Windswept_ The Story of Wind and Weather - Marq de Villiers [14]
Over a period of thousands of years, the rain accumulated as rivers and lakes and ocean basins. This left carbon dioxide, CO, as the main component of air. About half a billion years after its birth, give or take an eon or two, things had settled down enough for this carbon dioxide to react with water and other compounds to form rocks and minerals. The oldest known rocks are in Greenland, and have just celebrated their 3.9 billionth birthday. Earth was still aflame with volcanoes and bombarded by asteroids, meteorites, and whatever else was floating in the interstellar void and intersecting with our nascent planet, but those oldest rocks show signs of having been deposited in an environment already containing water.
At about the same time, methane began to increase. By our planet's billionth birthday, the atmosphere would have been 80 percent methane and related carbon compounds and less than 10 percent nitrogen. Nitrogen, which is not very chemically active, continued slowly to accumulate.
Nothing about this was unique. In the solar system the only body other than ours with an atmosphere is Saturn's largest moon, Titan. NASA's probes have found that Titan's "air" consists largely of nitrogen, hydrogen sulfide, and methane, with trace amounts of other gases—in effect, Titan's air is what we on Earth would call thick smog. Titans might thrive on it, but to us it would smell vile. Breathing it would quickly kill us. You can still get the sense of it by leaning (carefully) into a volcanic vent, if you do so between eruptions.
II
It can plausibly be argued that the most extraordinary event in Earth's long life, at least from the point of view of species now living, occurred when the planet was an adolescent, about three and a half billion years ago. This was the sudden, and still quite mysterious, appearance of photosynthetic bacteria, phytoplankton and algae.
These were not the planet's first life forms; yet more primitive sulfur-eating bacteria already existed, and indeed still do exist in miasmic sulfur vents in obscure places around the globe—bacteria that, for lack of a better theory, are thought to have emerged in a pool of soupy, chemical- and nutrientrich water, although a small but influential scientific subset believes life too might have come to us from space, ready-formed, cosmic nuggets among the infinite dross. "Runoff collected in a small volume is the most likely means of achieving the necessary concentration of ingredients," Gustaf Arrhenius, a geochemist at Scripps Institution of Oceanography, told National Geographic in March 1998. The presence of hydrogen in substantial concentrations would help to explain how these concentrated compounds attracted other molecules that then acted as catalysts in subsequent reactions. Hydrogen is a volatile and active gas, and in its presence sugar phosphates, possible precursors to organic life, would have been produced.
In any case, the new photosynthetic organisms began the laborious job of converting carbon dioxide to oxygen and of splitting water vapor into its component parts, and free oxygen began to trickle, molecule by molecule, into the air. As Richard Fortey put it in his book The Earth: An Intimate History, "Three billion years of photosynthesis, much of it achieved by very simple organisms like blue-green bacteria . . . the minute rod- or thread-like remains of the bacteria that changed the world have been found among the earliest fossils. They formed sticky mats, which left their record in finely layered, crimped or cushion-like fossils called stromatolites, looking like piles of petrified flaky pastry" If you were alive at this period, Fortey wrote, "[you would see] a hot spring bubbling and hissing at your feet, surrounded by brilliant orange and purple livid stains. They feel slick to the touch. The colors are painted by bacterial life that forms a slimy film; the lively