From The Lives of a Cell
Viewed from the distance of the moon, the astonishing thing about the Earth, catching the breath, is that it is alive. The photographs show the dry, pounded surface of the moon in the foreground, dead as an old bone. Aloft, floating free beneath the moist, gleaming membrane of bright blue sky, is the rising Earth, the only exuberant thing in this part of the cosmos. If you could look long enough, you would see the swirling of the great drifts of white cloud, covering and uncovering the half-hidden masses of land. If you had been looking for a very long, geologic time, you could have seen the continents themselves in motion, drifting apart on their crustal plates, held afloat by the fire beneath. It has the organized, self-contained look of a live creature, full of information, marvelously skilled in handling the sun.
It takes a membrane to make sense out of disorder in biology. You have to be able to catch energy and hold it, storing precisely the needed amount and releasing it in measured shares. A cell does this, and so do the organelles inside. Each assemblage is poised in the flow of solar energy, tapping off energy from metabolic surrogates of the sun. To stay alive, you have to be able to hold out against equilibrium, maintain imbalance, bank against entropy, and you can only transect this business with membranes in our kind of world.
When the Earth came alive it began constructing its own membrane, for the general purpose of editing the sun. Originally, in the time of prebiotic elaboration of peptides and nucleotides from inorganic ingredients in the water on the Earth, there was nothing to shield out ultraviolet radiation except the water itself. The first thin atmosphere came entirely from the degassing of the Earth as it cooled, and there was only a vanishingly small trace of oxygen in it. Theoretically, there could have been some production of oxygen by photodissociation of water vapor in ultraviolet light, but not much. This process would have been self-limiting, as Urey showed, since the wave lengths needed for photolysis are the very ones screened out selectively by oxygen; the production of oxygen would have been cut off almost as soon as it occurred.
The formation of oxygen has to await the emergence of photosynthetic cells, and these were required to live in an environment with sufficient visible light for photosynthesis but shielded as the same time against lethal ultraviolet. Berkner and Marshall calculate that the green cells must therefore have been about ten meters below the surface of water, probably in pools and ponds shallow enough to lack strong convention currents (the ocean could not have been the starting place).
You could say that the breathing of oxygen into the atmosphere was the result of evolution, or you could turn it around and say that evolution was the result of oxygen. You can have it either way. Once the photosynthetic cells had appeared, very probably counterparts of today’s blue-green algae, the future respiratory mechanism of the Earth was set in place. Early on, when the level of oxygen had built up to around one percent of today’s atmospheric concentration, the anaerobic life of the Earth was placed in jeopardy, and the inevitable next stage was on the emergence of mutants with oxidative systems and ATP. With this, we were off to an explosive developmental stage in which great varieties of respiring life, including the multicellular forms, became feasible.
Berkner has suggested that there were two such explosions of new life, like vast embryological transformations, both dependent on threshold levels of oxygen. The first, at one percent of the present level, shielded out enough ultraviolet radiation to permit cells to move into the surface layers of lakes, rivers, and oceans. This happened around 600 million years ago, at the beginning of the Paleozoic era, and accounts for the sudden abundance of marine fossils of all kinds in the record of this period. The second burst occurred when oxygen rose to ten percent of the present level. At this time, around 400 million years ago, there was a sufficient canopy to allow life out of the water and onto the land. From here on it was clear going, with nothing to restrain the variety of life except the limits of biologic inventiveness.
It is another illustration of our fantastic luck that oxygen filters out the very bands of ultraviolet light that are most devastating for nucleic acids and proteins, while allowing full penetration of the visible light needed for photosynthesis. If it had not been for this semipermeability, we could never have come along.
The Earth breathes, in a certain sense. Berkner suggests that there may have been cycles of oxygen production and carbon dioxide consumption, depending on relative abundances of plant and animal life with the ice ages representing periods of apnea. An overwhelming richness of vegetation may have caused the level of oxygen to rise above today’s concentration, with a corresponding depletion of carbon dioxide. Such a drop in carbon dioxide may have impaired the “greenhouse” property of the atmosphere, which holds in the solar heat otherwise lost by radiation from the Earth’s surface. The fall in temperature would in turn have shut off much of living, and, in a long sigh, the level of oxygen may have dropped by ninety percent. Berkner speculates that this is what happened to the great reptiles; their size may have been all right for a richly oxygenated atmosphere, but they had the bad luck to run out of air.
Now we are protected against lethal ultraviolent rays by a narrow rim of ozone, thirty miles out. We are safe, well ventilated, and incubated, provided we can avoid technologies that might fiddle with that ozone, or shift the levels of carbon dioxide. Oxygen is not a major worry for us, unless we let fly with enough nuclear explosives to kill off the green cells in the sea; if we do that, of course, we are in for strangling.
It is hard to feel affection for something as totally impersonal as the atmosphere, and yet there it is, as much a part and product of life as wine or bread. Taken all in all, the sky is a miraculous achievement. It works, and for what it is designed to accomplish it is as infallible as anything in nature. I doubt whether any of us could think of a way to improve on it, beyond maybe shifting a local cloud from here to there on occasion. The word “chance” does not serve to account well for structures of such magnificence. There may have been elements of luck in the emergence of chloroplasts, but once these things were on the scene, the evolution of the sky became absolutely ordained. Chance suggests alternatives, other possibilities, different solutions. This may be true for gills and swim-bladders and forebrains, matters of detail, but not for the sky. There was simply no other way to go.
We should credit it for what it is: for sheer size and perfection of function, it is far and away the grandest product of collaboration in all of nature.
It breathes for us, and it does another thing for our pleasure. Each day, millions of meteorites fall against the outer limits of the membrane and are burned to nothing by the friction. Without this shelter, our surface would long since have become the pounded powder of the moon. Even though our receptors are not sensitive enough to hear it, there is comfort in knowing that the sound is there overhead, like the random noise of rain on the roof at night.