In the Pilbara region of Western Australia, the Cleaverville Formation preserves some of the oldest intact sedimentary rocks on Earth, dating back roughly 3. 5 billion years. Among these strata lie Banded Iron Formations (BIFs)—striated layers composed of alternating silica-rich chert and iron oxide minerals like hematite and magnetite.
These vivid geological zebra stripes serve as a primary physical recording of early biological activity on primitive Earth. Before oxygen accumulated in the atmosphere, Earth's oceans were saturated with dissolved ferrous iron. When early photosynthetic organisms—likely ancestral cyanobacteria or primitive photoferrotrophs—began releasing oxygen as a metabolic byproduct, the free oxygen immediately reacted with dissolved iron in seawater.
This chemical reaction converted soluble iron into insoluble iron oxides, which precipitated out of the ocean and settled onto the seafloor as rusty red mud. During non-productive seasonal or biological cycles, light-colored silica chert accumulated instead. Over millions of years, these alternating pulses created the distinctive banded patterns, providing scientists with hard evidence of transient oxygenation events and biological innovation billions of years before the Great Oxidation Event.