Windswept_ The Story of Wind and Weather - Marq de Villiers [15]
Then, a mere billion years ago, the first eukaryotic organisms, the earliest ancestors of all of us, appeared. These were more "organized" cells than their predecessors, with DNA coding segregated into a nucleus instead of being jumbled up with everything else, and they were far more efficient at converting carbon dioxide to oxygen. The percentage of oxygen consequently began to climb, going up from mere traces about 600 million years ago to its present level of about 20 percent.
These processes, acting sequentially and then simultaneously, produced the delicate balance preserved in modern air, a combination of permanent gases (nitrogen at 78.084 percent of all air, and oxygen at 20.947 percent) and gases considered to be variable (gases that have changing concentrations over a finite period of time). Variable gases are essentially water vapor and aerosols (tiny liquid droplets that include ice crystals, smoke, sea salt crystals, dust, and volcanic emissions, suspended in the air), plus carbon dioxide, methane, and ozone, all of which are critical to maintaining planetary temperatures and therefore life (see Appendix 1).
This combination is not at all magical, except for the one simple fact that our lives are entirely dependent on it, and even a small deviation in its proportions would kill us and profoundly alter the planet's climate. But as Richard Fortey pointed out, nothing in evolutionary history made the photosynthetic bacterium that resulted in us "a dead cert" to emerge at all—the evolutionary winner could just as easily have been a bacterium that depends on sulfur, in which case we would never have evolved. Something else would have, but not us: No wonder evolution makes creationists nervous. It is not so much that we are descended from primeval slime that is disturbing, but that our whole existence is such a fluke.
Before the coming of life, then, Earth was a bleak place, a rocky globe with shallow seas and a thin band of gases—largely carbon dioxide, carbon monoxide, molecular nitrogen, hydrogen sulfide, and water vapor. It was a hostile and barren planet. This strictly inorganic state of Earth is called the geosphere, which is itself made up of the lithosphere (the rock and soil), the hydrosphere (the water), and the atmosphere (the air). Energy from the sun relentlessly bombarded the surface of the primitive Earth, and in time—millions of years—chemical and physical actions somehow produced the first evidence of life: formless, jellylike blobs that could collect energy from the environment and produce more of their own kind. Heritability was the real key to evolution, the ability to reproduce and therefore evolve. Thus was created the biosphere, the zone of life, an energy-diverting, entropy-fighting, rapidly evolving but perilously thin skin on Earth's surface that uses the matter of the earth to make living substance. It happened in water, and it happened in air. Air was the necessary predisposing factor, for all life depends on it.
III
So much we understand now; we can parse air to a fare-thee-well and measure particulate matter to a part per billion. But for millennia (from the beginning of recorded thought) and until very recently, air was the most mysterious of, well, substances, if that's what it can be called. In fact, there was considerable doubt that air existed at all; and when philosophers imagined it into being, there were no instruments able to measure it or prove its existence.
The first conceptual breakthrough came when the Greek philosophers grasped that air was something rather than nothing. It was, after all, far from self-evident. You couldn't see air, taste it, or measure it. True, there were clues—odors were a puzzle, and