium absorbed photons of different energies than the ones they absorb today.

In 1999, when Webb Rrst announced that alpha might have changed, physicists were more sceptical than worried. The first question on everyone's lips was: "Is it experimental error?" Banks was no exception. "My first reaction to this is that the observations should be rechecked very carefully," he says.

That's exactly what Webb and his team have been doing. They spotted the anomalies in the light because of an innovative analysis technique first used in the 1999 results. This improved the sensitivity of the Keck telescope in Hawaii, where all of their observations have been made, by a factor of 10.

They have now collected and analysed light from more than a hundred quasars, and so far every observation backs up the shift in alpha. Webb intends to publish the new data later this year. "These results are going to cause quite a fuss when they come out," he says.

He is anything but complacent, however. His team is checking for a systematic error introduced by their equipment. "We're doing our best to find out what we could have done wrong, and what that systematic error could be," he says. So far they've found no error that might explain their result (Monthly Notices of the Royal Astronomical Society, vol 327, p 1208, 1223, 1237 and 1244).

As a final check they have just begun an analysis of quasar light caught by the European Southern Observatory's Very Large Telescope in the Atacama Desert in northern Chile. This involves a different instrument, different observers and a different technique for handling the data, so these results should expose any systematic errors. "My suspicion is that, if it's not a systematic effect, it's varying alpha," says Webb.

John Barrow of the University of Cambridge, who has been working with Webb, is more bullish. There would have to be an unimaginable sequence of coincidences to get such a consistent error, he says. The agreement in the value of alpha measured from so many quasars is "uncannyn, says Barrow, and a varying alpha is now pretty much undeniable. "Statistically, it's of vastly higher significance than you need to detect an elementary particle at CERN."

And if Webb's measurements are correct, every area of physics touched by alpha will need to be revisited. Alpha is a conglomerate of other constants, given by the formula 2~2/hc, where h is Planck's constant, e is the charge of an electron and c is the, speed of light. At first, the notion that alpha could vary set researchers wondering whether they could accommodate the idea. For example, some have suggested that the electron's charge or the speed of light has varied in the past. It's a hugely controversial idea, but it could explain the observed data and solve some other puzzles of cosmology (see "All change please").

For the theorists, though, simplistic solutions like this just won't wash. The trouble is that the value of alpha governs many other processes, such as the strength of the weak ntitear force that affects how the Sun burns and how radioactive beta decay occurs. It governs the "inflation" of the Universe after the big bang. Thus alpha provides a way to predict the rate at which these processes happen, and all the predictions based on today's alpha are supported by experiments.

Take the fluctuations in "empty" space- the quantum vacuum. In a vacuum, pairs of particles and antiparticles are constantly springing into existence and then being annihilated almost immediately. But the process of creating and destroying an electron-positron pair also creates and destroys a photon. "And so alpha controls the strength of these processes," says Douglas. Physicists can check their calculations for how much energy these processes need, and hence the rate at which they occur, against experimental evidence. And so far, the experiments fit with calculations using today's value of alpha.

But change alpha to be consistent with Webb's observations, or invoke an alpha that changes over time, and physics simply cannot cope. Physicistst calculations would no longer agree with what they measure in experiments. The vacuum energy is particularly important because it is directly related to the cosmological constant, lambda, which describes the accelerating expansion of the Universe. Lambda is extremely sensitive to the strength of alpha and any variation would make physicists' theoretical early Universe blow up ridiculously fast. "All of conventional cosmology would be affected in a way that is grossly inconsistent with observation," says Banks.

It wouldn't be the first time physicists have been forced to indulge in a little creative thinking to make their ideas work, however. Theoretical physics predicts a value for lambda that is 10'2° times higher than the observed value. But, the scientists argue, this simply means there's an as yet undiscovered physical mechanism that reduces it to the observed value. When they incorporate this "fudge factor" into their models, they can reduce the vacuum energy to fit with observations without affecting anything else.

But that doesn't deal with a varying ~Ipha, because change alpha and you somehow have to juggle changes in several other parameters in the theoretical models-including the fudge factor that makes the cosmological constant fit so neatly with observations.

So if Webb's data and the theorists'prognosis hold up, there's only one possible outcome: we can wave goodbye to our "understanding" of the Universe. "If these observations are confirmed, one will have to invent some very exotic physics to explain them," Banks says.

It's not all gloom though. A varying alpha brings some benefits. It could solve cosmology's "horizon problem", for example.

Physicists have a hard time explaining why far-flung parts of the Universe are all at roughly the same temperature. It implies that these areas were once close enough for energy to pass between them, evening out their temperatures. Yet models of the early Universe prevent this from happening.

One way round this is if the speed of light was higher in the infant Universe than it is today, allowing energy in the form of light to pass between these areas (New Sciendst, 24 July 1999, p 28). And if the speed of light has decreased over the lifetime of the Universe, that would explain why alpha has increased.

This isn't the only long-standing puzzle that changes in alpha would resolve. Paul Langacker, Gino Segre and Matthew Strassler at the University of Pennsylvania have worked out how shifts in alpha could change the way elements such as helium formed in the early Universe. As the

Universe cooled after the big bang, a time came when there was no. Ionger enough energy for the weak nuclear force to change protons into neutrons and vice versa. From then onwards, the relative abundance of protons and neutrons was fixed.

Astronomers calculate that this event set a ceiling on the number of helium nuclei, each of which contains tWQ protons and two neutrons, that could be created. But the amount of helium floating around just after the big bang was vastly more than such theories predict-unless, that is, the strength of the weak nuclear force has changed. The value of alpha determines the effects of the weak interaction. If alpha changed, so would the relative abundance of helium and hydrogen created after the big bang. In fact, says Langacker, we might be able to use readings of how much helium was formed in the early Universe to work backwards

and find the value of alpha at that time. Langacker, himself a string theorist, doesn't believe the varying alpha is a prob lem for all our models of the Universe. He feels that Banks, Dine and Douglas are over reacting: string theories such as M-theory do allow physical "constants", such as alpha to vary over time, he points out. Examining the way these constants vary may even give us a clue to the more funda mental physics behind them, he argues. But Douglas disagrees: he believes varia tions of the kind Webb's data suggest would be a serious problem. While M-theory can describe varying coupling constants, this is one of its main problems when tested against real world observations, Douglas says. In many M-theory scenarios, varying coupling constants directly contradict exper imental observations. The business of developing ideas to fix this problem is a major subject of research. "Webb's observa tions-if true, and if they do not have some other explanation besides varying coupling s constants-have dire consequences for these ideas," Douglas says. ..

But he thinks that a varying alpha could ultimately prove to be a good thing. In the past, such crises have helped to refine physics. "It might well be the key to a real understanding of these issues," Douglas says. Banks is less optimistic: if alpha is varying, he doesn't see how physics can get us past the problem. "If the observations stand up, and the only explanation for them is variation of alpha, I think that it means our current theoretical uderstanding is seriously flawed," he says.

So, if physicists are as objective as they claim to be, and accept the observations as valid, we could well be living with some new "facts" by the end of the year: that a number of our fundamental constants are not constant after all; that M-theory fell at its first experimental hurdle; and that the standard model is a shoddy, incoherent explanation for the subatomic world. And all because of some aberrant light from across the cosmos. 1

"If there is some independent confirmation of these results, this issue will move to the forefront of physics," says Dine. i

"Some sort of surprising-and currently unknown-physical mechanism or princi ple must be at work." '5

We'll soon know if he's right.

Further reading: "Time-varying alpha and particle physics" by Thomas Banks, Michael Dine and

Michaet Douglas "Implications~of gauge~unification for time

variation of the fine structure constant" by Paul Langacker, Gino Segre and Matthew Strassler p h/O 112 2 3 3) rurtner reaolnq: --llme-varvmq alDna ano narrlcle

11 September and the biological roots of martyrdom

REVENGE might make us feel better, but in the end it is not the point. What matters is to make the world safer, and for that we need some inkling of why people who are neither mad nor stupid kill people they have never even met. We need a feel for the conditions that promote such behaviour and the mindset that leads to it. In short, we need some insight into that elusive quality known as human nature.

But what has emerged most starkly from 11 September is that we haven't a clue; or none, at least, that has enabled the world's leaders to improve on simple retaliation, which may be high-tech in execution but in strategy belongs to the Stone Age. To probe our own nature we should wherever possible bring science to bear, because its ideas are testable and so can be worked on and improved. With luck, it can move us beyond mere opinion. One obvious approach is to look at our own biology, for although we think of ourselves in ethereal terms- theologians speak of the soul, and Aristotle declared that we are "political animals"-we are also creatures of biology, evolved like any other, and it must be sensible to ask what that implies. For this we have to turn to the branch of biology known as evolutionary psychology.

Mere biology would scarcely seem up to the task of explaining why young men and women should sacrifice their own lives as suicide bombers. Their behaviour seems both shocking and incomprehensible, a cue for politicians to rage about "fanaticism" and resurrect the cold war concept of "brainwashing". Yet the term "fanaticism" taken alone has no explanatory value, and is often racist: the implication is that Muslims in general and Arabs in particular are prone to such behaviour, together perhaps with the northern Irish. This behaviour, it's implied, is innately pathological, and defies understanding. It cannot be seen as part of our evolved nature.

Can't it? A whole number of creatures from all sections of the phylogenetic tree, including many that have no brains to be washed and certainly follow no religion, are known to sacrifice themselves in all kinds of circumstances. The driving forces are clearly genetic, as it is possible in principle to show how such behaviours may follow Mendelian patterns of inheritance, and the underlying reasons can be analysed according to the rules of game theory.

The ubiquitous gut bact~erium Escherichia coli provides a case study (Selection, vol 1, p 51). Two scientists at Michigan State University, Richard Lenski and Greg Velicer, found that when nutrients are in short supply, differerrt strarns of the bacterium engage in chemical warfare. Individuals within each strain produce toxins called colicins that kill bacteria of other strains but not their own. But there's a snag. Individual bacteria that produce the toxin kill themselves in the process, just as a honeybee does when it stings or, as Lenski and Velicer point out, like a "suicide bomber" (and they wrote their paper well before September 2001). Selfsacrifice makes evolutionary sense because the gene that promotes the behaviour is also contained within other bacteria of the same genotype. All individuals of a given genotype will contain the colicin gene, but only a few actually produce the toxin and so sacrifice themselves on behalf of the others. Game theory analysis can show what proportion will emerge as suicide bombers.

Humans, of course, are not bacteria. It would, however, be a significant metaphysical conceit to suppose that we can learn nothing from other creatures. We have genes too, and they influence our behaviour. The rules of genetics are universal, so the same broad forces that shape the general behavioural strategies of all creatures must apply to us too. Specifically, broad biological principles tell us that, whatever the species, it is not surprising that some individuals in a society-and not just those who are pathologically prone to "fanaticism"-might sacrifice themselves for the rest.

In our own species, suicide bombers and hijackers willing to die for the cause are more often young men rather than women. Female suicide bombers have recently struck several times in Israel, and the Tamil Tigers, who have carried out many more suicide bombings than the Palestinians, have also employed women bombers, but young male bombers afe in the great majority. No one is surprised by this, because among all categories of human being-male and female, young and old-it's young men who are the biggest risk-takers, not just in matters of physical prowess but in all aspects of life.

This tendency may well derive directly from reproductive physiology. Females who are fertile and want to have a baby are generally able to do so. Males can in principle impregnate a great many mates but this increases the competition between them, so they can easily finish up with no mates at all, and offen do. Males can't be certain of reproducing without taking risks: they cannot simply wait for a mate to call. Those who risk all might die in the attempt, but this is no worse genetically speaking than sitting around and dying childless; they might, by risking all, do very well indeed. Faint heart never won fair lady.

Young men's propensity for risk is easily quantified in violent crime. Thus in their classic study over several decades, Martin Daly and Margo Wilson of McMaster University in Ontario showed that in all societies, young men commit the overwhelming majority of murders. Of course, human behaviour is extremely flexible and the environment has a huge bearing on the likelihood that any one person, male or female, young or old, will commit murder. More women commit murder in Philadelphia than men do in Iceland. Set up the right comparison and you can find a group of women who are more violent than a group of men. But still, male murderers in Philadelphia outstrip Philadelphia's women murderers by 100 to 1, and the same ratio of men to women is found in the Philippines, and Scotland, and Denmark, and indeed in Iceland, although the number of murders there is so small that the statistics hardly register.

Evolutionary psychologists have also studied the other side of the coin: not conflict and violence, but sociality. Not all creatures are social but those that are clearly benefit in many ways. Common sense, observation and all manner of theories agree that sociality typically involves some measure of unselfishness, and this by definition implies that sociality exacts a price. The various theories of altruism throw light on where unselfishness comes from. The late evolutionary biologist Bill Hamilton's notion of kin selection neatly explained why creatures are liable to help their own kin, even at cost to themselves, and for any creature its immediate kin will inevitably form a significant part of its society. His ideas were taken further by Bob Trivers, an anthropologist at Rutgers University in New Jersey, to show why evolution might also favour individuals that help others they are not related to, in particular through "reciprocal altruism": one helps another in the hope and expectation that at some time the favour will be returned.

More progress has been made on the back of these fundamental ideas. John Maynard Smith's analyses of behavioural strategy through game theory, developed at the University of Sussex, are particularly important. His work shows that societies which appear to provide what Jeremy Bentham some two centuries ago called "the greatest happiness for the greatest number" may not be evolutionarily stable. Thus societies which contain only altruistic "doves" are as peaceful as can be since nobody will fight anyone else. But all-dove societies are liable to be upset by some mutant "hawk" that swaggers in and takes everything without opposition. The hawks will thrive and multiply until there are so many that they start clashing with other hawks, which is bad news for the hawks. So the hawks are more or less bound to appear among societies of doves, but their own hawkishness stops them becoming too numerous.

More generally, all societies require individuals to behave with some degree of altruism, and that in turn means that they are all at risk from freeloaders who take the goodies and do nothing in return. Trivers predicted that social creatures should be adept at detecting cheats and take a dim view of them. Evolutionary psychologistsJohn Tooby and Leda Cosmides of the University of California, Santa Barbara, have tested this prediction in humans; and so it turns out. In laboratory tests they show that people find it hard to detect violations of an "if/then" rule laid out in logic but can do so easily and accurately if the violation represents cheating in a social situation. The conclusion is that our brains have specifically evolved to spot cheating. We have not simply evolved a general ability to think logically.

The other side of this coin has been explored by Robert Frank of Cornell University among others. In his books, including The Winner-Take-AII Society, he develops the notion that humans have evolved a tremendously strong sense of justice. On the one hand we are acutely aware of being done down; on the other we go to great lengths to establish our reputations as good, honest people. In this, we go far beyond what might seem to be necessary: for example, we leave tips for waiters in restaurants we will never visit again, because we can't stop proving to the world how reliable we are.

Such notions can be tested indirectly. In Switzerland, Ernst Fehr of the University of Zurich and Simon Gachter of the University of St Gallen gave students a cooperative task of the "prisoner's dilemma" kind: all benefit if everyone plays honourably, but those who cheat benefit more provided they don't get found out (New Scientist, 12 January, p 11). The students were rewarded with real money if they did well and fined if they did not. They were also able to punish fellow players by imposing fines but only by paying a penalty themselves, so those who punished others a lot were liable to finish up with less money than those who punished very little. Surprisingly-though perhaps not so surprising to those versed in evolutionary psychology-the students tended to punish cheats severely, even though they lost out by doing so. People seem to hate cheats so much that they are prepared to incur significarit penalties to punish them. Again, this tendency does not seem confined to humans; it is not just a part of our own specific "cultural overlay". Jennifer Scott at the Wesleyan University in Connecticut has found comparable behaviour in gorillas. Even the alpha males, huge and dominant though they are, are liable to be given a bad time by their subordinates if they appear to behave unjustly.

One more line of thought, this time deriving from Darwin's idea of sexual selection, seems to sew these disparate observations into a neat story. Darwin proposed the notion that selection did not act only on characteristics that made creatures better able to survive, but also on those that made them better able to attract mates. His ideas were neglected until Amotz Zahavi took them up in the 1970s to explore why animals do weird and wonderful things for the opposite sex that are quite useless or even damaging for day-to-day survival. Indeed, Zahavi proposed that mating displays with feathers and antlers and thunderous bass voices are impressive precisely because they are so costly in time and energy. It is as though the animal were saying: "What wonderful genes I must have to able to do this and survive too."

Geoffrey Miller, in The Mating Mind, suggests that all the greatest achievements of humans, including fine art and music and the ability to do calculus and speculate on God, began as displays of prowess. One way to stand out from the crowd is simply to be passionate, and young men are particularly prone to passion; indeed, Daly and Wilson have proposed that the extreme violence so often evident in young males can be construed as a form of showing off.

So as we draw together the various threads of evolutionary psychology it becomes easy to see how young men in particular want both to display their bravery, and are deeply offended by injustice, and on both counts may risk their own lives even to the point of certain death. When these people are on our side we call them heroes and martyrs; when they are not, we label them terrorists.

What does all this add up to? The general contribution of evolutionary psychology so far is to suggest that even flying an aeroplane full of passengers into a building full of ordinary people is not beyond human understanding. It should not be described simply in the mystifying language of psychopathology. It wastes everybody's time simply to smother the whole event and the people behind it in words like "wicked" and "evil". This is the vocabulary of desperation: expressions of intellectual and moral abdication, as if such acts must forever be beyond understanding, so there is nothing sensible to be done except to purge the world of their perpetrators, with as many bombs as it takes. Once we perceive that even such extreme behaviour is in principle comprehensible, and that it is very probably rooted in the deep, human, evolved sense of justice and injustice, and perpetrated by young people who feel done down and have a yen for martyrdom, then we at least have the basis for sensible and perhaps effective strategy.

Of course, evolutionary psychology does not provide an all-embracing theory of human nature. It is not a solo turn, to replace all other ways of looking at ourselves. But it should be accepted alongside other sources of insight, including psychology, sociology, philosophy, literature. And the common canard of the critics, that the hypotheses of evolutionary psychology are not testable and therefore are not science, is simply not true. These are only the beginnings, too. Evolutionary psychology is not a panacea, or an algorithm, but it surely is a significant what should be humanity's quest for enlightenment.

Colin Tudge s latest books, In Mendel's Footnotes and The Variety of Life, are now available in paperback. The author is especially grateful to Oliver Curry at the London School of Economics for his help in preparing this article.