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Living Systems in Evolution

Elisabet Sahtouris

copyright © 1999 by Elisabet Sahtouris


Problems for Earthlife


Imagining Gaia as a beautiful goddess dancing gives us a poetic metaphor for nature's living beauty. But real life is often hard and troublesome, as we know from our own experience. And Earth had big problems right from the time its dance of life began. In more scientific terms, we might say the probability that Gaia -- our name for Earthlife as a whole -- would continue to evolve was rather low during its early stages, or that a stable autopoietic Gaian system evolved only under considerable threat to its existence.

Even when its crust was already coming alive with microbes the young Earth, whirling more than twice as fast as it does now, still hissed with steam, cracked so that its lava flowed like blood and was endlessly bombarded by meteors belting in through the thin atmosphere, raising dark clouds of dust as they wounded its still tender body. The embryonic Earth's continuing life was not at all a sure thing; Gaia was not yet a secure, stable being able to maintain itself.

The constant hail of meteors, leaving craters such as we see on the Moon, was a serious threat. Though meteors may have contributed important molecules such as lipids to the formation of microbes, they might also have killed them off again. Every day these space rocks of all sizes came hurling from the sky like bullets. If nothing had happened to protect the Earth from them, it might well have ended up as lifeless and pockmarked as the Moon and our neighboring planets.

There may also have been another problem, though scientists differ on this matter. The Sun's energy was most helpful in splitting molecules so that new ones could form, but as the first microbes formed and multiplied, the strong Sunlight may have been too much for many of them to stand, putting them in need of protection from the burning part of Sunlight we call ultraviolet radiation. Some ultraviolet is good for living creatures, but too much can burn them, and our young Sun probably produced far more ultraviolet than it does today, when we are concerned about our own threat to the shield of ozone protecting us from it -- a shield that did not exist at all around the early Earth, though the smoggy early atmosphere may have offered some protection.

In any case, the first microbes seem to have formed on the seafloor, in seawater or wet mud deep enough to filter out the dangerous rays. There, as we saw in the last chapter, bits of a rich soup of organic molecules and seawater were probably trapped in liposome spheres where the molecules could move about and begin new kinds of chemical cycles. These would have included, had they not already been formed elsewhere, the construction of the giant RNA and DNA molecules that became useful as a storage system for information life needed.

In the self-production and reproduction cycles that gradually evolved, RNA lined up with DNA to copy its information, then lined up with amino acids to produce the proteins coded for, which in turn helped DNA split apart to copy itself, and so on around the loop. But giant RNA and DNA molecules could be broken by ultraviolet light, and so one of life's earliest inventions, not long after reproduction itself, was the repair of DNA with special enzymes.

Early microbes, were now becoming full-fledged bacteria of the type we call archae, simply meaning ancient. Lipid walls enclosing them permitted the entry of new raw materials and the disposal of wastes. Every living being or system has to cycle and recycle supplies. As Earth's weather cycles circulate water from sky to ground and sea and back to sky, rock is dissolved in running water and swept to the sea. Atmospheric gases are also cycled and their balance regulated. The planet's temperature is determined by all these processes, with a strong role played by its variable cloud cover.

Meanwhile, as we will see in more detail later, dissolved rock used up in forming the bodies of sea creatures ends up buried on the sea bottom in sediments pressed back into rock. Later that seafloor rock may end up as dry surface land in new plate upheavals, only to begin the cycle again.

Just so, the liposome microbes formed in the Earth's crust developed internal cycles for circulating their own supplies and carrying out the business of life. Gradually they replaced their tiny spherical capsules with larger, more flexible cell membranes and evolved into bacteria. They still depended on seawater to float supplies to them, or to float them to supplies, and to float away wastes they could no longer use. By trial and error they learned to use these supplies to grow themselves, to repair themselves when they suffered damage, and to reorganize themselves as needed, keeping records of their new discoveries in their DNA.

Every living creature must get materials and energy from its environment to form itself and to keep itself alive. What is left of these supplies after the useful parts and the energy have been taken from them, along with whatever else was part of the creature but is no longer of use to it, is waste that must be gotten rid of by returning it to the creature's environment. This is why no living creature can ever be entirely independent -- it is always a holon within larger holons, including ecosystems, depending on them for its very life.

As author/scientist/philosopher Arthur Koestler put it, a holon has at once the autonomy -- in Greek, self-rule -- of a whole in its own right and the dependence of a part embedded within larger holons. Koestler grappled with this concept of dependence along with relative independence, referring to it as an integrative tendency, or even as self-transcendence. Let us call it a holon's holonomy -- the rule of the greater whole or holon that must be balanced with its self-ruling autonomy. Physicist David Bohm used the word holonomy in exactly this sense when describing how the autonomy of every subatomic particle is stabilized and tempered by the rule of all other particles around it -- by its holonomy. Recall our earlier discussion of bootstrap theory in physics, which also expressed this concept.

Any holon containing smaller holons, such as an Earth full of bacteria or a body made of cells, tempers the individual autonomy of its components with its own autonomy, which is their holonomy. Any individual human, for example, must transcend simple self-rule and integrate him- or herself with the rules of family and society, while human society must transcend its autonomy and integrate itself with the holonomy imposed by the autonomy of the planet. The balance between any holon's autonomy and holonomy must be worked out as mutual consistency if the holon is to survive as part of a holarchy, and it cannot survive in any other way if we accept the fundamental notion of mutual consistency as described in Chapter 2 and as illustrated in later chapters.

These concepts of embeddedness or holarchy, and of the autonomy at every level of holarchy always tempered by holonomy are extremely important to understanding how life works. We humans, for example, fight about whether to seek individual interest or community interest, whether to develop locally or globally. This is because we fail to understand life's fundamentally holarchic nature -- always a dialogue among relatively autonomous embedded holons, all of which are critical to the function of the holarchy.

Bacteria are holons within larger holons consisting of their complex communities and even worldwide networks, as well as within their broader ecosystems. While we are talking definitions, let us use the term ecosystem to refer to systems of related organisms in their habitats.

Bacteria are technically called monera -- the first kingdom of living things in our present evolutionary classification scheme. Monera include the archae and their later descendents of many types. (Later we will see that bacteria are also called prokaryotes, but let that come in time.) Each moneron is a single cell, and yet it is also a whole organism or creature. The tiny monera that were Earth's first creatures were thus the first relatively independent holons within the Earth holon -- in Lewis Thomas' view, tiny cells within a huge cell.

Fortunately for these early monera, the sea was full of supply molecules, ranging from small dissolved rock salts to the larger sugar and acid molecules needed to build DNA and protein. So the bacteria could grow and divide and grow again, spreading themselves thickly throughout the seas. As they multiplied, winds and water driven by the Sun's energy swirled this rich chemical soup about, stirring it into ever greater activity. So prolific were these microbes, that their colonies, including the habitats they assembled, formed entire continental shelves long before corals evolved. Even today, bacteria, or monera, are by far the most numerous creatures of the Earth.

The more bacteria there were to suck up supplies and blow out their wastes, the more the whole chemistry of the Earth changed -- sometimes the worse for life, sometimes the better, as we will see.

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Many early monera were getting their energy by breaking up supply molecules in a process we call fermentation. The bacteria we use to make cheese, yogurt, and wine still work the same way today. Yeasts, such as those we use to make bread, do it, too. Fermenting bacteria can be thought of as bubblers, since they make bubbles of waste gases, like the bubbles you see in risen bread and in cheese. Whenever you see bubbles rising in mud or stagnant waters, fermenting bacteria are probably at work.

Breaking up molecules by fermentation or in other ways frees the energy that held them together. The bubblers stored this energy in a special kind of molecule we call adenosine triphosphate ( ATP). At first they may have found ready-made ATP molecules in their surroundings, but eventually they learned how to make them. The bubblers kept the energy-loaded ATP handy until the energy was needed for building, repair, and other work. Every living thing on Earth since then has been using the ATP energy storage system invented by the bubblers, though bacteria later discovered faster, better ways of making ATP than by fermentation. ATP is thus often called the energy currency of life.

In addition to energy, of course, the bubblers needed building supplies, and for a long time, as we said, large sugar and acid molecules were plentiful in the environment, ready to be split up or used as they were. To reproduce, some monera copied their DNA and then split themselves down the middle in the process we call mitosis, building two offspring monera from their own split halves. Others budded off smaller bits of themselves containing copied DNA to start their offspring. When supplies got low here and there, some bacteria learned to pack their DNA and a bit of protein into solid little spores with tough shells. These spores floated about doing nothing at all till they came to places where supplies were plentiful and they could grow into proper monera.

Over time, monera built new kinds of protein and new enzymes and invented new chemical processes and cycles, new parts for themselves, new lifestyles. More than three billion years ago, then, bubbler monera were multiplying and dividing into different strains, forming a thick soup or surface scum, living off ready-made supplies of large sugar and acid molecules. Some strains of bacteria learned to use the acid and alcohol wastes of others, and to set up efficient cycles of using one another's wastes as supplies. Some learned to make the nitrogen of the atmosphere usable by combining it with other elements. Had they not, life would have died out from nitrogen starvation, as nitrogen is one of the six basic elements needed to build living things.

Still, as competition for large-molecule food supplies increased, a new crisis developed. As if Gaia didn't have enough problems already, it began to look as though her first tiny creatures might die for lack of supplies.

But they didn't. Life is far too inventive to give up so easily.

What happened to the monera back then is rather like what is happening to us humans today. We have been making much of the energy we need to live in our human societies from the coal and oil supplies found ready-made in our environment. Now these supplies are running out, and we must find new ways to produce energy. A very important way of doing so involves the use of Sunlight, or solar energy.

This is exactly what some monera began doing as their supplies ran low. Some elements they had to have in order to build their living bodies were all around them, but like the atmospheric nitrogen, they were not in usable form. Others were hard to get at, such as the nitrogen locked into the salty nitrates of the sea or the carbon locked up in the carbon dioxide gas of the atmosphere. There was plenty of carbon and nitrogen all around, but the bubblers had to invent special ways to unlock the carbon and nitrogen and then `fix' them by turning them into usable bodybuilding molecules.

Perhaps the bubblers' most important discovery was finding ways to harness solar energy -- to trap Sunlight and turn it into ATP energy, which they did by using certain light-sensitive chemicals such as the porphyrins that make our blood red and the chlorophyll that makes grass and leaves green. They could then use this energy to split molecules of carbon dioxide gas, water, and rock salts into atoms, which could be rebuilt into food sugars, DNA parts, and more ATP for the work of growing, repairing, and reproducing. This process is, of course, photosynthesis -- in Greek `making with light' -- the use of light in the manufacture of food.

Some of the photosynthesizing monera are called blue-green bacteria because of the color their photosynthesizing chemicals gave them. Let's call them bluegreens for short. Their new way of life was very successful, so they multiplied quickly. After all, the blue-greens, unlike the bubblers, needed no special supplies. Water full of dissolved rock salts was what they lived in, and the atmosphere was full of light and carbon dioxide.

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There was only one problem: the bluegreens' wonderful new way of making their own food and energy was also creating pollution.

Both the bubblers and the bluegreens made waste gases as they worked, but light-making food from water and carbon dioxide gas produced a very poisonous waste -- so poisonous that it killed living things. This poisonous waste gas was oxygen!

We are used to thinking of oxygen as good and necessary, as a life-giving and life-saving gas that we breathe. But for the first living creatures, it was deadly. It is oxygen that turns metals to rust and makes fires burn. Oxygen destroys the giant molecules of living things, burning them up just as ultraviolet and other kinds of radiation do. In fact, oxygen is more destructive than ultraviolet, for the large molecules needed to build the first living things could never have formed if the atmosphere had been as rich in oxygen then as it is now. So, when the bluegreens began making oxygen, they began making trouble.

Every molecule of carbon dioxide, or CO2, is made of one carbon atom and two oxygen atoms -- di meaning two. And every molecule of water, H2O, is made of two atoms of hydrogen and one of oxygen. It takes six molecules of carbon dioxide and six molecules of water to make one molecule of food sugar. But when the sugar molecule is built from carbon, hydrogen, and oxygen atoms, it only needs twelve oxygen molecules, so six are left over as waste.

This is the oxygen that began polluting the early Earth after photosynthesis began. At first the free oxygen combined harmlessly with dissolved rock minerals such as iron, making them rust, and built itself into rock. When these crustal materials had absorbed all they could, the oxygen began piling up in the atmosphere.

It was as if a giant pump had been turned on. Bacteria were pumping carbon dioxide out of the atmosphere, using the carbon and pumping some of the pure oxygen back into it. They also pumped nitrogen out of nitrate sea salts, fixed some of it for their use, and pumped useless nitrogen gas into the atmosphere. The living Earth was bringing its own special nitrogen- and oxygen-rich atmosphere into being.

Our nearest planet neighbors, Venus and Mars, have atmospheres made almost entirely of carbon dioxide, just as was Earth's, very likely, when this great pump got going. But now our atmosphere is almost all nitrogen and oxygen -- because life made it so. But how did life survive the poisonous oxygen?

Much of it didn't survive. Some kinds of bubblers that didn't need to be near light dug themselves down into mud where the poisonous oxygen could not get at them. Their fermenter descendants still live today by hiding from oxygen in mud or in other safe places such as the stomachs of cows, where they help digest hay, or the bead-like root nodules of peas and beans, where they fix nitrogen to enrich the soil. But many, if not most, kinds of early bacteria must have been killed as the oxygen piled up around them.

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What was Gaia to do? Her dance of life had produced a rich array of living bacteria despite the dangers of meteors and ultraviolet light. Now most of the early kinds were dying just because some had discovered a new and better way to live. It was a lesson Gaia learned more than once, that new experimental forms of life may seriously endanger the whole dance and that other improvisations may be required to rebalance it. Gaia had no human brain to assess her experiment and think up strategies. Nevertheless, our evolving planet developed, as it still does, the kind of `body wisdom' physiologists attribute to our bodies.

By this body wisdom, living systems operate and maintain themselves, somehow knowing what to do on a momentary and daily basis as well as in most cases when things go wrong. We are used to it in our bodies and we count on it, but to learn that the Earth behaves the same way is still news to many people. Not at all long ago it was scientific heresy to attribute intelligence to nature. Now, as we said, our worldview is evolving rapidly. Many scientists, including Lynn Margulis, the leading researcher on microbial evolution and the author of the greatest change in our `tree of evolution' since we first devised it, see consciousness and intelligence present in the earliest microbes. If we recall that some physicists now see cosmic consciousness as the source of all matter, then it is not so surprising to find intelligent body wisdom evolving in our Gaian Earth.

Though clever species of bubblers survived by hiding from oxygen, they were no longer the main kind of monera. Bluegreens invented enzymes, which made the oxygen they produced harmless to themselves. Some also learned to make ultraviolet Sunscreens, as we make Sunglasses and chemicals to protect ourselves from Sunburn. These were able to live successfully in stronger Sunlight, where they could make plenty of food.

Others solved the problem of ultraviolet burn by living together in thick colonies. Those on top were burned to death, but the dead cells made good filters, absorbing the burning rays while letting the rest of the light reach those that needed it below. This was another way in which some lives were given for others, and a good reason for bacteria to live as cooperative life teams rather than as independent individuals.

You can see such colonies of bacteria beginning as a greenish brown scum on damp walls or muddy ground. Near the sea, they trap sand and other particles, forming thick muddy masses in shallow waters as live bacteria multiply and keep climbing toward the top. In some places we can still see this mass harden into rocks called stromatolites. In ancient stromatolites, the bacteria that have turned to rock can still be seen and identified.

The number of such rock colonies formed billions of years ago, sometimes extending into entire continental shelves, tells us just how successful the oxygen makers have been. This also shows clearly how rocks that rearranged themselves into living creatures can rearrange themselves back into rock.

For about two billion years -- almost half of Earth's life until now -- the bluegreen oxygen makers were her most successful creatures. They multiplied into thousands of different kinds all through her waters and muds, making more and more oxygen in their ever-growing colonies. And then they made yet another dramatic discovery. Some of them learned how to use the waste oxygen they created in making food molecules -- they used it to burn those very food molecules for energy.

This process of burning food with oxygen is what we call respiration -- the third way of making ATP, after the fermentation of the bubblers and the photosynthesis of the bluegreens. It is the most efficient way of all. In respiration, the destructive energy of oxygen is used to break up food molecules and thereby free both their parts and their energy for use. It is a much more powerful way to do this than fermentation. Soon other kinds of bacteria learned this method of using poisonous oxygen to good advantage. Since we call the intake of oxygen for breaking up or burning food molecules breathing, let's call the new respiring bacteria breathers.

Like fermentation and photosynthesis, respiration produces waste gas. But this time the waste gas is carbon dioxide -- the very gas needed for photosynthesis. What an incredible new opportunity. Respiration completed a cycle by leaving a supply of carbon dioxide with which to start photosynthesis anew.

Looking closely into the green and brown living `scum' on the tops of stromatolites, or in your kitchen sink, for that matter, with our newest and least intrusive microscopes, we are astounded! For up close, the scum, often containing vast numbers of bubblers, bluegreens and breathers, resolves itself into the most amazing cityscapes populated by all sorts of bacteria doing different tasks cooperatively while living different lifestyles. These cities look like Manhattan's skyscrapers, as one scientist put it, or like Hollywood sets for cities of the future, with buildings like tall balls on stems or cone-shaped and linked by endless canals, bridges, and other transport systems. Thus our ancient bacterial forbears built infrastructures for their communities much as we do today.

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Let's look further at how tiny bacteria managed to accomplish so many innovations and lifestyles. Like our own human world, the developing world of bubblers, bluegreens, and breathers in the Great Bacterial Age made its progress through many technological inventions. Simple as bacteria are in comparison with later evolved creatures, they are remarkably ingenious, and we still have a great deal to learn from them. Several of our own greatest recent technological advances, such as the DNA recombination we call genetic engineering, were learned from them. Bacteria discovered this process, actually the secret of their wild success, billions of years ago, while we have just caught on and learned to get them to do it for us in ways that we intend.

Scientific research has shown for over half a century, beginning with Barbara McClintock's work on corm plants, that DNA reorganizes intelligently in response to specific problems faced by living organisms. It happens in life forms from microbes to very large multicelled creatures. But this evidence had to pile up very convincingly over so much time, because it so flew in the face of the official dogma that it could not be so. Evolution was only supposed to proceed by accident and `selection.' Scientists firmly believed that changes in the forms of living creatures could happen only as a result of accidental mistakes in copying DNA, or by accidental breakage and recombination of DNA at certain times, such as when it is struck by fast-flying nuclear particles that zoom through the atmosphere and through us without our notice. But now scientists see that many, if not most, DNA changes are anything but accidental.

Modern bacteria have obviously been able to change very quickly in ways that protect them against our lethal antibiotics -- in Greek, `against life.' To do this, they have to make changes in their DNA. Life, it seems, does not just wait around for lucky accidents to solve problems and improve things, but is quite inventive, especially under survival pressure. But just how do bacteria do it?

We can easily see with modern microscopes that bacterial DNA is a very long complex molecule formed into a loose loop inside the tiny creature. We can also see that bacteria come very close to one another and then dissolve parts of their cell walls long enough to create a hole through which they exchange bits of DNA. One or both of them leaves this encounter with a new combination of DNA from the two though no reproduction had taken place.

This information exchange, or communication system, of ancient (and modern) bacteria is at least as remarkable as any of their other inventions and no doubt is what made the rest of their innovations possible. We are just beginning to learn how it works and to recognize it as original sex! -- something we thought had been invented much later in evolution.

Sex is by definition the production of creatures by a combination of DNA from more than one individual. Every time bacteria receive bits of DNA called genes from others, they are engaging in sex by making themselves the product of two bacterial sources even though they are not reproducing. This sexual communication system apparently belongs to virtually all bacteria of all strains, so that bacteria can -- and do -- trade their DNA genes with one another all over the Earth to this day!

. Thus these tiny ancient beings actually created the first WorldWideWeb of information exchange, trading genes as we trade our own messages from computer to computer around the world. We have speeded up their web by carrying them around the world on our ships and airplanes, to make contact in far places they might not have reached by wind and waters so quickly.

All bacteria can be thought of as one great holon with a common pool of DNA genes -- a single live network or system covering our entire planet, even extending deep under its polar ice covers and into its below-surface fissures. Throughout this system the bacteria trade and recombine genes according to need and experiment. And their `Internet' probably includes larger creatures, including ourselves, as we can see bacteria (and viruses, which may be their survival devices) coming into plants and animals to trade bits of DNA. Even before we made this discovery, we knew that no other form of life could survive today without bacteria. Why this is so will become clear as we watch the dance of life develop.

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The young Earth's bacterial gene pool or web made it possible to spread resistance to oxygen by sharing blueprints for various protective devices, as well as to spread the use of oxygen for breathing, or burning food molecules. Of all Gaia's creatures, the blue-green breathers that harnessed solar energy were the most independent ever to evolve, and they are still going strong today, billions of years later. They make their own food and burn it, using only the simplest supplies. If they drift away from light, they work in the dark. They fix their own carbon and nitrogen. Living in water, they do not even risk drying up, as do the land plants that evolved long after them to carry on the double lifestyle that the bluegreen breathers invented.

The ancient bubblers, bluegreens, and breathers had invented the only three ways that living beings all over the Earth, even today, make their ATP energy currency, for as we will see, all larger creatures are their descendants. The recycling of carbon dioxide and oxygen that began with them -- the simplest and tiniest of Earth's creatures -- was so successful that it has been an essential part of the Gaian life system ever since. In time the two parts of this cycle -- photosynthesis and respiration -- became ways of life for different kinds of one-celled creatures coming up in our story. Then, much later, plants and animals evolved to cooperate in producing carbon dioxide and oxygen for each other's use, or in recycling each other's waste, depending on how you look at it.

Again we are reminded of lessons people are learning today. First the ancient bacteria solved their energy crisis by developing solar technology, then they discovered that recycling supplies is the best way to avoid running out of them.

As the oxygen piled up in and thickened the atmosphere, it not only created new problems requiring new solutions, but was itself a solution to the old problem of ultraviolet burn. Destructive as oxygen was to so many kinds of microscopic monera, it actually helped form a protective blanket of air around the living Earth. Just as in our ancient myth Gaia first formed the seas in her dance of life and then created a protective atmosphere, so it was in reality.

Our atmospheric blanket of air seems very thin to us. We can just barely feel it by waving our arms around in it. But what we feel against our arms would be much harder if our arms were waving much faster. Meteors move so fast that the air is quite solid to them. And rubbing hard against something solid produces heat, as you can easily demonstrate by rubbing your hand hard over a table. Meteors rub up against air so hard that the heat, together with the oxygen, ignites them and burns them up. The more oxygen there was, the more meteors burned up, until so few of them got through the atmosphere that life was much safer on Earth.

Nor were meteors the only outside dangers oxygen protected Gaia against. The oxygen in our air is made of twin oxygen atoms dancing together as free-floating molecules. As ultraviolet rays strike these molecules, they break up the pair, leaving separated twins to join other oxygen pairs as triplet molecules. Such triplet molecules are no longer oxygen gas; they are ozone -- O3. And it is very difficult for ultraviolet rays to pass through ozone because it absorbs them. When there was plenty of oxygen, a whole layer of ozone collected in the middle of the atmosphere, shielding the Earth from dangerous amounts of ultraviolet radiation.

Microbe-produced oxygen probably even played a role in preventing the seas from drying up, because atmospheric oxygen can trap evaporating lightweight hydrogen as water, thus preventing it from escaping into space and allowing it instead to fall back into the seas in the form of rain. Methane-producing fermenters may also help hold the oceans onto Earth, as atmospheric methane decomposed by ultraviolet rays creates the tropopause lid, which is another barrier to the escape of hydrogen into space.

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From this early history of the Gaian dance of life we can see that great problems are great challenges, and that living things are very inventive when faced with challenges. Maybe that is one of the most important things we can learn from evolution.

It remains to be seen whether we humans will prove as creative as ancient bacteria in the face of the problems we create. Over billions of years, most of the carbon dioxide was pumped from Earth's atmosphere by photosynthesis, while nitrogen, oxygen, and rarer gases produced by living creatures replenished it. Over this long time, life worked out exactly the right balance of gases that was best for it. Now we are changing that balance in dangerous ways.

Our use of coal and oil, for example, is creating a very serious double problem. Not only are we using up these important fuel supplies but we are also polluting the atmosphere with too much carbon dioxide in burning them. Coal and oil are made of ancient forests that built much carbon into their plant and animal life. As they were pressed underground over time, the carbon was buried and transformed into natural fuels. That is why we sometimes playfully call oil `dinosaur blood.' When we dig up these fossil fuels and burn them, the carbon is released back into the atmosphere as carbon dioxide.

At the same time, we are also burning today's growing forests to clear land for our use. This releases even more carbon into the air while killing the very plant life that uses up carbon dioxide to make oxygen, thus preserving the balance.

Billions of years ago oxygen was the great danger. Now the danger is too much carbon dioxide. The result, among other things, is that our planet is heating up, for too much carbon dioxide prevents its normal loss of heat through the atmosphere. When our own bodies heat up in this way, we have a fever. If we don't solve our energy and production problems very soon in ways that are healthful for life, the Earth will have to solve the problem itself, restoring its balance as best it can. Atmospheric carbon dioxide is rapidly approaching levels it apparently reached previously just before the ice ages. Perhaps Gaia will cool her man-made fever with a new ice age, destroying most of what we have built and forcing us into retreat -- like the ancient bubbler bacteria -- to safer environments.

Inconvenient as another ice age would be, we at least know humans have survived a number of them by moving to the tropics where new land is exposed as ocean water is removed to form snow and ice. Far worse would be Gaia's other alternative, which is now looking ever more likely: to reset her thermostat at a higher planetary temperature, thus regaining the Gaian system's stability as a whole at our expense. Humans, other land animals, trees and other land life, would all succumb to the increased heat and the loss of almost all dry land if polar caps melted and flourishing oceans rose dramatically in a heat age.

This is what we are learning: to understand that the Gaian life system has evolved in such a way that it takes care of itself as a whole, and that we humans are only one part of it. Gaia goes on living, that is, while her various species come and go. We used to believe that we were put here to do whatever we wanted to with our planet, that we were in charge. Now we see that we are natural creatures which evolved inside a great Earthlife system. Whatever we do that is not good for life, the rest of the system will try to undo or balance in any way it can. That is why we must learn Gaia's dance and follow its rhythms and harmonies in our own lives.

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