NIck Lane --The Vital Question, Post #3 -- June 12, 2019

From page 28:

In all forms of photosynthesis, the energy of light is used to strip electrons from an unwilling donor. The electrons are then forced on to carbon dioxide to form organic molecules.

The various forms of photosynthesis differ in their source of electrons, which can come from all kinds of different places, most commonly dissolved (ferrous) iron, hydrogen sulphide, or water. In each case, electrons are transferred to carbon dioxide, leaving behind the waste: rusty iron deposits, elemental sulphur (brimstone) and oxygen, respectively. The hardest nut to crack, by far, is water.

By 3.2 billion years ago, life was extracting electrons from almost everything else. Life, as biochemist Albert szent-Györgyi observed, is nothing but an electron looking for a place to rest. Quite when the final step to extracting electrons from water took place is contentious. Some claim it was an early event in evolution, but the weight of evidence now suggests that 'oxygenic' photosynthesis arose between 2.9 and 2.4 billion years ago, not so long before a cataclysmic period of global unrest, the earth's midlife crisis. Worldwide glaciations, known as a 'snowball earth', were followed by the widespread oxidation of terrestrial rocks, around 2.2 billion years ago, leaving rusty 'red beds' as a definitive sign of oxygen in the air -- the 'Great Oxidation Event.' Even the global glaciations indicate a rise of atmospheric oxygen.

By oxidising methane, oxygen removed a potent greenhouse gas fro the air, triggering the global freeze. [This methane was producedby methanogenic bacteria, or more specifically archaea, which if carbon isotope signatures are to be believed (methanogens produdce a particularly strong signal), were thriving before 3.4 billion years ago. As noted earlier, methane was not a significant constituent of the earth's primordial atmosphere.]

With the evolution of oxygenic photosysnthesis, life's metabolic took kit was essentially complete.

Three big points:

• life arose very early, probably between 3.5 and 4 billion years ago, if not earlier, on a water world not unlike our own
• by 3.5 to 3.2 billion years ago, bacteria had already invented most forms of metabolism, including multiple forms of respiration and photosynthesis
• everything was in place, except oxygen
• with the rise of oxygen, 2.4 billion years ago, life transfigured our planet to the point that this thriving bacterial world could have been detected as a living planet from life