Photosynthesis, Oxygen, And The Early Earth


Today, Oxygen comprises almost 21 percent of the atmosphere.  The rest is mostly Nitrogen.  There is no known chemical or biological process, Nick Lane tells us in his very informative book, OXYGEN: The Molecule That Made The World, that could have “produced an atmosphere so rich in Nitrogen.”  He believes that erupting volcanoes may have provided much of it.   Although, I have to admit, I’m always a little suspicious when scientists pull out volcanoes or meteorites to explain the otherwise inexplicable.

Lane tells us that for the first two billion years or so of the Earth’s existence, Oxygen made-up less than one percent of the atmosphere.  Bacteria were what thrived best in such a low-Oxygen environment.  Plant and animal cells (made from more complex Eukaryotic cells) would have to wait a few billion years to make their appearance.

At some point, these uni-cellular critters began to harness the energy arriving from the Sun in an early form of photosynthesis, but not the kind that plants and algae use today.  Lane says that the source of the Hydrogen in those days was not water but Hydrogen Sulfide (probably the original form of photosynthesis) or Iron Salts or, later, Hydrogen Peroxide; but as these resources became depleted, the Hydrogen source of choice eventually became water.

Lane describes water-based photosynthesis as a process of using the Sun’s energy to rip away the proton and electron forming the Hydrogen atoms of water (water of course equaling two Hydrogens plus one Oxygen) and “leaving the husk—the Oxygen—to be jettisoned.”  Lane says that “the energy required to extract protons and electrons from water is much higher […] than that needed to split Hydrogen Sulfide,” but the early photosynthesizers had little choice but to make the switch:  water was abundant, and Hydrogen Sulfide had become not so.

Once water-based photosynthesis began to spread over the world, the stage was set for the rock-n-roll show that we call Life.  About 543 million years ago (a pretty specific date when it comes to geological time), scientists tell us that there was a great leap in the population of multi-cellular life-forms on Earth.  “The whole of creation as we know it exploded into being,” exudes Lane.

Lane suggests that the growing amount of atmospheric Oxygen produced by the new photosynthesizers may account for this push toward multi-cellular life.  Life-forms, not yet adapted to this new level of Oxygen, would have found it beneficial to clump together and take advantage of each other’s rudimentary Oxygen defenses:  if the guy next to you is creating a space with less Oxygen toxicity in it, then why not elbow inside that relatively safer bubble?  And it’s not as selfish as it sounds; by combining resources, you are both better off.

Even as Life was evolving to better handle the extra Oxygen in the air, the presence of increased atmospheric Oxygen was already having a profound positive effect on Earth for Life as we know it:  it was helping the planet to hang on to its water supply.

Left unchecked, says Lane, the powerful ultraviolet rays of the Sun would eventually burn away our stock of water, sending Hydrogen off into space.  But the presence of free Oxygen in the atmosphere stops the loss of water by reacting with the rising Hydrogen and reforming water, and so preserving the Earth’s oceans.

The text was unclear as to what happens to the Oxygen part of the molecule after the sunlight splits apart water; seems to me that whatever Oxygen did not get bound into iron oxides and such on the surface would also become free Oxygen in the air, and thus would work to halt the escape of Hydrogen into Space by recombining with it to form water again .  Perhaps, without the Oxygen created by Photosynthesis, the rate of recombination to re-form water simply could not keep up with the rate of the loss from the destructive energy of the Sun.  Regardless, Lane credits the evolution of Oxygen-producing photosynthesis with keeping Earth from becoming a Mars-like desert.

James Lovelock, the scientist credited with coming up with “Gaia Hypothesis” of Earth (which basically states that the Earth operates as a unified, self-regulating system), says that even with photosynthesis, Hydrogen still escapes into Space at the loss-rate of 300,000 tons per year.  But don’t worry.  At this rate, says Lovelock, after another four billion years, the Earth will still have lost only one percent of its oceans.  Maybe there’ll be a smarter species around to deal with things then.


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