My desk sits in front of a window. When I should be working, it is far too easy to raise my head and idle away the hours staring out at a peaceful prospect of animals and plants.

Of course, I realize that such peace is largely an illusion. There is a war - a whole series of wars - going on out there. Invisible to me, caterpillars are munching on foliage, and birds are feasting on caterpillars and insects. Below ground, grubs suck on tree roots and grass roots, while moles tunnel along eating whatever animal protein they come across along the way. Other organisms, many of them fungi, take whatever dies and breaks it down to forms useful to plants. The whole cycle begins anew.

We were born to this, and it all seems natural and even inevitable. The process goes on as much in the seas as on the land, as much in tropical forests as in searing deserts. A hundred years ago this was seen as the "balance of nature," a system in which each organism worked for the eventual good of all. Today, viewing events from rather a different perspective, we are more likely to say that each element works only for its own good. A plant or animal maximizes the amount of its own DNA present in the world, knowing and caring nothing for the fate of any other species.

Both ways of looking at the process are valid. Over the nearly four billion years in which life has been present on the Earth, enormous self-interest and huge inter-dependencies have both developed. We see this "war of the species" everywhere that we look.

At the same time, a quite different global war is going on. To see the nature of that war, we first need to recognize the very odd nature of the Earth's atmosphere. About 23 percent of it by weight is oxygen. As we look around the solar system, we find nowhere else remotely like it, and for a very good reason: oxygen is a powerfully reactive substance, combining whenever possible with other elements. Free oxygen should not exist.

We are very aware of oxygen reactions - burning - in the form of fire; we are much less aware of the more muted flames burning away inside us, because enzymes inside our bodies control the process of combination with oxygen, allowing it to proceed at far lower temperatures than those of, say, a forest fire. But the digestion and use of our food is, from every viewpoint, just one more form of combustion. Oxygen wants to combine, whether it is inside us or outside us.

How is it possible, that this process of using oxygen can go on, millennia after millennia and eon after eon, and there is still plenty left? We have known the answer to that question for a long time. It can be used up, because it is constantly being regenerated. Plants employ the process of photosynthesis, which takes the energy of sunlight and uses it to break up molecules that contain oxygen. To the plant, the oxygen is an unwanted by-product, produced while it is building up its own tissues. It is, of course, absolutely vital to all forms of animal life.

If you have been reading along comfortably and agree with that last sentence, I should now point out that it is totally false. Life has indeed existed on Earth for close to four billion years. However, for half of that time there was little or no oxygen in the atmosphere. The first atmosphere of the planet may have been chemically neutral, or it may have contained some free hydrogen. But in either case, to the organisms living at that time, oxygen was a deadly poison.

So what happened? We can only conjecture. But at some point, an organism - call it a plant if you want, though it surely little resembled the plants of today - discovered photosynthesis. It had no use for the by-product of that process, which was oxygen, and would have been harmed by it had it been retained. It therefore discharged the free oxygen into the air. A little oxygen would have been swallowed up rapidly in reactions with other elements, but as organisms employing photosynthesis thrived and spread across the world, a critical balance point was reached. The air of the time changed, from one that was predominantly reducing (high in hydrogen, little oxygen) to one that was oxidizing (high in oxygen, low in hydrogen).

To most living creatures, that change was an absolute disaster. Oxygen, fiercely reactive, would sear and destroy their living tissues, and they simply could not stand it.

There was global war between the old regime and the new. However, it was a one-sided war, because the new toxin was everywhere in the air. Evolutionary pressure required one of two courses of action. Either you could adapt, learning to live with oxygen and ultimately to depend on it; or you could retreat to places beyond the reach of the poisonous gas.

Both avenues were pursued. One of them led to us and beings like us, comfortable with and absolutely dependent on oxygen. Not too much of it, however. An atmosphere with today's pressure and hundred percent oxygen, as the Apollo 1 accident tragically proved, makes most everyday substances ignite and go up in flames. The flammability of vegetation provides a natural upper bound on the amount of oxygen that can be tolerated in the air, and hence on the abundance of photosynthesizing plant life.

Meanwhile, the older life forms retreated deep underground, or to places where they could be safe from the attacks of oxygen. They persist today, as anaerobic forms of bacteria. No anaerobic life has ever evolved to become a multi-cellular organism. From our point of view, "we" - the aerobic forms, the oxygen-producing and oxygen-using forms - have "won." We inhabit the whole surface, we are the biggest, we are the most visible, we are the most widespread.

On the other hand, there are reasons to make us temper our boastfulness. The anaerobic forms were here long before the aerobic ones; they are here still. It's possible that in total mass they are the winners, because no one knows how deep beneath the surface Earth's anaerobic biosphere may extend, or how well-populated it may be. And, precisely because they are so deep and protected, they could probably withstand planetary surface changes that would kill us and all our kind.

The anaerobic life forms were here two billion years before the aerobic forms. Maybe they will be here two billion years after us.