Genesis of the Planet’s Oxygen
(Published in The EarthSphere Blog. Cover Image: Blue-Green Waves of Cyanobacteria by WM House; ArcheanArt; based in part on Public Domain photography by NASA Earth Observatory)
Prologue:
The Forgotten Origins collection of articles traced life’s early expansion as chemosynthetic organisms developed into the first empire in Earth’s primordial biosphere. This article picks up with evolution’s next step, the arrival of free oxygen.
Free Energy from the Sun
In the dank waters of Earth’s earliest oceans, single-celled chemosynthetic creatures carved out life’s first niche on a newly formed planet, the third planet from the Sun. Perhaps they scraped by against all the odds, but probably natural processes related to entropy and thermodynamics necessitated their existence. They were alive by all definitions but woefully inefficient, pecking around in the dark and cold, looking for tasty inorganic compounds to consume and break apart. They stole energy from these compounds and used it to create food for growth and reproduction.
It wasn’t exactly a cozy existence, and there is a reasonable chance these early bacteria spent their days crowded around volcanic vents and superheated hydrothermal flows, fighting over the best pickings. But life was life, and one did the best one could under the circumstances. Occasionally these chemosynthetic floaters might have been fortunate enough to hitch a ride on an underwater current heading towards the surface. On a particularly good day, they might even bask in the glow of the young star we now call the Sun. Warm radiant energy streaming down from above like mana from heaven would catch them in its ethereal glow. If the radiation didn’t kill them, they could absorb some of its heat.
No neurons were firing in contemplative thought at the time, but if they were, these tiny primitive cells might have despaired at the unfairness of it all. Floating around in the cold and dark and endlessly struggling for food, when above them flowed a virtually unlimited supply of free energy. Radiant sunlight was there waiting for the cleverest of clever to tap into its power and produce food for free.
Photosynthesis and Free Oxygen
Today’s autotrophs (organisms that produce their own food) rely on chloroplasts to take the Sun’s free energy and convert it into sugars for food. These magical engines of energy transformation reside within individual cells. But like all aspects of life, chloroplasts didn’t miraculously appear; they evolved into their current form. In a time long gone, some of the single-celled, chemosynthetic prokaryotes figured out how to tap into the Sun by processing near-infrared light to produce sulfides or sulfate compounds.
The story most often told is that when the world was young, some archaea or bacteria managed to ingest other photosynthetic prokaryotes and steal their trick of converting sunlight to food. This turned out to be a repeating pattern in evolution; find something useful and quickly steal it for your personal use. My story is speculative since we have no direct evidence of what happened, but it seems to be a reasonable working hypothesis. The pigments from these early sunbathers were the initial predecessors to today’s chlorophyll.
But oxygen was not yet part of the equation. Bacteria produced food from the Sun, but their reactions used sulfides, not water. The revolution started when some of the bacteria figured out how to use water and sunlight to produce food by taking CO2 + H2O + sunlight and producing the sugars needed for life. However, all actions have unintentional consequences, and a byproduct of this particular version of photosynthesis was free oxygen (O2).
Cyanobacteria
The first organisms to master this new technique were cyanobacteria, also known as blue-green algae. Despite being nicknamed ‘blue-green algae,’ cyanobacteria are not algae at all. Cyanobacteria, and bacteria in general, are prokaryotes, whereas true algae are eukaryotes, and real algae developed long after the first cyanobacteria. But the use of water in photosynthesis and the production of free oxygen as a byproduct forever altered the course of life’s evolution. The lowly cyanobacteria started the blue-green revolution.
Since investigations of early life on Earth often involve more hypothetical thinking than hard evidence, there is considerable debate about when this blue-green revolution started. Conservative thinking puts the development of cyanobacteria at 2.7 billion years ago, a mere 300 million years before the great oxygenation event when the atmosphere gained significant amounts of free oxygen.
Other bolder lines of thought push the date back to between 3.4 to 3.7 billion years ago. These dates rely on evidence of algal mats in the geological record. Stromatolite fossil structures in Australia and Greenland pin down these dates. But there is a leap of faith in these interpretations. The fossil stromatolites are assumed to be formed by shallow-marine cyanobacteria bioherms, similar to the ones we see today in places like Shark Bay Australia. But maybe there were non-oxygen-producing precursors to cyanobacteria who liked hanging around together in bioherms.
The best estimate of when cyanobacteria became a significant part of the biosphere is rooted in the geological evidence of free oxygen. Free oxygen itself didn’t fossilize, but its high chemical reactivity sparked various identifiable chemical changes in the geosphere and biosphere. The next article in the series looks at how we can unravel the rock record to detect the first signs of a world with significant free oxygen.
(Excerpts from Vanishing Origins, read the book on Wattpad as it unfolds)
Or, see my current set of Medium articles as I chronologically trace the development of life on our planet: EarthSphere Page — Forgotten Origins
The EarthSphere Blog: Exploring life and the planet supporting it.
More from ArcheanWeb:
ArcheanWeb: Exploring the environment, art, science, and more
ArcheanArt: Innovative digital art
ArcheanWeb On Medium:
EarthSphere Publication — Science and the environment
Dropstone Publication — Stories, life observations, art, and more
Books:
Reflections on life’s journey and thoughts on the Tao Te Ching — In Search of a Path
A fictional adventure about the origins of life — The Strings of Life
Stories in progress on WattPad
Sources:
When Did Earth’s First Whiffs of Oxygen Emerge? (by Becky Oskin; Live Science)
Timeline of Photosynthesis on Earth (Source: Scientific American) — https://www.scientificamerican.com/article/timeline-of-photosynthesis-on-earth/
The origins of life on Earth (Source: Australian Academy of Science)
