快猫短视频

Venus is hot enough to melt lead. Mars is colder than the South Pole. Yet for 3.5 billion years conditions on Earth have been just right for life. The Gaia hypothesis provides one possible explanation why

THE EARTH is a fine place for life. The air is breathable, temperature and pressure are just right, and the atmosphere screens us from ultraviolet radiation from the Sun which would otherwise fry us in minutes. Did this match between Earth鈥檚 environment and life come about because life on Earth adapted to the conditions it found? Or did life alter the Earth鈥檚 environment to make it 鈥� and to keep it 鈥� habitable?

Adaptation by life must be part of the explanation, but it cannot be the whole story. We know that many of the conditions we and other living things need to survive are produced by living things. For example, plants made all the oxygen during photosynthesis, the process by which they convert sunlight into chemical energy for growth. Nearly all the nitrogen and carbon dioxide in the atmosphere today have biological sources, though in the early history of Earth they were produced by volcanoes. So living things must sometimes have altered the environment as well as adapted to it.

Without life, the Earth鈥檚 atmosphere would be like that of Mars or Venus with a lot of carbon dioxide and no oxygen. The temperature would probably be different, too. The greenhouse effect of gases in the atmosphere keeps the Earth warm by trapping some energy when sunlight which has been absorbed by the Earth is re-radiated back into space. But some gases are better at this than others. Since the mixture of gases on Earth would be different without life, so would the temperature of the Earth. Our planet could easily be either too hot (like Venus) or too cold (like Mars) for life to exist at all. Nonetheless, despite the fact that the Sun has long been getting hotter and the Earth has been hit by many huge meteorites, life has thrived here for an unimaginable length of time 鈥� 3.5 billion years.

The Gaia hypothesis seeks to explain the survival of life on Earth for billions of years by treating life and the global environment as two parts of a single system. The system, sometimes also called simply 鈥淕aia鈥�, has developed so that it can regulate and repair itself. Regulation means life actively keeps the global environment comfortable for life to continue. And, says the hypothesis, if Gaia is knocked dangerously off balance, it can repair itself.

Consequently, the system as a whole is nearly immortal, able to survive even major crises such as the impact of huge meteorites 鈥� something which has probably happened many times during the history of life on Earth. This does not mean that any individual species, such as human beings, is guaranteed to survive. It does mean that life as a whole is very robust 鈥� particularly simple bacteria, which until 1 billion years ago were the only type of life on the planet.

Gaia (pronounced 鈥済uy-ah鈥�) was the brainchild of Jim Lovelock, an independent-minded scientist and inventor who first published his idea in 1972. It was named after the 鈥渕other Earth鈥� goddess of the ancient Greeks and, confusingly, the word is used not only for the hypothesis but also for the system the hypothesis describes. The Gaia hypothesis is still controversial with both strong supporters and strong opponents, including some who charge that it is not properly science at all, but belongs more to philosophy or even religion. I shall give the main arguments for and against the hypothesis, and leave you to make up your mind.

Arguments for Gaia

Regulation and repair

THE EARTH on which life first appeared 3.5 billion years ago was totally unlike the planet we know today. The planet was only just past the final stages of formation from a nebula of gas, dust and rocks. Cataclysmic impacts with planetesimals, huge meteorites tens of kilometres in diameter, were still occurring frequently (see Inside Science No 30). Though we do not know for certain what the composition of the atmosphere was, it was dominated by carbon gases, probably thick carbon dioxide but possibly methane. It certainly contained no oxygen.

The Sun was also different. Assuming that it has developed in the same way as other stars of its size and composition, it must have been 25-30 per cent less hot than it is now. If you were to turn down the Sun this much today, the Earth would cool so much that the oceans would freeze. But we know this has not happened since life appeared because practically all life requires temperatures above freezing to thrive. Warm-blooded animals which regulate their own temperature only evolved comparatively recently. The common explanation for this paradox is that the greenhouse effect of the early Earth was stronger, probably because of the greater amount of carbon dioxide (see Inside Science No 13). Over time, the greenhouse gases have been reduced in the atmosphere, and the Sun has become brighter, so that the temperature at the surface has remained comfortable for life. Gaia鈥檚 supporters believe this is evidence for the existence of Gaian regulation mechanisms 鈥� that life on the planet actively regulated the temperature to make this happen.

THE SECOND idea is that of repair. If Earth was a peaceful place there would be no need for Gaia. But Earth is not peaceful. It is continually bombarded by fragments of rock from space. Almost all the rocks are small and burn up in the atmosphere to form shooting stars but a few are big enough to reach the ground. About once every 100 million years nowadays a planetesimal may hit the Earth, shooting enough dust and gas 15 kilometres or more into the atmosphere to block out the Sun over most of the planet. These cosmic collisions are thought to do enormous damage, wiping out huge numbers of species.

The Earth has probably been hit by planetesimals several times and the most recent one is thought to have wiped out the dinosaurs 65 million years ago. There is no reason to believe that collisions with planetesimals should be less common the further back in time one goes.

But life survived these catastrophes. In fact, on a geological time scale, new species arose quickly to replace the old ones 鈥� as, for example, after the dinosaurs were wiped out. Supporters of Gaia argue that this is evidence in support of the hypothesis. They say it shows the joint system of life and the environment on Earth is both robust and able to repair itself quickly. Though biological control of the global environment may break down immediately after such a disaster, life rapidly regains control after these events and begins to regulate again.

This does not mean the planet is left unchanged. The species on Earth would be different after the catastrophe, causing the new environment to differ, too.

Arguments against Gaia

Evolution and luck

CRITICS of Gaia say there is no mechanism by which the system could have evolved. Evolution of species occurs by competition between both individuals and their genes. Genes determine the characteristics of an organism and are passed on to offspring. Genes which help their own organism to survive will tend to be successful and spread more widely. Genes which help other organisms at the expense of their own will tend to die out.

There are examples of altruistic behaviour by organisms which apparently favour others at their own expense. But most apparently altruistic behaviour actually results in concrete benefits. For example, a chimpanzee grooming its neighbours for parasites seems to be performing a service for no reward, but it is helping to establish its membership of the group of animals on which, when it comes to feeding or mating, it depends. So although the behaviour seems altruistic at first, it is really selfish and helps the genes which cause the behaviour to become more common.

Opponents argue that Gaia requires that life has evolved to be altruistic on a global scale, with organisms all over the world cooperating for all their mutual benefit. But it is difficult to see how the processes of evolution could produce this result. For example, what is to stop the organism which cheats, putting no effort into this grand scheme but letting the others do all the work? This freeloader will have more energy to spare for its own immediate survival and as a result will leave more offspring. Its genes will survive at the expense of more altruistic organisms and the cooperation will soon break down.

If regulatory mechanisms do exist on a global-scale, this argument shows that they cannot have arisen through the process of evolution by natural selection acting on individual genes. How then did they arise? Lovelock and his supporters have suggested that perhaps they did not evolve in the sense in which biologists use that word. They suggest that regulatory behaviour simply developed as a property of the complex processes which link organisms to their environment. Simple models of these processes such as Daisyworld (see box) explain how this might work.

ALTERNATIVELY, it has been argued that the survival of life on Earth for 3.5 billion years could be just luck. Imagine there were originally millions of planets like Earth spread through the Universe, each with life evolving soon after the formation of the planet. After 3.5 billion years, we could expect that all life has gone extinct on all except a few of them, perhaps on all except one. But only on that planet has enough time elapsed for intelligent animals to evolve. These animals look back on the history of their own planet, and unaware of the extinction occurring on the other planets, conclude that some guiding principle called Gaia must exist which has kept their environment suited to life. They would be wrong of course: they forgot that, if life had become extinct, they would never have evolved to ask the question in the first place.

This argument is impossible to refute, but it is not so much an argument against Gaia as a warning against reasoning from false premises: the system that the Gaia hypothesis proposes may exist, but the long survival of life on Earth is not by itself a proof of it. To prove or disprove the hypothesis we have to look to see whether the processes of regulation and repair which the hypothesis proposes do, in fact, exist.

Gaian lessons for Earth

The carbon dioxide pump

ONE of the results of the Gaia hypothesis is that scientists have looked more carefully at the mechanisms which regulate the Earth鈥檚 environment. It is early days for theories about these mechanisms but two such theories demonstrate well the complexity of the Earth鈥檚 processes and the difficulty of studying them. Neither of these theories depends explicitly on the Gaia hypothesis, but both were to some extent inspired by 鈥淕aian thinking.

We believe the amount of carbon dioxide in the atmosphere has controlled the temperature of the Earth throughout its history. If Gaia exists then we would expect that atmospheric carbon dioxide was under biological control. Life should try to remove more carbon dioxide from the atmosphere as the temperature goes up so as to act as a stabilising influence.

Over millions of years, the amount of carbon dioxide in the air is controlled by a balance between the rate at which it is released bv volcanoes and the rate at which it is taken up by the chemical weathering of rocks containing silica. Weathering is the process by which minerals are slowly eroded by chemical and physical reactions. Neither weathering nor the eruption of volcanoes apparently involves life, so how can life control atmospheric carbon dioxide?

The weathering of silicate minerals, found in granite for example, takes place in the soil and uses up carbon dioxide. Plants and microorganisms 鈥� such as bacteria 鈥� absorb carbon dioxide from the air and pump it into the soil either through their roots or by causing decay in the soil. So typically there is about forty times as much carbon dioxide available in soil as there is in air, and this speeds up the weathering. As a result, more carbon dioxide ends up in the soil and less in the atmosphere. Eventually, carbon is laid down as carbonate rocks such as limestone.

What would happen as the Sun became hotter? An increase in the Earth鈥檚 temperature would probably expand the tropics at the expense of polar regions. This would increase the amount of biological activity on the Earth and the amount of carbon dioxide pumped into the soil. Consequently, carbon dioxide would be removed from the atmosphere, reducing the greenhouse effect and tending to reduce the original rise in temperature.

This is a 鈥淕aian鈥� temperature regulation mechanism, in the sense that it relies on a property of life for its effectiveness. However, some scientists have proposed purely non-biological mechanisms for temperature regulation by way of atmospheric carbon dioxide. For example, in common with almost all chemical reactions, the weathering of silicate minerals by carbon dioxide will go faster at higher temperatures, and the increased rainfall associated with this increase in temperature would also tend to accelerate the inorganic reaction. Proponents of Gaia argue that organisms process and cycle carbon so rapidly that such inorganic feedbacks would be ineffective compared with biological mechanisms.

In a cloud, every water droplet starts life by condensing around a microscopic particle of water-absorbing material called a cloud condensation nucleus or CCN. If there is a shortage of CCN, as sometimes occurs in very clear air, then clouds form with fewer, but larger, water droplets. If there are plenty of CCN there will be more but smaller cloud droplets. Smaller drops make for whiter clouds and reflect more of the Sun鈥檚 light back into space, thus cooling the Earth. Potentially, therefore, the climate of the whole planet can be altered by changing the number of CCN in the atmosphere.

Clouds and plankton

Sunlight changes

IT is possible that over large parts of the oceans the main source of CCN is dimethylsulphide gas 鈥� DMS for short 鈥� emitted by algal plankton that live close to the surface. The gas is generated in the surface oceans, from which it moves into the atmosphere. Sunlight changes the DMS into acids which form CCN. 快猫短视频s have calculated that the algae release more sulphur in DMS than all the power stations in the world. And the pattern of CCN over large areas of the oceans is roughly consistent with their being derived from DMS.

According to the 鈥渃loud-algae鈥� theory, the algae exert an important influence on the temperature of the Earth through their effect on the whiteness of clouds. The emission of DMS thus helps to keep the planet cool 鈥� another Gaian-like regulator.

But even if the algae do influence global temperatures in this way, it is not clear how strong the influence is or whether it is a regulatory mechanism. We do not know whether algae produce more or less DMS as temperatures rise. For the whole system to tend to stabilise the temperature, the cloud-algae feedback has to be negative 鈥� DMS production and the whiteness of clouds must rise as global temperatures go up.

Recent measurements made in Antarctic ice up to 15 000 years old suggest the opposite. As the temperature increased at the end of the last Ice Age, the amount of DMS-derived sulphur in atmospheric particles seems to have fallen. This would mean that the relationship between clouds and algae tends to destabilise global temperature. This seems inconsistent with Gaia. However, it is possible that the sudden warming at the end of the Ice Age represents a temporary breakdown of the normal regulators.

A living planet

Inventive ideas

IS the Gaia hypothesis right or wrong? We may never be able to give a black-or-white answer. But whether it is correct or not, Gaia has acted as a powerful stimulus for creative thinking in the study of the global environment. The cloud-algae hypothesis is a good example of an inventive idea, directly inspired by Gaia, which may be of great importance in climate studies but may not in the end actually support the original hypothesis at all. Gaia has contributed to a realisation among scientists and non-scientists alike that the special environment at the surface of the Earth depends fundamentally on life. In a real sense, the Earth is a 鈥渓iving planet鈥�.

Testing the Gaia hypothesis

THE DIFFERENCE between a theory in science and an idea in philosophy or religion is that a scientific theory must be capable of being tested. Normally, the way a theory is tested is by making predictions from it, and then finding out whether the predictions are correct or not. 快猫短视频s still dispute whether Gaia counts as a 鈥渢heory鈥�. Some testable predictions can be made from Gaia but, as with many theories dealing with the history of the planet, they are not easy to investigate. We may have to await advances in our understanding of the remote past of Earth and other planets of the Solar System before we can test them properly. Here are two examples of these predictions.

There was never any life on Mars: We know from the unmanned NASA missions to Mars that today it is a cold and lifeless planet. However, we also know that Mars was once much warmer than it is now. Some features on the surface were eroded by abundant running water, which implies that temperatures there were once above freezing. This has led some scientists to speculate that early in the evolution of the Solar System there may have been life on Mars, but that it became extinct as the planet lost its early thick atmosphere and cooled down. From the Gaia hypothesis this would be unlikely. If it is a property of life that it quickly gains control over the planetary environment and regulates it, a slow cooling of the planet should not wipe out all life (though perhaps a truly awesome collision with a planetesimal could). This prediction could be tested by a manned mission to Mars, which looked for evidence of early life.

Life and the environment evolve in synchrony and in sudden bursts: If life and the environment are really as closely linked as Gaia suggests, they should always evolve together. Large changes in the planetary ecosystem should be accompanied by large shifts in the global environment, such as the composition of the atmosphere. According to Gaia, the system is normally stable and unchanging. Changes when they occur, happen suddenly during abnormal periods, when the system is strained beyond its capacity to regulate. Fortunately, fossils provide a good record of how species evolved and show when there were sudden jumps in the pattern of life on the Earth.

If, as we find out more about the history of the Earth, it becomes clear that variables such as atmospheric composition normally vary widely and smoothly, without sudden jumps, this will be evidence against Gaia. But if the environment tends to change suddenly, at the same time as breaks in the fossil record, this would be consistent with Gaia, though of course it does not provide an absolute proof.

Daisyworld environment

DAISYWORLD is an imaginary planet spinning on its own axis and orbiting Daisysun. The only kind of life on the planet are daisies of white and black hue, and the only thing that affects them is temperature. Like all organisms, the daisies will not grow if the temperature is too high or too low. They grow best at 20 掳C and will not grow if temperature falls below 5 掳C and wilt and die if it exceeds 40 掳C. The mean temperature of a planet is a balance between the heat received from its sun and the heat lost to the cold depths of space in the form of radiation. But on Daisyworld the daisies can influence the temperature of their surroundings. Their whiteness reflects sunlight away and tends to cool the planet, while their blackness absorbs heat and warms it up.

As it ages, Daisysun, just like our Sun, warms and becomes brighter. To begin with, Daisysun is less luminous so only a narrow band warms enough for growth. Here, the daisies germinate and flower. We can assume the first crop of daisies to be an equal mix of light and dark species. Even before the first season鈥檚 growth is over, though, the dark daisies are favoured. Their greater absorption of sunlight keeps them warm above 5 掳C. Most of their pale cousins die because they reflect the sunlight and cool below the critical 5 掳C.

The dark daisies have a head start the next season, for their seeds are the most abundant. Soon, their presence warms not just the plants themselves but, as they grow and spread, also the soil, and then the air, locally to begin with and then regionally. With the rise of temperature, their rate of growth exerts a positive feedback and leads to the colonisation of most of the planet by dark daisies. Their spread becomes limited by a rise of global temperature to levels above the optimum for their growth. Any further spread would lead to a reduced yield in seeds. In addition, white daisies grow and spread in competition with dark ones because they are able to keep cool. Gradually white daisies exert a negative feedback, causing the global temperature to drop.

As Daisysun grows older and hotter the proportion of light to dark daisies changes until the heat flux is so great that even the white daisies cannot keep the planet below the critical 40 掳C upper limit for growth. As Lovelock says, 鈥淎t this time flower power is not enough. The planet quite suddenly becomes barren again and becomes hotter and hotter鈥�.

Systems with negative feedback 鈥� that is, those which react to changes in their surroundings in such a way as to reduce the effect of the change 鈥� are naturally stable. In planets, such a feedback would help to ensure the survival of life. Positive feedback, where the reaction to changes in the surroundings enhances the effect of a change, can be useful but cannot by itself create stable conditions.

Further Reading

The Ages of Gaia: a Biography of Our Living Earth, by James Lovelock (Oxford University Press, 1988); Gaia: the Growth of an Idea, by Lawrence E. Joseph (St Martins Press, 1990); An Atlas of Planet Management, edited by Norman Myers (Doubleday, 1982); The Breathing Planet: A 快猫短视频 Guide, edited by John Gribbin (Basil Blackwell, 1986).