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December 3, 2004 2:00 am

Gaia theory's new signs of life

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Mention Gaiato some scientists and you can almost see the red mist descend. "Fairy story" is of the more polite descriptions used in public; "new-age claptrap" comes closer to summing up their private views.

Small wonder. For what else should one think of a hypothesis put forward by a self-employed inventor who says the Earth is a planet- sized organism?

But James Lovelock is not your standard autodidact in the potting shed. He may be a scientific outsider working from home in Cornwall, but he is a Fellow of the Royal Society, a Companion of Honour and renowned in academia for his invention of exquisitely sensitive chemical detectors.

As for his Gaia hypothesis, Professor Lovelock may well have been ill-advised in naming it after the Greek goddess of the Earth, but there is nothing flaky about the evidence now backing its basic claim: that living organisms continually modify our planet to keep it fit for life.

A few weeks ago, the journal Nature carried the results of a study that pointed to a hitherto unsuspected link between the global climate and plankton. The researchers found that the tiny ocean-going organisms release vast quantities of organic material that floats up into the air and seeds the formation of clouds. This, in turn, affects the amount of sunlight bounced back into space - and the surface temperature of the Earth.

How the "plankton effect" might influence the debate on global warming is unclear; the effect has yet to be included in any of the supercomputer models used to predict the Earth's climate.

However, it is becoming hard to dismiss the view that life does not merely exist on the planet but actively modifies it to suit itself.

For Prof Lovelock it has been a long haul. The idea behind Gaia came to him in the mid-1960s, while he was a freelance consultant to Nasa. The space agency was trying to think up ways of identifying life on other planets, and Prof Lovelock suspected its scientists were parochial in their view of what alien life would be like. He argued for a more general test, focused on the fact that all living organisms extract nutrients from their environment, chemically alter them, and dump their waste products back out again. A tell-tale sign of this activity, Prof Lovelock realised, would be a complex atmosphere replete with reactive gases like oxygen and methane.

Nasa rejected Prof Lovelock's test for alien life, not least because it did not bode well for missions to Mars and Venus, whose atmospheres are choked with carbon dioxide (see box below). But it prompted Prof Lovelock to wonder about other ways in which a planet could be affected by the life on it, and led to what he called (at the suggestion of William Golding, the novelist) the Gaia hypothesis.

The initial response of most of the scientific community was lukewarm. Some biologists insisted the hypothesis directly contravened Darwin's theory of evolution. According to this, life has to adapt to whatever environment it finds through a combination of random mutation and the survival of those best able to pass on their genes to the next generation.

Critics such as Richard Dawkins, author of The Selfish Gene, argued that Gaia required organisms to collaborate without necessarily benefiting themselves, a form of "global altruism" hard to square with Darwin's theory.

Such apparently compelling arguments look increasingly facile as evidence grows that the Gaia hypothesis and Darwin's theory complement each other. This is one of the conclusions from a review of the current status of Prof Lovelock's theory* by an international team of scientists.

The evidence comes principally from what has become the sine qua non of "real" science: sophisticated mathematical analysis and computer modelling. Much of the work builds on Prof Lovelock's attempt to capture the essence of his theory in a computer model known as Daisyworld.

Developed in 1981, Daisyworld is a virtual Earth orbiting a young, sun-like star. Its surface consists of bare soil in which two types of "daisy" can grow: black and white. As the young star warms up, natural selection favours black daisies - which are better at absorbing sunlight - until the temperature becomes high enough for white, heat-reflecting daisies to do better. Prof Lovelock showed that despite the changing brightness of the star, the temperature of the planet would be kept constant, held there by the ebb and flow of daisies with different heat-reflecting properties.

The review of progress with the Gaia hypothesis makes clear that far more sophisticated computer simulations have since been performed, but the upshot is the same: the idea that organisms actively modify their host planet complements, rather than contradicts, Darwinian principles.

At the same time, evidence continues to mount that the real Earth exhibits Gaia-like behaviour. It takes the form of the discovery of various "feedback mechanisms" in which organisms modify their environment in ways that, in turn, affect the organisms.

One of the most important centres on sulphur, an essential element for living organisms. Marine algae have been found to give off a sulphur-bearing gas, dimethyl sulphide, which seeds the formation of clouds whose rainfall delivers nutrients to land-based organisms. These, in turn, accelerate erosion, and the resulting run-off delivers nutrient back to the sea-going algae to complete the cycle. Meanwhile, the rain clouds affect the amount of solar energy striking the Earth, acting on the growth of the organisms and keeping the whole feedback loop under control.

The newly discovered "plankton effect" is the latest example of the elegant natural cycles that are prompting growing numbers of scientists to take the Gaia hypothesis seriously. Overall, the picture now emerging is of a complex web of non-linear effects, where even the tiniest organisms can exert global control.

The moral of Prof Lovelock's "fairy story" seems to be that the meek shall inherit the Earth, but it is the microscopic who run it.

*Scientists debate Gaia: The Next Century, edited by Stephen Schneider et al; Massachusetts Institute of Technology Press, London.

Robert Matthews is Visiting Reader in Science at Aston University, Birmingham

MARS METHANE PUTS LOVELOCK IN THE LIMELIGHT

* The Gaia hypothesis came into being 40 years ago when James Lovelock realised that atmospheric gases could give signs of life on other worlds. Now his idea is centre stage as scientists try to make sense of the discovery of methane in the atmosphere of Mars.

* Produced on Earth almost entirely by organisms, the first hints that methane was present on the red planet emerged in measurements taken last year by Europe's Mars Express orbiter. These have been confirmed by observations made with Earth-based telescopes, which detected the optical signature of the gas.

* The tiny concentrations found, combined with the instability of methane, suggest either that it has been dumped there recently - perhaps by a comet - or that it is being continually produced by some process.

* What scientists cannot yet decide is whether that process is biological or merely chemical. Intriguingly, the highest concentrations have been found seeping out of the ground close to the relatively warm Martian equator, one of the more hospitable regions of the planet.

* Astronomers are hoping to cast further light on the mystery by looking for ethane, another gas that is often produced by organisms along with methane - on Earth, at least.

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