Ancient Rock Find Supports Early Date for First Photosynthetic Life
Diagram of a typical Prokaryotic microbe
At some point in the geologic history of this planet, primitive, unicellular organisms (prokaryotes) emerged and proliferated. These primitive microbes were able to harness the Sun’s energy and convert it to food. The metabolic “waste product” of this photosynthetic (light-making) activity–Oxygen (O)–filled the Earth’s atmosphere over the course of vast time scales. This is sometimes referred to as the Great Oxidation Event (GOE). This geologically long event enabled the “explosion” of oxygen-breathing life forms in nearly every environment where it was present.
However, the precise date (within a few million years or so) of this event has been a point of contention amongst scientists for decades. Most have held that such life did not emerge until (no earlier than) 2.4 billion years ago. A few have radically asserted an even earlier date of nearly three and half billion years ago.
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A recent find of hematite crystals (produced by the oxidization of Iron [Fe2O3]) in Australian rock formations has been dated to 3.46 billion years. The oxidation of Iron occurs via exposure to atmospheric Oxygen (O2)–produced as a by-product of photosynthesis. Thus, this discovery and dating lends strong support to the more ancient date estimation for the emergence of unicellular life forms.
This first photosynthesizing microbe could have been a simple bacterium like the cyanobacterium–also known as blue-green algae–believed to be amongst the most primitive (having no nucleus) forms of microbial life that persists to this day. Cyanobacteria were the evolutionary precursors to eukaryotic (nucleus containing) algae and chloroplast in plants.
Earlier in the decade, hematite was detected on Mars, and then subsequently verified by the second Opportunity rover, indicating the presence of abundant atmospheric oxygen on Mars at some point in its past, or perhaps, evidence of abundant surface water (as hematite is sometimes formed in aqueous conditions).
The discovery was made by M. Hoashi, et al, and reported in the March 15 edition of the journal Nature Geoscience (Primary haematite formation in an oxygenated sea 3.46 billion years ago), and referenced in the May-June edition of American Scientist
photo: Prokaryote Microbe diagram - Public Domain
“…or perhaps, evidence of abundant surface water (as hematite is typically formed in aqueous conditions).”
Typically forms in aqueous conditions? This is an all too common misconception. Hematite (Fe2O3) rarely, if ever, forms directly in “aqueous conditions”… liquid water. It forms as the result of a dehydration of a number of precursor hydrous iron oxides (goethite, ferrihydrite)… the water is gone! Thus, the presence of hematite, especially the “coarse-grained gray hematite” seen from space over broad areas on Mars, means that water was present at some time in the past and the rocks were dehydrated later. Not exactly a beacon in the search for life or even life processes. The hematite found by the Mars rovers was in rocks billions of years old and it’s hard enough to find evidence of life on Earth in rocks that old.
Thank you for the clarification. I altered my phrasing to reflect this. However, that said, the precursor “hydrous iron oxides” that you mention (and which are subsequently “dehydrated”) could very well have once resided–prior to their dehydration–in an aqueous environment. The mistake was in not elucidating the more involved chemo-morphological sequence that you note. Another factor in this may be the amount of dissolved Oxygen in said liquid (assuming it to be water).