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Take a thousand eggs...
Mass-produced clones could soon be rolling off the production line

A CHIP that will automatically create hundreds of cloned embryos at a time is being developed by a Californian biotech company, New Scientist has learned.

If it lives up to its promise, the chip should help make cloning cheap and easy enough for companies to mass-produce identical copies of the best milk or meat producing animals for farmers. It might even be used for cloning human embryos.

The chip automates the laborious process of nuclear transfer, the key step in cloning. At present it takes hours of painstaking work with a microscope to remove the nucleus of an egg cell and replace it by fusing the denucleated egg with another cell.

"If somebody's got something like that, obviously it would make everybody's life easier," says Tanja Dominko of Advanced Cell Technology, the Massachusetts company that caused a stir late last year when it announced that it had created cloned human embryos.

In animals, cloning is still very wasteful. At best, around half of cloned embryos develop to the point where they can be implanted, and only a tenth of these survive to birth. Often more than a hundred nuclear transfers must be carried out to create a single clone.

Scientists usually start with a batch of 150 eggs, and denucleate them one at a time before moving on to the next step. That means eggs can be left sitting around for several hours, a delay that may reduce success rates.

But the nuclear transfer array developed at Aegen Biosciences, by the company's founders Richard Kuo and Gregory Baxter, could handle hundreds or even thousands of eggs at once. Kuo says they can routinely denucleate 30 to 50 sea urchin eggs at a time. They plan to start testing cow eggs in the next few weeks.

The prototype is a thin silicon slice a few centimetres across etched with hundreds of tiny wells, one for each egg. The trick is to spin the chip in a centrifuge, forcing the eggs' dense nuclei through a small hole at the bottom of each well. About 90 per cent of the eggs can be successfully denucleated this way, Kuo says. Kuo and Baxter are now working on the next step, which is to fuse a donor cell with the denucleated egg. A lid with appropriately positioned donor cells will be placed on top of the eggs. "Then they're ready to fuse,' says Kuo, although he won't reveal-details of the method. After fusion, eggs that develop far enough could be implanted manually into an animal's womb as normal.

"If it works with cow [eggs], that would be very neat," says Rudolph Jaenisch of MIT, who studies problems with cloning. But just because it works with sea urchins doesn't guarantee that it will work with the eggs of other species, he warns. And Randall Prather of the University of Missouri, whose team recently announced the cloning of miniature pigs, says the chip won't help solve other problems, such as ensuring that the eggs you use have been kept in the right conditions. He thinks it might also be too expensive for many labs.

Kuo admits there is much work still to be done on the chip, but he believes it's worth the effort. One could submit different batches of eggs to various treatments, to flnd out which conditions improve success rates in cloning, he says. Such studies could also help researchers identify the factors in eggs that reprogram the added nucleus. If the chip does improve success rates in animals, it is likely to be used to create cloned human embryos, where the problem is not dealing with many eggs at a time but getting hold of sufficient numbers of eggs. Companies such as Advanced Cell Technology hope to obtain embryonic stem cells from cloned embryos but have had only limited success (Ne,w Scientist, 1 December 2001, p 4).

The chips might also appeal to the mavericks who want to carry out human reproductive cloning despite all the warnings about the risks. The warnings are based on the health problems seen in the few clones that do survive, which have also prompted the FDA to ask companies not to sell food from clones until it has been proved be safe.

A lesson in tolerance
Can immune cells be taught to get along with transplanted organs?

A LITTLE re-education could work wonders for the cells that provoke the immune attack on a transplanted organ. They could be taught to tell the immune system to ignore the organ instead.

Activating either one of two genes in these cells does the trick, Nicole Suciu-Foca Columbia University in New York and her colleagues have discovered. The finding may also help explain why the immune system sometimes attacks its owner's body, and how certain viruses and bacteria evade it. "The implications are potentially enormous,' says Suciu-Foca.

Immune cells recognise other body cells as "self' because they all have the same MHC proteins on their surface. If foreign cells invade, antigen-presenting cells (APCS) take bits of the foreign proteins and display them on their surface, together with the body's own MHCS. When T helper cells see this combination, they tell other immune cells to hunt down and destroy any cells displaying the foreign proteins.

But in transplant patients, things can go horribly wrong because the APCs in the donor organ carry their original owner's MHC proteins. The host immune system interprets this as a signal to launch an attack on all cells bearing these MHCS, triggering rejection of the donor organ.

But Suciu-Foca has found that another kind of cell, called a T suppressor cell, can also come into play. When the right sup pressor cells are present, APCs persuade T helpers to ignore cells sporting foreign proteins instead of sounding the battle cry. Suciu Foca found they do this by get ting the APCs to switch on two genes: immunoglobulin-like tran script 3 and 4 (ILT-3 and ILT-4).

She thinks that it might be possible to prevent transplants being rejected if some way could be found to switch on the ILT-3 and ILT-4 in the APCs in donor organs before transplantation, or to generate a population of the right T suppressor cells. This would avoid the need to give transplant patients immuno suppressive drugs, which leaves them vulnerable to infections.

The big question is how to do this. But the team did find that this process might already hap pen naturally in some trans plant patients. They compared 10 heart transplant recipients who had experienced rejection episodes within eight months with five who had not. Only in these five were they able to find T suppressor cells capable of switching on ILT-3 and 4 in APCs from the donor.

"It's a very important insight for basic immunology,' says Guido Silvestri at Emory University in Atlanta, Georgia. But he warns that the picture is clouded by the fact that all 15 patients were taking immunosuppr6ssive drugs. To prove that having the right T suppressor cells really does make a difference, Suciu-Foca is planning a clinical trial to find out if they allow patients to get by with lower doses of immunosuppressive drugs. Philip Cohen, San Francisco More at: Nature immunology (DOI: 101038/ni760)

Reaping the reward
China's genetically modified crops are proving a success

IT'S not what campaigners against genetically modified crops want to hear. Not only are GM plants taking over China's rice paddies and cotton fields, but millions of poor farmers are benefiting as a result, according to a survey of the country's GM crop programme, the largest outside the US. China's government-employed biotechnologists have targeted many crops largely ignored by big Western companies. And, unhindered by eco warriors, they are moving swiftly from the lab to the field.

Scott Rozelle of the University of Califomia at Davis and jikun Huang from the Chinese Academy of Sciences in Beijing surveyed biotech labs and talked to farmers. They found that the country has already introduced more than 120 genes into about 50 plant species. But while Western companies mostly engineer crops to resist herbicides, Rozelle says that more than 90 per cent of Chinese field trials target insect and disease resistance, reducing the need for expensive and dangerous pesticides.

China has released rice varieties resistant to three major pests and done field trials on GM wheat. Other GM crops on sale include

pest and disease-resistant cotton, tomatoes and sweet peppers. In the pipeline are GM potatoes, rape, peanuts, cabbage, melons, maize, chillies, papaya and tobacco.

But Bt cotton, which carries a gene for a toxin that kills insects, is the big success story. China's own version has been on sale since 1997. Around 2 million Chinese cotton farmers now grow Bt cotton, in fields covering 7000 square kilometres. Farmers' production costs have dropped by 28 per cent and the average income has gone up by more than $150 per year. Use of toxic pesticides such as organophosphates has plummeted by 80 per cent and pesticide poisonings have gone down (see Table).

"the system is run by officials who at least are partly concerned about food security".

"There is a clear contrast with Western biotechnology," says Rozelle. In the West,
"We are concerned about genetic farm products, but there are so many other environmental issues in China for people to worry about," says Liang Congiie, president of Friends of Nature in Beiiing. But AdrianBebb of Friends of the Earth UK said the study ignored the environment. "It will only be a matter of time before insects develop resistance," he says. "The Chinese are just replacing one form of unsustainable farming with another." China could soon be exporting GM seeds and products to other developing countries, Rizelle says. But it plans to impose tough regulations on GM imports. Fred Pearce

More at: Science (vo[ 295, p 6T4)

The end of the world
How big must an asteroid be to destroy civilisation?

IF A collision with an asteroid is going to finish us off, it will have to be a lot larger than anyone thought, according to a controversial new study of the impact that wiped out the dinosaurs.

Virtually everyone agrees that the asteroid that hit Chicxulub in Mexico 65 million years ago killed the dinosaurs, but how it did so is unclear. A long-standing theory is that clouds of dust hung in the upper atmosphere for months, blocking sunlight and stopping plants growing. But no one is sure that this is really the reason, and finding out is critical for assessing the risk asteroids pose to humanity.

Now geologist Kevin Pope of Geo Eco Arc Research in Aquasco, Maryland, is claiming that dust can't have been to blame. Only dust grains smaller than a micrometre across stay suspended in the atmosphere, and Pope says that the 10-kilometre asteroid would not have created enough fine dust to have a global effect.

Instead he thinks sulphur ftom the rocks vaporised by the impact may have formed sulphate aerosols that blocked out the light. He says earlier overestimates of dust levels mean that the hazards from an asteroid impact today have been "greatly overstated".

The Chicxulub impact spread debris across the globe, which settled to form a layer averaging 3 millimetres thick -that's a few trillion tonnes of material. But having reviewed previous work on the subject, Pope says that more than 99 per cent of the layer is made up of spherules-droplets that condensed from vaporised rock. Only the remaining 1 per cent of the debris consisted of rock pulverised directly into dust.

It's still uncertain what the size distribution of that dust would have been, but from studies of volcanic dust, Pope deduces that less that I per cent of it consisted of particles smaller than 1 micrometre. That's only 100 million tonnes-about 10 times as much dust as was released by the 1991 eruption of Mount Pinatubo, which had a barely measurable effect on global climate.

But other researchers aren't convinced that the impact produced so little dust. Jan Smit of the Free University in Amsterdam points out that volcanic dust isn't formed in the same way as impact dust, so the particle sizes wouldn't necessarily be the same. He says his studies of iridium in the impact layer suggest that at least half of it is in particles smaller than 0. 1 micrometres.

Even if Pope is right, we can't rest easy just yet. "Other things will get you," says Brian Toon, an atmospheric scientist from the University of Colorado in Boulder. He believes the effects of an asteroid impact would be apocalyptic-filling the entire sky with fiery meteors as the debris rained back down onto the atmosphere. "Everything on the surface is going to catch fire," he predicts.

But despite all the debate, much still depends on guesswork. "We know so little about impacts," says theoretical geophysicist jay Melosh of the University of Arizona. 'The uncertainties are at least a factor of five." Jeff Hecht More at: Geology (vot 30, p 99)

Out of the void
Even particles that don't exist can pull their weight

WE'D all like to get something for nothing. But harnessing the energy of empty space?

It sounds crazy, but the idea is not so farfetched, thanks to a strange force that comes out of nothing. Researchers have persuaded this force, called the Casimir effect, to slide tiny gold plates past each other. "This should help us exploit this fundamental force on a tiny scale," says Umar Mohideen, a physicist at the University of California, Riverside.

The Casimir effect depends on the fact that on tiny scales, empty space isn't empty at all. Even a total vacuum is filled with a quantum froth of virtual particles that pop in and out of existence. They don't last long enough for us to detect them directly, but in 1948 the Dutch physicist Hendrik Casimir predicted that if you put two parallel plates close enough together so that the largest particles can't squeeze into the gap, then the net pressure ftom the extra particles outside would push the plates together.

It took until 1996 for physicists to measure the effect directly-dramatically demonstrating that bizarre quantum goings-on really can affect large-scale objects in the "real world". But pushing plates together isn't much use to anyone, and the effect even threatens to jam up moving parts in micromachines (New Scientist, 17 February 2001, p 22).

Then, in 1997, MIT physicist Mehran Kardar worked out that the shape of the plates could have a startling effect. The Casimir force always acts perpendicular to the plane of the plate, so he reasoned that using corrugated plates should persuade the force to move them sideways past each other.

Last week, Mohideen and his team announced that they had measured this lateral force. They placed two corrugated gold plates a few hundred nanometres apart with their peaks and troughs aligned (see Diagram). When they moved the plates slightly out of alignment, they detected a force of a few piconewtons that pushed them back into position.

You don't get out any more energy than you put in, but it's the first time that virtual particles have been cajoled into doing work in this way. The researchers are now trying to generate other effects, such as a repulsive force and a "dynamic Casimir effect" that moves plates back and forth.

The team's measurements could also pick up signs of other as yet undiscovered fundamental forces, as well as evidence of extra spatial dimensions that some theorists predict are 'curled up" on a tiny scale. "We should be able to place limits on the size of these effects," says Mohideen. Justin Mullins More at: http://arxivorg/abs/quant-ph/0201087

On the brink
The tiniest change can push the Earth's climate over the edge

OUR planet is balancing on a knife edge. Even small, random events such as surging glaciers or sudden floods can trigger fundamental changes in the Earth's climateprovided they happen at the right moment.

Several times during the last ice age our planet warmed by up to 10 'C in little more than a decade. Typically these sudden warmings, known as Dansgaard-Oeschger events, happen every 1 500 years, but sometimes they miss one or two beats, reappearing after 3000 or 4500 years. Andrey Ganopolski and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany developed a computer model of the warmings to try to explain this mysterious pattern, and have come up with what climatologists are this week hailing as the most likely answer-random events coinciding with a subtle and otherwise benign climatic pulse.

In Dansgaard-Oeschger warmings, ocean circulation flips abruptly between two stable states. Its state at a given time depends on the salinity of the far northern Atlantic Ocean near Greenland. When that water is salty and dense it sinks to the ocean floor, driving a vigorous ocean circulation that keeps the planet cool. But if salinity falls, the circulation slows or even halts and our world warms.

The Earth's climate system seems to have a natural pulse every 1 500 years, say Rahmstorf and Ganopolski, probably caused by regular fluctuations in the energy output of the Sun. By itself this weak rhythm would have little effect on our climate. But if random geological events on Earth happen to coincide with the peak of the cycle, the cumulative effect can be enough to tip the balance and cause a flip from one state to the other. This effect is called stochastic resonance, and it comes into play in a variety of systems, from radio reception to the stock market. Climatologist Richard Alley of Penn State University says the finding is a breakthrough. It's not proof-no climate model can provide that, he says. "But this is right at the front of important ideas driving this research. It gives us hope of understanding the instability of climate." Since the last ice age ended, the planet has been fixed in a relatively stable cool phase. But, says Ganopolski, "the ocean circulation is not unconditionally stable in present conditions. It simply means that our modern climate needs stronger perturbations for it to be disturbed." And global warming could do it. Changing rainfall patterns and melting ice in Greenland mean that the water in the north Atlantic is already becoming fresher. Ganopolski believes that a flip in the oceanic system could boost temperatures still further as early as the beginning of the 22nd century. "That's not a prediction. But it is something more than pure speculation," he says. Alley agrees: "The more the climate is forced to change, the more likely it is to hit some unforeseen threshold that can trigger quite fast, surprising and perhaps unpleasant changes." Fred Pearce More at: Physical Review Letters (vol 88, p 038501)

Mother wish

TAKE A GOOD LONG LOOK AT YOUR PARTNER. REMIND YOU OF ANYONE? LYNN DICKS DESCRIBES UNSETTLING RESEARCH THAT SUGGESTS WE'RE ATTRACTED TO PEOPLE WHO LOOK LIKE OUR PARENTS

EVERYONE KNOWS the adage that when it comes to eyeing up prospective mates, opposites attract. Well, not in David Perrett's lab they don't. In the quiet confines of the psychology department at the University of St Andrews in Scotland, like attracts like with unerring regularity. And while there's no actual sex involved-just a lot of students gazing into the faces of hypothetical mates on computer screens-the results have been creating quite a stir.

Perrett is a cognitive psychologist and well-known manipulator of human faces (British readers may recall his famous stepwise, digital morphing of Margaret Thatcher into John Major). And for more than a decade he and his team have been sitting people, usually students, in front of computers, showing them a stream of faces and asking them simple but telling questions like, who do you fancy most?

Their attempts to pin down the essence of facial attractiveness have been reported by so many newspapers in recent years that most of the findings have a familiar ring. Like other psychologists, for instance, Perrett and his team find that being average is a good way to attract. Blend lots of faces together and you get a bland yet curiously attractive composite which people rate more highly than even the most appealing of the individual faces.

Also like psychologists elsewhere, Perrett and his team find that they can get even higher ratings by exaggerating certain features of these fanciable, average faces. Make the man's brow and chin more imposing and the jaw larger, and in a trice he becomes as alluring as a young Brando. Make the woman's face a tad flatter and longer, her chin smaller and cheek bones more prominent (think Audrey Hepburn) and she too beats the average hands down. All of which suggests our beauty detectors are finely calibrated to respond to some sort of "ideal" face.

'GENETIC VARIETY IS SUPPOSED TO BE THE KEY TO PRODUCING BIOLOGICAL FITNESS. SO SURELY WE SHOULD BE PROGRAMMED TO FIND OPPOSITE TYPES ATTRACTIVE?'

But here's the rub. When Perrett and his colleagues photograph the face of the person doing the rating, subtly change its sex using their clever morphing program, and then flash that image up on the screen, people really go for this sex-changed version of their own face. They never recognise it as a feminised or masculinised form of themselves. There's just something about it that appeals. And now the scientists think they know what that something is. We are attracted to such faces, it seems, not because they unconsciously remind us of ourselves, but because they remind us of our mum or dad.

In other words, there may be a grain of truth in the Freudian idea that we learn what to look for in a partner by gazing into the faces of our parents during our impressionable years. And if it sounds like just another quirky psychology theory, think again. The evidence to date may not be exactly cast-iron, but Perrett and his team believe they have stumbled on something that could be central to the way we play the mating game. If so, it's a force that has shaped human evolution. It may even help explain why racial characteristics vary more than adaptation to different environments alone would suggest-an idea that Perrett and his team are now testing. Many readers will already be reaching for their pens. Yet the basic observation that people tend to choose mates who are boringly similar to themselves is actually nothing new. Psychologists have known for decades that total strangers can pair up married couples from photographs of the individuals with eerie accuracy. What wasn't so clear in the past was just how strong and discriminating the attraction between lookalikes is. It may not always trump the desire for a perfect jaw or chin, but, according to Perrett's research, it's a force to be reckoned with. And it's a force, too, whose evolutionary purpose remains something of a mystery. Like may well attract like. But to what end? The fact that there are ideal faces we all rate highly is easier to explain. In recent years, evolutionary theorists have produced several plausible (if not exactly proven) suggestions as to why strong jawlines and prominent brows are so attractive in men, and small chins and high cheekbones such a boon to women. To take just one example, strong jawlines are thought to result from an unusually strong surge of testosterone during puberty. Why should that be attractive? Because-so the argument runs-only men with robust immune systems are able to tolerate such a surge. A firm jawline, in other words, is an outward sign of hidden biological fitness. But apply such Darwinian thinking to the special attraction we supposedly feel toward our lookalikes and you draw a blank. After all, genetic variety, not sameness, is supposed to be the key to producing biologically fit offspring-hence the well known hazards of inbreeding. So surely we should be programmed to find opposite types attractive?

One possibility is that our fondness for people who look like us is not what it seems. There are all sorts of reasons we might end up with someone like ourselves that have little to do with biology. Marriages, for instance, usually unite people of the same religion, educational background and socioeconomic status. What's more, to look for a mate is to enter a high-stakes marketplace in which the safest tactic is to settle for-someone at about the same level as you in the pecking order of attractiveness. Making a play for someone way above or below ourselves in that hierarchy is bound to end in tears. Perhaps the perceived similarity of couples is a reflection of just that: people finding their level. Perrett and his team doubt it. They say that couples tend to match on a wide range of characteristics-such as size, eye colour and strength-that are not universal currencies of attractiveness. In these cases we must be choosing partners who look like ourselves ... or members of our family. It was this realisation that drew Perrett's team into investigating whether our parents' looks might influence out choice of mate. To test the idea, the researchers presented undergraduates with a computer-generated image of an average face at different ages and asked them to rate it for attractiveness. The results were striking. Although all students preferred younger faces to old, those whose parents were older than 30 when they were born were significantly more attracted to older faces than were students born to young parents. So it seems the older your parents when you're growing up, the more likely you are to prefer older partners later in life. That doesn't prove people are responding to their parents' looks. As well as having more wrinkles, older parents are often wealthier and able to offer greater stability. Perhaps that's what makes older faces more alluring for the children of older parents? It's unlikely. In addition to rating faces, Perrett' students were asked to give an age range for their 'ideal partner"-in other words the kind of person with whom they'd want to share their lives, rather than just have sex. This was no higher than normal for those with older parents, indicating they really were judging by looks in the earlier study and not by the material resources that age represents. By choosing to focus on age in their experiments, the researchers were sidestepping another potential pitfall. Thanks to genes, we share many physical features with our parents. So many, in fact, that it was always going to be hard to know whether the students had learned what's attractive by looking at their parents, or at themselves in the mirror. That's why the team chose age. Because we can never be the same age as our parents, Perrett explains, "there is no chance of self-inspection affecting the choice".

That's hardly the case with noses, chins and jawlines, which we may well have inherited. So proving these exert the same sort of influence is more complicated. Even so, Perrett's colleague Tony Little is currently studying hair and eye colour. 'You need huge samples and complicated statistics to control for the effect of self," he says, "but so far the results look promising." His preliminary findings suggest we are most attracted to people with eyes the same colour as those of our opposite-sex parent. A hint, perhaps, of a biological basis for Freud's Oedipus complex? 'The appearance of opposite-sex biases is highly suggestive of Freudian patterns," says Perrett. To settle the matter once and for all, you would have to compare the preferences of people brought up by biological and adoptive parents. If Perrett and his team are right, adopted children should show a preference for faces that are similar to those of their adoptive parents, not their biological ones. Although the team has no plans to test this at the moment, there is a second, more distant line of reasoning they can draw on.

'THE PRELIMINARY FINDINGS GIVE A HINT, PERHAPS, OF A BIOLOGICAL BASIS FOR FREUD'S OEDIPUS COMPLEX'

The idea that we learn what's attractive from our parents when we are growing up may seem radical to us, but it's no revelation for biologists studying other animals. From looking at their parents, many learn very early in life who they should mate with later on. And they can be easily hoodwinked. A duck brought up in a goose family will try to mate with geese when it reaches maturity. The lavanese mannikin, normally a drab brown bird, can be fooled into preferring mates with red crests simply by gluing red feathers onto its parents' heads.

Even mammals have been shown to be in thrall to this "sexual imprinting'. In 1998, a team led by Keith Kendrick of the Babraham Institute in Cambridge persuaded ewes to adopt newborn goats, and nanny goats to adopt newborn lambs. In adulthood, when they were led into a barn and given a choice between a sheep and a goat of each sex, the fostered animals didn't hesitate. "It was clear v@ quickly that cross-fostered animals chose to mate and socialise with their [adopted] mother's species,', says Kendrick.

Of course, sexual imprinting in these animals is acting at a pretty crude levelthey're merely using their parents to decide which species to mate with. AS humans, we don't need to examine our parents to learn to fancy people rather than chimpanzees. So if sexual imprinting happens in people, it must have a subtler role, skewing our choice of mate towards a parental "type".

Even if it does, though, this still doesn't address the bigger mystery of what purpose such fine-tuning of our beauty detectors might serve. Sexual imprinting may explain how we learn to prefer faces that are similar to our own or those of our parents. But why learn at all? What's the reproductive pay-off?

One possibility, according to Pat Bateson of the University of Cambridge, is that inbreeding is not always such a bad thing. Mating with a relative, rather than a complete stranger, traps harmful genetic traits in a population's gene pool. But at the same time, it might preserve combinations of genes that are successfully adapted to your particular local environment (New Scientist, 26 January, p 13). It might also increase the proportion of your genes that you share with your children, making it even more worth your while-from the standpoint of your selfIsh genes-to invest time looking after them.

That's why Bateson believes there's a balance to be struck between reckless outbreeding and obsessive inbreeding. And in animals at least, he believes this is achieved by sexual imprinting. It's a precise mechanism for improving the quality of offspring, he says.

'SEXUAL IMPRINTING MAY EXPLAIN HOW WE LEARN TO PREFER FACES THAT ARE SIMILAR TO THOSE OF OUR PARENTS. BUT WHY LEARN AT ALL? WHAT'S THE PAY-OFF?'

In the early 1980s, Bateson showed that Japanese quails brought up in foster families with artificial adornments like spots painted on their chests would later choose mates with similar, but not exactly the same adornments. In fact, the degree of similarity most preferred was one that would be found between first cousins. In the light of these results, it's perhaps no surprise that marriages between first cousins are not just allowed but encouraged in many human cultures.

Not everyone goes along with the idea that a bit of inbreeding is a good thing. Bill Amos, also at Cambridge University, has recently shown that in albatrosses, pilot whales and seals, outbreeding is always better. In these creatures, the offspring of less closely related parents are always more fertile and hence biologically fitter. In Amos's view, a complete stranger is always best. So how does he explain our attraction to people who look like us?

It all depends on the dangers of inbreeding in a given population, he says, and the haz ards are greater for animals like whales than people. 'In our exponentially expanding global population, the effects of inbreeding are minimised, " he says. "The desire for some one who looks like your father becomes a good strategy because if you have reached the stage of choosing a mate, your father was obviously good at producing young."

The twist, adds Amos, is that the degree of outbreeding in the population as a whole that will make this a safe option hasn't always been there, and still isn't in some parts of the world. 'I suspect you wouldn't find this effect if you went to small hunter gatherer tribes in Africa."

For the rest of us, however-in our highly mobile, ethnically diverse world-the risks of inbreeding may be less important. In fact, selecting a strange mate from such a bewilderingly diverse market could pose greater genetic and social dangers. Whether these are big enough to make us uncon sciously prefer parental lookalikes remains to be seen. But if they are, then unprepos sessing people everywhere can draw some comfort from the idea.

After all, why worry if you lack the perfect Brando jawline when there's an alternative route to someone's heart. Simply make yourself look like them-or better still, like their mum or dad.
Lynn Dicks is a freeiance writer based in Cambridge

Further reading: "Facial attractiveness judgements reflect learning of parental age characteristics" by Dave Perrett and others, Proceedings of the Royal Society B, in press. Visit www.perceptiontab.com to take part in oniine experiments on facial attractiveness and other studies organised by Perrett and his team.

SENSATIONAL MINDS WILL THE DAY COME WHEN WE CAN RUN A BRAIN SCAN OR TAKE A BLOOD SAMPLE AND SAY, THAT'S A CERTAIN TYPE OF CONSCIOUSNESS AT WORK?

SUSAN GREENFIELD THINKS IT WILL. HERE SHE PROPOSES A NEW WAY TO LOOK AT THIS MOST SUBJECTIVE OF EXPERIENCES

HOW does a wrinkled lump of grey matter weighing little more than a kilogram manage to think, love, dream and feel such widely different sensations as raw pleasure and numbing depression? Philosophers, physicists and computer modellers have been pondering these questions for decades, wondering how your brain creates your consciousness-your personal inner world of thoughts and feelings. Thus far, their deliberations have not been entirely fruitful. My own view is that we should put this big question-the "water into wine" problem of how the bump and grind of brain cells translates magically into subjective experience-to one side for the moment, and concentrate on a much less glamorous approach. I think we can try to establish a correlate of consciousness-the particular physical state of the brain that always accompanies a subjective feeling. If we could do so we may at last be ready to develop a testable model of what happens in the brain when you are conscious.

My suggestion is that the depth of consciousness varies according to the number of brain cells working together at any moment in time. At its most basic level I am proposing that consciousness is synonymous with raw emotions, and at its fullest extent with inner reflection and self-awareness. Consciousness is like a dimmer switch, it grows as brains grow, but it also varies from moment to moment as neurons are coordinated into vast but highly evanescent working assemblies. These assemblies are modified in turn by feedback from the body, and communicate their state to it. Hence, consciousness, in my view, is also a dialogue between the three great control systems in the body: the nervous, hormonal and immune systems.

'EVEN IF YOU'RE AN IDENTICAL TWIN, YOUR MIND WILL BE UNIQUE. YOU SEE THE WORLD IN TERMS OF THINGS THAT HAVE HAPPENED TO YOU ALONE'

Soon we may even be able to monitor this dialogue, or at least to measure these assemblies as an index of consciousness, and so perhaps gain a better understanding of what other people or animals are experiencing. Most usefully, this model might also suggest new ways to treat mental illnesses, many of which I see as caused by an inappropriate degree of consciousness at any one time. The best way to begin to explain consciousness is to draw up a shopping list of the features or properties we expect. Then neuroscientists can go back to their labs and see how the brain could deliver. First, I don't believe we should be looking for one special brain region. Many regions are active while you are awake, but as you become unconscious, they all shut down in a fairly uniform way. When someone has been anaesthetised, there's no one region that lights up or gets extinguished. There is no single specialised "centre for consciousness'. Secondly, although consciousness comes from more than one brain area, at any one moment you have only one consciousness. The world seems of a piece. So we can expand the first item on the list to say that while consciousness is distributed all over the brain, somehow the activities of the different regions are coordinated. And if there's no special centre or neurons for consciousness then the neurons and areas that generate it must do other jobs as well. The physical manifestation of consciousness must be something that happens in or to ordinary brain cells at certain times, but not others. Also on my shopping list is the notion that the more complex the brain the deeper the consciousness. The idea of degrees of consciousness helps answer questions such as when a fetus becomes conscious, and which other animals are conscious. I can't see a physical Rubicon when the brain of a developing fetus changes suddenly, nor any obvious cutoff in the animal kingdom between a nervous system that generates consciousness and one that does not. We should think instead of a continuum: a rat is conscious but not as conscious as a dog; a dog is conscious but not as conscious as a primate; and so on. Even an ant will have a tiny modicum of consciousness. If you think of consciousness like this-as something that varies by degree-there are two interesting consequences. The first is that we may be more conscious at some times than at others, hence our experience of states of "heightened awareness", and the conviction that we can "raise" or "deepen" our consciousness. The second, crucial consequence is that we will have finally converted consciousness ftom a qualitative to a quantitative phenomenon. We can then look for a measure of the depth of our consciousness as it varies from one moment to the next, and search the brain for something that contracts or expands with it. I think that the most logical place to look is in very large networks-"assemblies"-of brain cells.

You're born with pretty much all the brain cells you'll ever have, but as you mature these cells develop more interconnecting branches. Our brains are incredibly plastic, and these connections grow and change with every experience. Babies evaluate the world in purely sensory terms-how sweet, how fast, how cold, how loud. But gradually these abstract sensations coalesce into people and objects with meaning and associations. It's these personal connections and associations that I think of as the "mind". The mind is your personalised brain, which allows you to see the world in terms of what you have experienced already. Even if you're a clonethat is, an identical twin-your mind will be unique. You see the world in terms of things that have happened to you alone. If we see a familiar person, our visual system activates a "hub" of brain cells that corresponds not only to the shapes, movements and colours of a face, but to all the associations set up in our mind by our experiences of that person. That can all happen without our being aware of it. Consciousness, I believe, is generated as this active, hard-wired hub corrals huge numbers of other brain cells around it to form a vast working assembly that lasts for just a trice. The image I have is like throwing a stone into a puddle, producing ripples of consciousness. We now know the brain to be capable of forming such highly transient assemblies. Amiram Grinvaid at the Weizmann Institute in Rehovot, Israel, has shown that in response to a flash of light, as many as IO million brain cells become active together, coordinated into a working assembly that lasts for less than a quarter of a second-exactly the space and time scales I think we should be exploring.

The assembly will be slightly different every time. Partly it will depend upon the size and strength of the stimulation of the hub, but also on the levels of a variety of chemical messengers-neurotransmitterswhich change moment by moment. These transmitters "modulate" the activity of large groups of cells and mediate arousal levels, your sleep-wake cycle and your dreaming. In physiological terms, these put cells on "red alert"-they can predispose brain cells to be recruited into the working assembly, triggering lots of covert associations. I think it is the activity of these transient neuronal assemblies that correlates with the depth of your consciousness at any one moment. To test the model, let's take some examples of the different types of assemblies formed and see how they relate to different types of consciousness. One time you'd expect to see unusually small cell assemblies would be when you didn't have much connectivity in the first place, as in a young child's brain. VVhat do we know about an infant's consciousness? One feature is that their centre of attention varies depending entirely on the sensory quality of what they're seeing. They live in the press of the moment. in a rather abstract world with little meaning, reacting to everything in a simple, emotional way. Infants are like little sensory sponges: they lack any accumulated experience with which to interpret the world. They haven't yet forged multiple connections-they haven't yet developed a "mind". Each burst of brain activity will come from only a small hub of cells, which will create small, short-lived ripples of consciousness.

'MY OWN VIEW IS THAT EMOTIONS ARE THE BUILDING BLOCKS OF CONSCIOUSNESS, AND THAT YOU CAN'T HAVE CONSCIOUSNESS WITHOUT SOME SORT OF EMOTION'

This is, I believe, the most primitive kind of consciousness we have, with a small assembly associated with strong emotions and an immature mind. So my own view is that emotions are the building blocks of consciousness, and that you can't have consciousness without some sort of emotion. That's why I for one don't put much of a premium on computer models of consciousness: such models focus on tasks such as learning and memory, which an ordinary PC can do without subjective inner states. There are times when adults too have diminished consciousness. You would have small assemblies, as in childhood, when you're dreaming. However, the reason would be different. In this case you have no strong sensory input, so there's little to stimulate the neuronal hubs, and you're dependent on internal residual neuronal activity. This perhaps explains why dreams have a disconnected, flimsy narrative. At the time they seem very real, with high emotional content, but in retrospect we wake up and judge our dreams as irrational with the checks and balances of our cognitive adult minds.

We can chemically alter our level of consciousness, too. So a third situation in which you might have a small assembly would be if the work of the brain's chemical messengers was disrupted, affecting the ease with which the working assemblies formed. Taking drugs such as ecstasy can interfere with one such chemical, serotonin. And in schizophrenia, levels of another messenger, dopamine, are effectively in excess. In both cases the ease with which assemblies form would change, the net size would be smaller and consciousness would seem childlike or dreamy. People may take the world at face value, see it in sensory terms and display flimsy logic. Another time you would find only small assemblies is when you are in a rapidly changing environment with such competition that the assemblies don't have a chance to form properly. Fast-paced sports like white-water rafting, bungee-jumping or skiing would do it, as would a rave. The opposite of such states would be a large cell assembly, where one would expect the outside world to seem remote. Your senses would be reduced, you might feel emotionally numb, yet extremely self-conscious. You would have a highly logical, perhaps persistent train of thought. These symptoms often occur in clinical depression. Perhaps depression is due to the malfunctioning of the chemical modulators, resulting in overly large assemblies. We know the drug Prozac and related agents influence those chemicals.

Although my theory seems to predict what to expect in different types of consciousness, the assemblies of neurons I'm positing do not all on their own generate consciousness. Assemblies are merely an index-a correlate of your prevailing inner state. Something else must happen. I believe assemblies report to the rest of the body, and the rest of the body reports back to them, and this iteration somehow translates into subjective consciousness.

Neuroscientists, at their peril, often ignore the fact that the brain is in a body. We know that feedback from the rest of the bodymost noticeably the immune system and hormones-can influence our state of mind, and similarly our state of mind can influence other control systems like our immune status. And we know that the nervous, endocrine and immune systems are interlinked. I think the links must be chemical, and for my money peptides are very good candidates. These substances coexist with traditional transmitters, but are only released under special circumstances, as neurons become more active. There are many different peptides, so you would never have exactly the same amounts or combinations twice. Moreover, we know that peptides can interface with the immune, nervous and endocrine systems: some peptides are also hormones, and this puts them in a good position to be, if you like, trilingual.

The way I see it is that at any one moment, transiently formed cell assemblies would release a signature profile of peptides into the body. These peptides influence the endocrine and immune systems, and in return the systems would release peptides that would determine the size of the brain assemblies. That iteration of peptides between the three great control systems of the body is, in my view, what happens when you are conscious. One day it may be possible to test this hypothesis, by recording profiles of peptide availability in the blood and trying to correlate these with the prevailing state of consciousness. I think at a clinical level this exercise would be useful. It might suggest new ways of treating conditions such as depression with novel types of drugs, or of developing non-drug treatments that might drive the formation of a certain size of assembly and alter the type of consciousness in a beneficial way. How this all translates into the elusive subjective inner state of consciousness is a completely different question, and I'm not pretending to have answered it. On the other hand, I do think that we can use this model in the future to design experiments and help us understand depression and emotions, why people take drugs, and perhaps most mystifyingly, why people go bungee-jumping.

Baroness-Susan Greenfield is a crossbencher in the House of Lords, Professor of Pharmacology at the University of Oxford and director of the Royal Institution of Great Britain

Further reading: The Private Life of the Brain by Susan Greenfield is published by Penguin (2000)